Cognitive Complexity

Cognitive Complexity is a measure of how difficult a unit of code is to intuitively understand. Unlike Cyclomatic Complexity, which determines how difficult your code will be to test, Cognitive Complexity tells you how difficult your code will be to read and comprehend.

A method's cognitive complexity is based on a few simple rules:

• Code is not considered more complex when it uses shorthand that the language provides for collapsing multiple statements into one
• Code is considered more complex for each "break in the linear flow of the code"
• Code is considered more complex when "flow breaking structures are nested"

Further reading

• Cognitive Complexity docs
• Cognitive Complexity: A new way of measuring understandability

disallow bitwise operators (no-bitwise)

The use of bitwise operators in JavaScript is very rare and often & or | is simply a mistyped && or ||, which will lead to unexpected behavior.

var x = y | z;

Rule Details

This rule disallows bitwise operators.

Examples of incorrect code for this rule:

/*eslint no-bitwise: "error"*/

var x = y | z;

var x = y & z;

var x = y ^ z;

var x = ~ z;

var x = y << z;

var x = y >> z;

var x = y >>> z;

x |= y;

x &= y;

x ^= y;

x <<= y;

x >>= y;

x >>>= y;

Examples of correct code for this rule:

/*eslint no-bitwise: "error"*/

var x = y || z;

var x = y && z;

var x = y > z;

var x = y < z;

x += y;

Options

This rule has an object option:

• "allow": Allows a list of bitwise operators to be used as exceptions.
• "int32Hint": Allows the use of bitwise OR in |0 pattern for type casting.

allow

Examples of correct code for this rule with the { "allow": ["~"] } option:

/*eslint no-bitwise: ["error", { "allow": ["~"] }] */

~[1,2,3].indexOf(1) === -1;

int32Hint

Examples of correct code for this rule with the { "int32Hint": true } option:

/*eslint no-bitwise: ["error", { "int32Hint": true }] */

var b = a|0;

Source: http://eslint.org/docs/rules/

disallow bitwise operators (no-bitwise)

The use of bitwise operators in JavaScript is very rare and often & or | is simply a mistyped && or ||, which will lead to unexpected behavior.

var x = y | z;

Rule Details

This rule disallows bitwise operators.

Examples of incorrect code for this rule:

/*eslint no-bitwise: "error"*/

var x = y | z;

var x = y & z;

var x = y ^ z;

var x = ~ z;

var x = y << z;

var x = y >> z;

var x = y >>> z;

x |= y;

x &= y;

x ^= y;

x <<= y;

x >>= y;

x >>>= y;

Examples of correct code for this rule:

/*eslint no-bitwise: "error"*/

var x = y || z;

var x = y && z;

var x = y > z;

var x = y < z;

x += y;

Options

This rule has an object option:

• "allow": Allows a list of bitwise operators to be used as exceptions.
• "int32Hint": Allows the use of bitwise OR in |0 pattern for type casting.

allow

Examples of correct code for this rule with the { "allow": ["~"] } option:

/*eslint no-bitwise: ["error", { "allow": ["~"] }] */

~[1,2,3].indexOf(1) === -1;

int32Hint

Examples of correct code for this rule with the { "int32Hint": true } option:

/*eslint no-bitwise: ["error", { "int32Hint": true }] */

var b = a|0;

Source: http://eslint.org/docs/rules/

disallow bitwise operators (no-bitwise)

The use of bitwise operators in JavaScript is very rare and often & or | is simply a mistyped && or ||, which will lead to unexpected behavior.

var x = y | z;

Rule Details

This rule disallows bitwise operators.

Examples of incorrect code for this rule:

/*eslint no-bitwise: "error"*/

var x = y | z;

var x = y & z;

var x = y ^ z;

var x = ~ z;

var x = y << z;

var x = y >> z;

var x = y >>> z;

x |= y;

x &= y;

x ^= y;

x <<= y;

x >>= y;

x >>>= y;

Examples of correct code for this rule:

/*eslint no-bitwise: "error"*/

var x = y || z;

var x = y && z;

var x = y > z;

var x = y < z;

x += y;

Options

This rule has an object option:

• "allow": Allows a list of bitwise operators to be used as exceptions.
• "int32Hint": Allows the use of bitwise OR in |0 pattern for type casting.

allow

Examples of correct code for this rule with the { "allow": ["~"] } option:

/*eslint no-bitwise: ["error", { "allow": ["~"] }] */

~[1,2,3].indexOf(1) === -1;

int32Hint

Examples of correct code for this rule with the { "int32Hint": true } option:

/*eslint no-bitwise: ["error", { "int32Hint": true }] */

var b = a|0;

Source: http://eslint.org/docs/rules/

Disallow Use of Chained Assignment Expressions (no-multi-assign)

Chaining the assignment of variables can lead to unexpected results and be difficult to read.

(function() {
    const foo = bar = 0; // Did you mean `foo = bar == 0`?
    bar = 1;             // This will not fail since `bar` is not constant.
})();
console.log(bar);        // This will output 1 since `bar` is not scoped.

Rule Details

This rule disallows using multiple assignments within a single statement.

Examples of incorrect code for this rule:

/*eslint no-multi-assign: "error"*/

var a = b = c = 5;

const foo = bar = "baz";

let a =
    b =
    c;

Examples of correct code for this rule:

/*eslint no-multi-assign: "error"*/
var a = 5;
var b = 5;
var c = 5;

const foo = "baz";
const bar = "baz";

let a = c;
let b = c;

Related Rules

• [max-statements-per-line](max-statements-per-line.md) Source: http://eslint.org/docs/rules/

enforce consistent line breaks inside braces (object-curly-newline)

A number of style guides require or disallow line breaks inside of object braces and other tokens.

Rule Details

This rule enforces consistent line breaks inside braces of object literals or destructuring assignments.

Options

This rule has either a string option:

• "always" requires line breaks inside braces
• "never" disallows line breaks inside braces

Or an object option:

• "multiline": true requires line breaks if there are line breaks inside properties or between properties. Otherwise, it disallows line breaks.
• "minProperties" requires line breaks if the number of properties is at least the given integer. By default, an error will also be reported if an object contains linebreaks and has fewer properties than the given integer. However, the second behavior is disabled if the consistent option is set to true
• "consistent": true (default) requires that either both curly braces, or neither, directly enclose newlines. Note that enabling this option will also change the behavior of the minProperties option. (See minProperties above for more information)

You can specify different options for object literals, destructuring assignments, and named imports and exports:

{
    "object-curly-newline": ["error", {
        "ObjectExpression": "always",
        "ObjectPattern": { "multiline": true },
        "ImportDeclaration": "never",
        "ExportDeclaration": { "multiline": true, "minProperties": 3 }
    }]
}

• "ObjectExpression" configuration for object literals
• "ObjectPattern" configuration for object patterns of destructuring assignments
• "ImportDeclaration" configuration for named imports
• "ExportDeclaration" configuration for named exports

always

Examples of incorrect code for this rule with the "always" option:

/*eslint object-curly-newline: ["error", "always"]*/
/*eslint-env es6*/

let a = {};
let b = {foo: 1};
let c = {foo: 1, bar: 2};
let d = {foo: 1,
    bar: 2};
let e = {foo() {
    dosomething();
}};

let {} = obj;
let {f} = obj;
let {g, h} = obj;
let {i,
    j} = obj;
let {k = function() {
    dosomething();
}} = obj;

Examples of correct code for this rule with the "always" option:

/*eslint object-curly-newline: ["error", "always"]*/
/*eslint-env es6*/

let a = {
};
let b = {
    foo: 1
};
let c = {
    foo: 1, bar: 2
};
let d = {
    foo: 1,
    bar: 2
};
let e = {
    foo: function() {
        dosomething();
    }
};

let {
} = obj;
let {
    f
} = obj;
let {
    g, h
} = obj;
let {
    i,
    j
} = obj;
let {
    k = function() {
        dosomething();
    }
} = obj;

never

Examples of incorrect code for this rule with the "never" option:

/*eslint object-curly-newline: ["error", "never"]*/
/*eslint-env es6*/

let a = {
};
let b = {
    foo: 1
};
let c = {
    foo: 1, bar: 2
};
let d = {
    foo: 1,
    bar: 2
};
let e = {
    foo: function() {
        dosomething();
    }
};

let {
} = obj;
let {
    f
} = obj;
let {
    g, h
} = obj;
let {
    i,
    j
} = obj;
let {
    k = function() {
        dosomething();
    }
} = obj;

Examples of correct code for this rule with the "never" option:

/*eslint object-curly-newline: ["error", "never"]*/
/*eslint-env es6*/

let a = {};
let b = {foo: 1};
let c = {foo: 1, bar: 2};
let d = {foo: 1,
    bar: 2};
let e = {foo: function() {
    dosomething();
}};

let {} = obj;
let {f} = obj;
let {g, h} = obj;
let {i,
    j} = obj;
let {k = function() {
    dosomething();
}} = obj;

multiline

Examples of incorrect code for this rule with the { "multiline": true } option:

/*eslint object-curly-newline: ["error", { "multiline": true }]*/
/*eslint-env es6*/

let a = {
};
let b = {
    foo: 1
};
let c = {
    foo: 1, bar: 2
};
let d = {foo: 1,
    bar: 2};
let e = {foo: function() {
    dosomething();
}};

let {
} = obj;
let {
    f
} = obj;
let {
    g, h
} = obj;
let {i,
    j} = obj;
let {k = function() {
    dosomething();
}} = obj;

Examples of correct code for this rule with the { "multiline": true } option:

/*eslint object-curly-newline: ["error", { "multiline": true }]*/
/*eslint-env es6*/

let a = {};
let b = {foo: 1};
let c = {foo: 1, bar: 2};
let d = {
    foo: 1,
    bar: 2
};
let e = {
    foo: function() {
        dosomething();
    }
};

let {} = obj;
let {f} = obj;
let {g, h} = obj;
let {
    i,
    j
} = obj;
let {
    k = function() {
        dosomething();
    }
} = obj;

minProperties

Examples of incorrect code for this rule with the { "minProperties": 2 } option:

/*eslint object-curly-newline: ["error", { "minProperties": 2 }]*/
/*eslint-env es6*/

let a = {
};
let b = {
    foo: 1
};
let c = {foo: 1, bar: 2};
let d = {foo: 1,
    bar: 2};
let e = {
    foo: function() {
        dosomething();
    }
};

let {
} = obj;
let {
    f
} = obj;
let {g, h} = obj;
let {i,
    j} = obj;
let {
    k = function() {
        dosomething();
    }
} = obj;

Examples of correct code for this rule with the { "minProperties": 2 } option:

/*eslint object-curly-newline: ["error", { "minProperties": 2 }]*/
/*eslint-env es6*/

let a = {};
let b = {foo: 1};
let c = {
    foo: 1, bar: 2
};
let d = {
    foo: 1,
    bar: 2
};
let e = {foo: function() {
    dosomething();
}};

let {} = obj;
let {f} = obj;
let {
    g, h
} = obj;
let {
    i,
    j
} = obj;
let {k = function() {
    dosomething();
}} = obj;

consistent

Examples of incorrect code for this rule with the default { "consistent": true } option:

/*eslint object-curly-newline: ["error", { "consistent": true }]*/
/*eslint-env es6*/

let a = {foo: 1
};
let b = {
    foo: 1};
let c = {foo: 1, bar: 2
};
let d = {
    foo: 1, bar: 2};
let e = {foo: function() {
    dosomething();
    }
};
let f = {
    foo: function() {
    dosomething();}};

let {g
} = obj;
let {
    h} = obj;
let {i, j
} = obj;
let {k, l
} = obj;
let {
    m, n} = obj;
let {
    o, p} = obj;
let {q = function() {
    dosomething();
    }
} = obj;
let {
    r = function() {
        dosomething();
    }} = obj;

Examples of correct code for this rule with the default { "consistent": true } option:

/*eslint object-curly-newline: ["error", { "consistent": true }]*/
/*eslint-env es6*/


let empty1 = {};
let empty2 = {
};
let a = {foo: 1};
let b = {
    foo: 1
};
let c = {
    foo: 1, bar: 2
};
let d = {
    foo: 1,
    bar: 2
};
let e = {foo: function() {dosomething();}};
let f = {
    foo: function() {
        dosomething();
    }
};

let {} = obj;
let {
} = obj;
let {g} = obj;
let {
    h
} = obj;
let {i, j} = obj;
let {
    k, l
} = obj;
let {m,
    n} = obj;
let {
    o,
    p
} = obj;
let {q = function() {dosomething();}} = obj;
let {
    r = function() {
        dosomething();
    }
} = obj;

ObjectExpression and ObjectPattern

Examples of incorrect code for this rule with the { "ObjectExpression": "always", "ObjectPattern": "never" } options:

/*eslint object-curly-newline: ["error", { "ObjectExpression": "always", "ObjectPattern": "never" }]*/
/*eslint-env es6*/

let a = {};
let b = {foo: 1};
let c = {foo: 1, bar: 2};
let d = {foo: 1,
    bar: 2};
let e = {foo: function() {
    dosomething();
}};

let {
} = obj;
let {
    f
} = obj;
let {
    g, h
} = obj;
let {
    i,
    j
} = obj;
let {
    k = function() {
        dosomething();
    }
} = obj;

Examples of correct code for this rule with the { "ObjectExpression": "always", "ObjectPattern": "never" } options:

/*eslint object-curly-newline: ["error", { "ObjectExpression": "always", "ObjectPattern": "never" }]*/
/*eslint-env es6*/

let a = {
};
let b = {
    foo: 1
};
let c = {
    foo: 1, bar: 2
};
let d = {
    foo: 1,
    bar: 2
};
let e = {
    foo: function() {
        dosomething();
    }
};

let {} = obj;
let {f} = obj;
let {g, h} = obj;
let {i,
    j} = obj;
let {k = function() {
    dosomething();
}} = obj;

ImportDeclaration and ExportDeclaration

Examples of incorrect code for this rule with the { "ImportDeclaration": "always", "ExportDeclaration": "never" } options:

/*eslint object-curly-newline: ["error", { "ImportDeclaration": "always", "ExportDeclaration": "never" }]*/
/*eslint-env es6*/

import {foo, bar} from 'foo-bar';
import {foo as f, bar} from 'foo-bar';
import {foo,
    bar} from 'foo-bar';

export {
   foo,
   bar
};
export {
   foo as f,
   bar
} from 'foo-bar';

Examples of correct code for this rule with the { "ImportDeclaration": "always", "ExportDeclaration": "never" } options:

/*eslint object-curly-newline: ["error", { "ImportDeclaration": "always", "ExportDeclaration": "never" }]*/
/*eslint-env es6*/

import {
    foo,
    bar
} from 'foo-bar';
import {
    foo, bar
} from 'foo-bar';
import {
    foo as f,
    bar
} from 'foo-bar';

export { foo, bar } from 'foo-bar';
export { foo as f, bar } from 'foo-bar';

Compatibility

• JSCS: requirePaddingNewLinesInObjects
• JSCS: disallowPaddingNewLinesInObjects

When Not To Use It

If you don't want to enforce consistent line breaks inside braces, then it's safe to disable this rule.

Related Rules

• [comma-spacing](comma-spacing.md)
• [key-spacing](key-spacing.md)
• [object-curly-spacing](object-curly-spacing.md)
• [object-property-newline](object-property-newline.md) Source: http://eslint.org/docs/rules/

Require Dot Notation (dot-notation)

In JavaScript, one can access properties using the dot notation (foo.bar) or square-bracket notation (foo["bar"]). However, the dot notation is often preferred because it is easier to read, less verbose, and works better with aggressive JavaScript minimizers.

foo["bar"];

Rule Details

This rule is aimed at maintaining code consistency and improving code readability by encouraging use of the dot notation style whenever possible. As such, it will warn when it encounters an unnecessary use of square-bracket notation.

Examples of incorrect code for this rule:

/*eslint dot-notation: "error"*/

var x = foo["bar"];

Examples of correct code for this rule:

/*eslint dot-notation: "error"*/

var x = foo.bar;

var x = foo[bar];    // Property name is a variable, square-bracket notation required

Options

This rule accepts a single options argument:

• Set the allowKeywords option to false (default is true) to follow ECMAScript version 3 compatible style, avoiding dot notation for reserved word properties.
• Set the allowPattern option to a regular expression string to allow bracket notation for property names that match a pattern (by default, no pattern is tested).

allowKeywords

Examples of correct code for the { "allowKeywords": false } option:

/*eslint dot-notation: ["error", { "allowKeywords": false }]*/

var foo = { "class": "CS 101" }
var x = foo["class"]; // Property name is a reserved word, square-bracket notation required

allowPattern

For example, when preparing data to be sent to an external API, it is often required to use property names that include underscores. If the camelcase rule is in effect, these snake case properties would not be allowed. By providing an allowPattern to the dot-notation rule, these snake case properties can be accessed with bracket notation.

Examples of correct code for the sample { "allowPattern": "^[a-z]+(_[a-z]+)+$" } option:

/*eslint camelcase: "error"*/
/*eslint dot-notation: ["error", { "allowPattern": "^[a-z]+(_[a-z]+)+$" }]*/

var data = {};
data.foo_bar = 42;

var data = {};
data["fooBar"] = 42;

var data = {};
data["foo_bar"] = 42; // no warning

Source: http://eslint.org/docs/rules/

Require Brace Style (brace-style)

Brace style is closely related to indent style in programming and describes the placement of braces relative to their control statement and body. There are probably a dozen, if not more, brace styles in the world.

The one true brace style is one of the most common brace styles in JavaScript, in which the opening brace of a block is placed on the same line as its corresponding statement or declaration. For example:

if (foo) {
  bar();
} else {
  baz();
}

One common variant of one true brace style is called Stroustrup, in which the else statements in an if-else construct, as well as catch and finally, must be on its own line after the preceding closing brace. For example:

if (foo) {
  bar();
}
else {
  baz();
}

Another style is called Allman, in which all the braces are expected to be on their own lines without any extra indentation. For example:

if (foo)
{
  bar();
}
else
{
  baz();
}

While no style is considered better than the other, most developers agree that having a consistent style throughout a project is important for its long-term maintainability.

Rule Details

This rule enforces consistent brace style for blocks.

Options

This rule has a string option:

• "1tbs" (default) enforces one true brace style
• "stroustrup" enforces Stroustrup style
• "allman" enforces Allman style

This rule has an object option for an exception:

• "allowSingleLine": true (default false) allows the opening and closing braces for a block to be on the same line

1tbs

Examples of incorrect code for this rule with the default "1tbs" option:

/*eslint brace-style: "error"*/

function foo()
{
  return true;
}

if (foo)
{
  bar();
}

try
{
  somethingRisky();
} catch(e)
{
  handleError();
}

if (foo) {
  bar();
}
else {
  baz();
}

Examples of correct code for this rule with the default "1tbs" option:

/*eslint brace-style: "error"*/

function foo() {
  return true;
}

if (foo) {
  bar();
}

if (foo) {
  bar();
} else {
  baz();
}

try {
  somethingRisky();
} catch(e) {
  handleError();
}

// when there are no braces, there are no problems
if (foo) bar();
else if (baz) boom();

Examples of correct code for this rule with the "1tbs", { "allowSingleLine": true } options:

/*eslint brace-style: ["error", "1tbs", { "allowSingleLine": true }]*/

function nop() { return; }

if (foo) { bar(); }

if (foo) { bar(); } else { baz(); }

try { somethingRisky(); } catch(e) { handleError(); }

if (foo) { baz(); } else {
  boom();
}

if (foo) { baz(); } else if (bar) {
  boom();
}

if (foo) { baz(); } else
if (bar) {
  boom();
}

if (foo) { baz(); } else if (bar) {
  boom();
}

try { somethingRisky(); } catch(e) {
  handleError();
}

stroustrup

Examples of incorrect code for this rule with the "stroustrup" option:

/*eslint brace-style: ["error", "stroustrup"]*/

function foo()
{
  return true;
}

if (foo)
{
  bar();
}

try
{
  somethingRisky();
} catch(e)
{
  handleError();
}

if (foo) {
  bar();
} else {
  baz();
}

Examples of correct code for this rule with the "stroustrup" option:

/*eslint brace-style: ["error", "stroustrup"]*/

function foo() {
  return true;
}

if (foo) {
  bar();
}

if (foo) {
  bar();
}
else {
  baz();
}

try {
  somethingRisky();
}
catch(e) {
  handleError();
}

// when there are no braces, there are no problems
if (foo) bar();
else if (baz) boom();

Examples of correct code for this rule with the "stroustrup", { "allowSingleLine": true } options:

/*eslint brace-style: ["error", "stroustrup", { "allowSingleLine": true }]*/

function nop() { return; }

if (foo) { bar(); }

if (foo) { bar(); }
else { baz(); }

try { somethingRisky(); }
catch(e) { handleError(); }

allman

Examples of incorrect code for this rule with the "allman" option:

/*eslint brace-style: ["error", "allman"]*/

function foo() {
  return true;
}

if (foo)
{
  bar(); }

try
{
  somethingRisky();
} catch(e)
{
  handleError();
}

if (foo) {
  bar();
} else {
  baz();
}

Examples of correct code for this rule with the "allman" option:

/*eslint brace-style: ["error", "allman"]*/

function foo()
{
  return true;
}

if (foo)
{
  bar();
}

if (foo)
{
  bar();
}
else
{
  baz();
}

try
{
  somethingRisky();
}
catch(e)
{
  handleError();
}

// when there are no braces, there are no problems
if (foo) bar();
else if (baz) boom();

Examples of correct code for this rule with the "allman", { "allowSingleLine": true } options:

/*eslint brace-style: ["error", "allman", { "allowSingleLine": true }]*/

function nop() { return; }

if (foo) { bar(); }

if (foo) { bar(); }
else { baz(); }

try { somethingRisky(); }
catch(e) { handleError(); }

When Not To Use It

If you don't want to enforce a particular brace style, don't enable this rule.

Further Reading

• Indent style Source: http://eslint.org/docs/rules/

Require Dot Notation (dot-notation)

In JavaScript, one can access properties using the dot notation (foo.bar) or square-bracket notation (foo["bar"]). However, the dot notation is often preferred because it is easier to read, less verbose, and works better with aggressive JavaScript minimizers.

foo["bar"];

Rule Details

This rule is aimed at maintaining code consistency and improving code readability by encouraging use of the dot notation style whenever possible. As such, it will warn when it encounters an unnecessary use of square-bracket notation.

Examples of incorrect code for this rule:

/*eslint dot-notation: "error"*/

var x = foo["bar"];

Examples of correct code for this rule:

/*eslint dot-notation: "error"*/

var x = foo.bar;

var x = foo[bar];    // Property name is a variable, square-bracket notation required

Options

This rule accepts a single options argument:

• Set the allowKeywords option to false (default is true) to follow ECMAScript version 3 compatible style, avoiding dot notation for reserved word properties.
• Set the allowPattern option to a regular expression string to allow bracket notation for property names that match a pattern (by default, no pattern is tested).

allowKeywords

Examples of correct code for the { "allowKeywords": false } option:

/*eslint dot-notation: ["error", { "allowKeywords": false }]*/

var foo = { "class": "CS 101" }
var x = foo["class"]; // Property name is a reserved word, square-bracket notation required

allowPattern

For example, when preparing data to be sent to an external API, it is often required to use property names that include underscores. If the camelcase rule is in effect, these snake case properties would not be allowed. By providing an allowPattern to the dot-notation rule, these snake case properties can be accessed with bracket notation.

Examples of correct code for the sample { "allowPattern": "^[a-z]+(_[a-z]+)+$" } option:

/*eslint camelcase: "error"*/
/*eslint dot-notation: ["error", { "allowPattern": "^[a-z]+(_[a-z]+)+$" }]*/

var data = {};
data.foo_bar = 42;

var data = {};
data["fooBar"] = 42;

var data = {};
data["foo_bar"] = 42; // no warning

Source: http://eslint.org/docs/rules/

enforce consistent line breaks inside braces (object-curly-newline)

A number of style guides require or disallow line breaks inside of object braces and other tokens.

Rule Details

This rule enforces consistent line breaks inside braces of object literals or destructuring assignments.

Options

This rule has either a string option:

• "always" requires line breaks inside braces
• "never" disallows line breaks inside braces

Or an object option:

• "multiline": true requires line breaks if there are line breaks inside properties or between properties. Otherwise, it disallows line breaks.
• "minProperties" requires line breaks if the number of properties is at least the given integer. By default, an error will also be reported if an object contains linebreaks and has fewer properties than the given integer. However, the second behavior is disabled if the consistent option is set to true
• "consistent": true (default) requires that either both curly braces, or neither, directly enclose newlines. Note that enabling this option will also change the behavior of the minProperties option. (See minProperties above for more information)

You can specify different options for object literals, destructuring assignments, and named imports and exports:

{
    "object-curly-newline": ["error", {
        "ObjectExpression": "always",
        "ObjectPattern": { "multiline": true },
        "ImportDeclaration": "never",
        "ExportDeclaration": { "multiline": true, "minProperties": 3 }
    }]
}

• "ObjectExpression" configuration for object literals
• "ObjectPattern" configuration for object patterns of destructuring assignments
• "ImportDeclaration" configuration for named imports
• "ExportDeclaration" configuration for named exports

always

Examples of incorrect code for this rule with the "always" option:

/*eslint object-curly-newline: ["error", "always"]*/
/*eslint-env es6*/

let a = {};
let b = {foo: 1};
let c = {foo: 1, bar: 2};
let d = {foo: 1,
    bar: 2};
let e = {foo() {
    dosomething();
}};

let {} = obj;
let {f} = obj;
let {g, h} = obj;
let {i,
    j} = obj;
let {k = function() {
    dosomething();
}} = obj;

Examples of correct code for this rule with the "always" option:

/*eslint object-curly-newline: ["error", "always"]*/
/*eslint-env es6*/

let a = {
};
let b = {
    foo: 1
};
let c = {
    foo: 1, bar: 2
};
let d = {
    foo: 1,
    bar: 2
};
let e = {
    foo: function() {
        dosomething();
    }
};

let {
} = obj;
let {
    f
} = obj;
let {
    g, h
} = obj;
let {
    i,
    j
} = obj;
let {
    k = function() {
        dosomething();
    }
} = obj;

never

Examples of incorrect code for this rule with the "never" option:

/*eslint object-curly-newline: ["error", "never"]*/
/*eslint-env es6*/

let a = {
};
let b = {
    foo: 1
};
let c = {
    foo: 1, bar: 2
};
let d = {
    foo: 1,
    bar: 2
};
let e = {
    foo: function() {
        dosomething();
    }
};

let {
} = obj;
let {
    f
} = obj;
let {
    g, h
} = obj;
let {
    i,
    j
} = obj;
let {
    k = function() {
        dosomething();
    }
} = obj;

Examples of correct code for this rule with the "never" option:

/*eslint object-curly-newline: ["error", "never"]*/
/*eslint-env es6*/

let a = {};
let b = {foo: 1};
let c = {foo: 1, bar: 2};
let d = {foo: 1,
    bar: 2};
let e = {foo: function() {
    dosomething();
}};

let {} = obj;
let {f} = obj;
let {g, h} = obj;
let {i,
    j} = obj;
let {k = function() {
    dosomething();
}} = obj;

multiline

Examples of incorrect code for this rule with the { "multiline": true } option:

/*eslint object-curly-newline: ["error", { "multiline": true }]*/
/*eslint-env es6*/

let a = {
};
let b = {
    foo: 1
};
let c = {
    foo: 1, bar: 2
};
let d = {foo: 1,
    bar: 2};
let e = {foo: function() {
    dosomething();
}};

let {
} = obj;
let {
    f
} = obj;
let {
    g, h
} = obj;
let {i,
    j} = obj;
let {k = function() {
    dosomething();
}} = obj;

Examples of correct code for this rule with the { "multiline": true } option:

/*eslint object-curly-newline: ["error", { "multiline": true }]*/
/*eslint-env es6*/

let a = {};
let b = {foo: 1};
let c = {foo: 1, bar: 2};
let d = {
    foo: 1,
    bar: 2
};
let e = {
    foo: function() {
        dosomething();
    }
};

let {} = obj;
let {f} = obj;
let {g, h} = obj;
let {
    i,
    j
} = obj;
let {
    k = function() {
        dosomething();
    }
} = obj;

minProperties

Examples of incorrect code for this rule with the { "minProperties": 2 } option:

/*eslint object-curly-newline: ["error", { "minProperties": 2 }]*/
/*eslint-env es6*/

let a = {
};
let b = {
    foo: 1
};
let c = {foo: 1, bar: 2};
let d = {foo: 1,
    bar: 2};
let e = {
    foo: function() {
        dosomething();
    }
};

let {
} = obj;
let {
    f
} = obj;
let {g, h} = obj;
let {i,
    j} = obj;
let {
    k = function() {
        dosomething();
    }
} = obj;

Examples of correct code for this rule with the { "minProperties": 2 } option:

/*eslint object-curly-newline: ["error", { "minProperties": 2 }]*/
/*eslint-env es6*/

let a = {};
let b = {foo: 1};
let c = {
    foo: 1, bar: 2
};
let d = {
    foo: 1,
    bar: 2
};
let e = {foo: function() {
    dosomething();
}};

let {} = obj;
let {f} = obj;
let {
    g, h
} = obj;
let {
    i,
    j
} = obj;
let {k = function() {
    dosomething();
}} = obj;

consistent

Examples of incorrect code for this rule with the default { "consistent": true } option:

/*eslint object-curly-newline: ["error", { "consistent": true }]*/
/*eslint-env es6*/

let a = {foo: 1
};
let b = {
    foo: 1};
let c = {foo: 1, bar: 2
};
let d = {
    foo: 1, bar: 2};
let e = {foo: function() {
    dosomething();
    }
};
let f = {
    foo: function() {
    dosomething();}};

let {g
} = obj;
let {
    h} = obj;
let {i, j
} = obj;
let {k, l
} = obj;
let {
    m, n} = obj;
let {
    o, p} = obj;
let {q = function() {
    dosomething();
    }
} = obj;
let {
    r = function() {
        dosomething();
    }} = obj;

Examples of correct code for this rule with the default { "consistent": true } option:

/*eslint object-curly-newline: ["error", { "consistent": true }]*/
/*eslint-env es6*/


let empty1 = {};
let empty2 = {
};
let a = {foo: 1};
let b = {
    foo: 1
};
let c = {
    foo: 1, bar: 2
};
let d = {
    foo: 1,
    bar: 2
};
let e = {foo: function() {dosomething();}};
let f = {
    foo: function() {
        dosomething();
    }
};

let {} = obj;
let {
} = obj;
let {g} = obj;
let {
    h
} = obj;
let {i, j} = obj;
let {
    k, l
} = obj;
let {m,
    n} = obj;
let {
    o,
    p
} = obj;
let {q = function() {dosomething();}} = obj;
let {
    r = function() {
        dosomething();
    }
} = obj;

ObjectExpression and ObjectPattern

Examples of incorrect code for this rule with the { "ObjectExpression": "always", "ObjectPattern": "never" } options:

/*eslint object-curly-newline: ["error", { "ObjectExpression": "always", "ObjectPattern": "never" }]*/
/*eslint-env es6*/

let a = {};
let b = {foo: 1};
let c = {foo: 1, bar: 2};
let d = {foo: 1,
    bar: 2};
let e = {foo: function() {
    dosomething();
}};

let {
} = obj;
let {
    f
} = obj;
let {
    g, h
} = obj;
let {
    i,
    j
} = obj;
let {
    k = function() {
        dosomething();
    }
} = obj;

Examples of correct code for this rule with the { "ObjectExpression": "always", "ObjectPattern": "never" } options:

/*eslint object-curly-newline: ["error", { "ObjectExpression": "always", "ObjectPattern": "never" }]*/
/*eslint-env es6*/

let a = {
};
let b = {
    foo: 1
};
let c = {
    foo: 1, bar: 2
};
let d = {
    foo: 1,
    bar: 2
};
let e = {
    foo: function() {
        dosomething();
    }
};

let {} = obj;
let {f} = obj;
let {g, h} = obj;
let {i,
    j} = obj;
let {k = function() {
    dosomething();
}} = obj;

ImportDeclaration and ExportDeclaration

Examples of incorrect code for this rule with the { "ImportDeclaration": "always", "ExportDeclaration": "never" } options:

/*eslint object-curly-newline: ["error", { "ImportDeclaration": "always", "ExportDeclaration": "never" }]*/
/*eslint-env es6*/

import {foo, bar} from 'foo-bar';
import {foo as f, bar} from 'foo-bar';
import {foo,
    bar} from 'foo-bar';

export {
   foo,
   bar
};
export {
   foo as f,
   bar
} from 'foo-bar';

Examples of correct code for this rule with the { "ImportDeclaration": "always", "ExportDeclaration": "never" } options:

/*eslint object-curly-newline: ["error", { "ImportDeclaration": "always", "ExportDeclaration": "never" }]*/
/*eslint-env es6*/

import {
    foo,
    bar
} from 'foo-bar';
import {
    foo, bar
} from 'foo-bar';
import {
    foo as f,
    bar
} from 'foo-bar';

export { foo, bar } from 'foo-bar';
export { foo as f, bar } from 'foo-bar';

Compatibility

• JSCS: requirePaddingNewLinesInObjects
• JSCS: disallowPaddingNewLinesInObjects

When Not To Use It

If you don't want to enforce consistent line breaks inside braces, then it's safe to disable this rule.

Related Rules

• [comma-spacing](comma-spacing.md)
• [key-spacing](key-spacing.md)
• [object-curly-spacing](object-curly-spacing.md)
• [object-property-newline](object-property-newline.md) Source: http://eslint.org/docs/rules/

disallow bitwise operators (no-bitwise)

The use of bitwise operators in JavaScript is very rare and often & or | is simply a mistyped && or ||, which will lead to unexpected behavior.

var x = y | z;

Rule Details

This rule disallows bitwise operators.

Examples of incorrect code for this rule:

/*eslint no-bitwise: "error"*/

var x = y | z;

var x = y & z;

var x = y ^ z;

var x = ~ z;

var x = y << z;

var x = y >> z;

var x = y >>> z;

x |= y;

x &= y;

x ^= y;

x <<= y;

x >>= y;

x >>>= y;

Examples of correct code for this rule:

/*eslint no-bitwise: "error"*/

var x = y || z;

var x = y && z;

var x = y > z;

var x = y < z;

x += y;

Options

This rule has an object option:

• "allow": Allows a list of bitwise operators to be used as exceptions.
• "int32Hint": Allows the use of bitwise OR in |0 pattern for type casting.

allow

Examples of correct code for this rule with the { "allow": ["~"] } option:

/*eslint no-bitwise: ["error", { "allow": ["~"] }] */

~[1,2,3].indexOf(1) === -1;

int32Hint

Examples of correct code for this rule with the { "int32Hint": true } option:

/*eslint no-bitwise: ["error", { "int32Hint": true }] */

var b = a|0;

Source: http://eslint.org/docs/rules/

disallow trailing whitespace at the end of lines (no-trailing-spaces)

Sometimes in the course of editing files, you can end up with extra whitespace at the end of lines. These whitespace differences can be picked up by source control systems and flagged as diffs, causing frustration for developers. While this extra whitespace causes no functional issues, many code conventions require that trailing spaces be removed before check-in.

Rule Details

This rule disallows trailing whitespace (spaces, tabs, and other Unicode whitespace characters) at the end of lines.

Examples of incorrect code for this rule:

/*eslint no-trailing-spaces: "error"*/

var foo = 0;//•••••
var baz = 5;//••
//•••••

Examples of correct code for this rule:

/*eslint no-trailing-spaces: "error"*/

var foo = 0;
var baz = 5;

Options

This rule has an object option:

• "skipBlankLines": false (default) disallows trailing whitespace on empty lines
• "skipBlankLines": true allows trailing whitespace on empty lines
• "ignoreComments": false (default) disallows trailing whitespace in comment blocks
• "ignoreComments": true allows trailing whitespace in comment blocks

skipBlankLines

Examples of correct code for this rule with the { "skipBlankLines": true } option:

/*eslint no-trailing-spaces: ["error", { "skipBlankLines": true }]*/

var foo = 0;
var baz = 5;
//•••••

ignoreComments

Examples of correct code for this rule with the { "ignoreComments": true } option:

/*eslint no-trailing-spaces: ["error", { "ignoreComments": true }]*/

//foo•
//•••••
/**
 *•baz
 *••
 *•bar
 */

Source: http://eslint.org/docs/rules/

Require Dot Notation (dot-notation)

In JavaScript, one can access properties using the dot notation (foo.bar) or square-bracket notation (foo["bar"]). However, the dot notation is often preferred because it is easier to read, less verbose, and works better with aggressive JavaScript minimizers.

foo["bar"];

Rule Details

This rule is aimed at maintaining code consistency and improving code readability by encouraging use of the dot notation style whenever possible. As such, it will warn when it encounters an unnecessary use of square-bracket notation.

Examples of incorrect code for this rule:

/*eslint dot-notation: "error"*/

var x = foo["bar"];

Examples of correct code for this rule:

/*eslint dot-notation: "error"*/

var x = foo.bar;

var x = foo[bar];    // Property name is a variable, square-bracket notation required

Options

This rule accepts a single options argument:

• Set the allowKeywords option to false (default is true) to follow ECMAScript version 3 compatible style, avoiding dot notation for reserved word properties.
• Set the allowPattern option to a regular expression string to allow bracket notation for property names that match a pattern (by default, no pattern is tested).

allowKeywords

Examples of correct code for the { "allowKeywords": false } option:

/*eslint dot-notation: ["error", { "allowKeywords": false }]*/

var foo = { "class": "CS 101" }
var x = foo["class"]; // Property name is a reserved word, square-bracket notation required

allowPattern

For example, when preparing data to be sent to an external API, it is often required to use property names that include underscores. If the camelcase rule is in effect, these snake case properties would not be allowed. By providing an allowPattern to the dot-notation rule, these snake case properties can be accessed with bracket notation.

Examples of correct code for the sample { "allowPattern": "^[a-z]+(_[a-z]+)+$" } option:

/*eslint camelcase: "error"*/
/*eslint dot-notation: ["error", { "allowPattern": "^[a-z]+(_[a-z]+)+$" }]*/

var data = {};
data.foo_bar = 42;

var data = {};
data["fooBar"] = 42;

var data = {};
data["foo_bar"] = 42; // no warning

Source: http://eslint.org/docs/rules/

Require Brace Style (brace-style)

Brace style is closely related to indent style in programming and describes the placement of braces relative to their control statement and body. There are probably a dozen, if not more, brace styles in the world.

The one true brace style is one of the most common brace styles in JavaScript, in which the opening brace of a block is placed on the same line as its corresponding statement or declaration. For example:

if (foo) {
  bar();
} else {
  baz();
}

One common variant of one true brace style is called Stroustrup, in which the else statements in an if-else construct, as well as catch and finally, must be on its own line after the preceding closing brace. For example:

if (foo) {
  bar();
}
else {
  baz();
}

Another style is called Allman, in which all the braces are expected to be on their own lines without any extra indentation. For example:

if (foo)
{
  bar();
}
else
{
  baz();
}

While no style is considered better than the other, most developers agree that having a consistent style throughout a project is important for its long-term maintainability.

Rule Details

This rule enforces consistent brace style for blocks.

Options

This rule has a string option:

• "1tbs" (default) enforces one true brace style
• "stroustrup" enforces Stroustrup style
• "allman" enforces Allman style

This rule has an object option for an exception:

• "allowSingleLine": true (default false) allows the opening and closing braces for a block to be on the same line

1tbs

Examples of incorrect code for this rule with the default "1tbs" option:

/*eslint brace-style: "error"*/

function foo()
{
  return true;
}

if (foo)
{
  bar();
}

try
{
  somethingRisky();
} catch(e)
{
  handleError();
}

if (foo) {
  bar();
}
else {
  baz();
}

Examples of correct code for this rule with the default "1tbs" option:

/*eslint brace-style: "error"*/

function foo() {
  return true;
}

if (foo) {
  bar();
}

if (foo) {
  bar();
} else {
  baz();
}

try {
  somethingRisky();
} catch(e) {
  handleError();
}

// when there are no braces, there are no problems
if (foo) bar();
else if (baz) boom();

Examples of correct code for this rule with the "1tbs", { "allowSingleLine": true } options:

/*eslint brace-style: ["error", "1tbs", { "allowSingleLine": true }]*/

function nop() { return; }

if (foo) { bar(); }

if (foo) { bar(); } else { baz(); }

try { somethingRisky(); } catch(e) { handleError(); }

if (foo) { baz(); } else {
  boom();
}

if (foo) { baz(); } else if (bar) {
  boom();
}

if (foo) { baz(); } else
if (bar) {
  boom();
}

if (foo) { baz(); } else if (bar) {
  boom();
}

try { somethingRisky(); } catch(e) {
  handleError();
}

stroustrup

Examples of incorrect code for this rule with the "stroustrup" option:

/*eslint brace-style: ["error", "stroustrup"]*/

function foo()
{
  return true;
}

if (foo)
{
  bar();
}

try
{
  somethingRisky();
} catch(e)
{
  handleError();
}

if (foo) {
  bar();
} else {
  baz();
}

Examples of correct code for this rule with the "stroustrup" option:

/*eslint brace-style: ["error", "stroustrup"]*/

function foo() {
  return true;
}

if (foo) {
  bar();
}

if (foo) {
  bar();
}
else {
  baz();
}

try {
  somethingRisky();
}
catch(e) {
  handleError();
}

// when there are no braces, there are no problems
if (foo) bar();
else if (baz) boom();

Examples of correct code for this rule with the "stroustrup", { "allowSingleLine": true } options:

/*eslint brace-style: ["error", "stroustrup", { "allowSingleLine": true }]*/

function nop() { return; }

if (foo) { bar(); }

if (foo) { bar(); }
else { baz(); }

try { somethingRisky(); }
catch(e) { handleError(); }

allman

Examples of incorrect code for this rule with the "allman" option:

/*eslint brace-style: ["error", "allman"]*/

function foo() {
  return true;
}

if (foo)
{
  bar(); }

try
{
  somethingRisky();
} catch(e)
{
  handleError();
}

if (foo) {
  bar();
} else {
  baz();
}

Examples of correct code for this rule with the "allman" option:

/*eslint brace-style: ["error", "allman"]*/

function foo()
{
  return true;
}

if (foo)
{
  bar();
}

if (foo)
{
  bar();
}
else
{
  baz();
}

try
{
  somethingRisky();
}
catch(e)
{
  handleError();
}

// when there are no braces, there are no problems
if (foo) bar();
else if (baz) boom();

Examples of correct code for this rule with the "allman", { "allowSingleLine": true } options:

/*eslint brace-style: ["error", "allman", { "allowSingleLine": true }]*/

function nop() { return; }

if (foo) { bar(); }

if (foo) { bar(); }
else { baz(); }

try { somethingRisky(); }
catch(e) { handleError(); }

When Not To Use It

If you don't want to enforce a particular brace style, don't enable this rule.

Further Reading

• Indent style Source: http://eslint.org/docs/rules/

Require === and !== (eqeqeq)

It is considered good practice to use the type-safe equality operators === and !== instead of their regular counterparts == and !=.

The reason for this is that == and != do type coercion which follows the rather obscure Abstract Equality Comparison Algorithm. For instance, the following statements are all considered true:

• [] == false
• [] == ![]
• 3 == "03"

If one of those occurs in an innocent-looking statement such as a == b the actual problem is very difficult to spot.

Rule Details

This rule is aimed at eliminating the type-unsafe equality operators.

Examples of incorrect code for this rule:

/*eslint eqeqeq: "error"*/

if (x == 42) { }

if ("" == text) { }

if (obj.getStuff() != undefined) { }

The --fix option on the command line automatically fixes some problems reported by this rule. A problem is only fixed if one of the operands is a typeof expression, or if both operands are literals with the same type.

Options

always

The "always" option (default) enforces the use of === and !== in every situation (except when you opt-in to more specific handling of null [see below]).

Examples of incorrect code for the "always" option:

/*eslint eqeqeq: ["error", "always"]*/

a == b
foo == true
bananas != 1
value == undefined
typeof foo == 'undefined'
'hello' != 'world'
0 == 0
true == true
foo == null

Examples of correct code for the "always" option:

/*eslint eqeqeq: ["error", "always"]*/

a === b
foo === true
bananas !== 1
value === undefined
typeof foo === 'undefined'
'hello' !== 'world'
0 === 0
true === true
foo === null

This rule optionally takes a second argument, which should be an object with the following supported properties:

• "null": Customize how this rule treats null literals. Possible values:
  • always (default) - Always use === or !==.
  • never - Never use === or !== with null.
  • ignore - Do not apply this rule to null.

smart

The "smart" option enforces the use of === and !== except for these cases:

• Comparing two literal values
• Evaluating the value of typeof
• Comparing against null

Examples of incorrect code for the "smart" option:

/*eslint eqeqeq: ["error", "smart"]*/

// comparing two variables requires ===
a == b

// only one side is a literal
foo == true
bananas != 1

// comparing to undefined requires ===
value == undefined

Examples of correct code for the "smart" option:

/*eslint eqeqeq: ["error", "smart"]*/

typeof foo == 'undefined'
'hello' != 'world'
0 == 0
true == true
foo == null

allow-null

Deprecated: Instead of using this option use "always" and pass a "null" option property with value "ignore". This will tell ESLint to always enforce strict equality except when comparing with the null literal.

["error", "always", {"null": "ignore"}]

When Not To Use It

If you don't want to enforce a style for using equality operators, then it's safe to disable this rule. Source: http://eslint.org/docs/rules/

Require === and !== (eqeqeq)

It is considered good practice to use the type-safe equality operators === and !== instead of their regular counterparts == and !=.

The reason for this is that == and != do type coercion which follows the rather obscure Abstract Equality Comparison Algorithm. For instance, the following statements are all considered true:

• [] == false
• [] == ![]
• 3 == "03"

If one of those occurs in an innocent-looking statement such as a == b the actual problem is very difficult to spot.

Rule Details

This rule is aimed at eliminating the type-unsafe equality operators.

Examples of incorrect code for this rule:

/*eslint eqeqeq: "error"*/

if (x == 42) { }

if ("" == text) { }

if (obj.getStuff() != undefined) { }

The --fix option on the command line automatically fixes some problems reported by this rule. A problem is only fixed if one of the operands is a typeof expression, or if both operands are literals with the same type.

Options

always

The "always" option (default) enforces the use of === and !== in every situation (except when you opt-in to more specific handling of null [see below]).

Examples of incorrect code for the "always" option:

/*eslint eqeqeq: ["error", "always"]*/

a == b
foo == true
bananas != 1
value == undefined
typeof foo == 'undefined'
'hello' != 'world'
0 == 0
true == true
foo == null

Examples of correct code for the "always" option:

/*eslint eqeqeq: ["error", "always"]*/

a === b
foo === true
bananas !== 1
value === undefined
typeof foo === 'undefined'
'hello' !== 'world'
0 === 0
true === true
foo === null

This rule optionally takes a second argument, which should be an object with the following supported properties:

• "null": Customize how this rule treats null literals. Possible values:
  • always (default) - Always use === or !==.
  • never - Never use === or !== with null.
  • ignore - Do not apply this rule to null.

smart

The "smart" option enforces the use of === and !== except for these cases:

• Comparing two literal values
• Evaluating the value of typeof
• Comparing against null

Examples of incorrect code for the "smart" option:

/*eslint eqeqeq: ["error", "smart"]*/

// comparing two variables requires ===
a == b

// only one side is a literal
foo == true
bananas != 1

// comparing to undefined requires ===
value == undefined

Examples of correct code for the "smart" option:

/*eslint eqeqeq: ["error", "smart"]*/

typeof foo == 'undefined'
'hello' != 'world'
0 == 0
true == true
foo == null

allow-null

Deprecated: Instead of using this option use "always" and pass a "null" option property with value "ignore". This will tell ESLint to always enforce strict equality except when comparing with the null literal.

["error", "always", {"null": "ignore"}]

When Not To Use It

If you don't want to enforce a style for using equality operators, then it's safe to disable this rule. Source: http://eslint.org/docs/rules/

Require === and !== (eqeqeq)

It is considered good practice to use the type-safe equality operators === and !== instead of their regular counterparts == and !=.

The reason for this is that == and != do type coercion which follows the rather obscure Abstract Equality Comparison Algorithm. For instance, the following statements are all considered true:

• [] == false
• [] == ![]
• 3 == "03"

If one of those occurs in an innocent-looking statement such as a == b the actual problem is very difficult to spot.

Rule Details

This rule is aimed at eliminating the type-unsafe equality operators.

Examples of incorrect code for this rule:

/*eslint eqeqeq: "error"*/

if (x == 42) { }

if ("" == text) { }

if (obj.getStuff() != undefined) { }

The --fix option on the command line automatically fixes some problems reported by this rule. A problem is only fixed if one of the operands is a typeof expression, or if both operands are literals with the same type.

Options

always

The "always" option (default) enforces the use of === and !== in every situation (except when you opt-in to more specific handling of null [see below]).

Examples of incorrect code for the "always" option:

/*eslint eqeqeq: ["error", "always"]*/

a == b
foo == true
bananas != 1
value == undefined
typeof foo == 'undefined'
'hello' != 'world'
0 == 0
true == true
foo == null

Examples of correct code for the "always" option:

/*eslint eqeqeq: ["error", "always"]*/

a === b
foo === true
bananas !== 1
value === undefined
typeof foo === 'undefined'
'hello' !== 'world'
0 === 0
true === true
foo === null

This rule optionally takes a second argument, which should be an object with the following supported properties:

• "null": Customize how this rule treats null literals. Possible values:
  • always (default) - Always use === or !==.
  • never - Never use === or !== with null.
  • ignore - Do not apply this rule to null.

smart

The "smart" option enforces the use of === and !== except for these cases:

• Comparing two literal values
• Evaluating the value of typeof
• Comparing against null

Examples of incorrect code for the "smart" option:

/*eslint eqeqeq: ["error", "smart"]*/

// comparing two variables requires ===
a == b

// only one side is a literal
foo == true
bananas != 1

// comparing to undefined requires ===
value == undefined

Examples of correct code for the "smart" option:

/*eslint eqeqeq: ["error", "smart"]*/

typeof foo == 'undefined'
'hello' != 'world'
0 == 0
true == true
foo == null

allow-null

Deprecated: Instead of using this option use "always" and pass a "null" option property with value "ignore". This will tell ESLint to always enforce strict equality except when comparing with the null literal.

["error", "always", {"null": "ignore"}]

When Not To Use It

If you don't want to enforce a style for using equality operators, then it's safe to disable this rule. Source: http://eslint.org/docs/rules/

Require === and !== (eqeqeq)

It is considered good practice to use the type-safe equality operators === and !== instead of their regular counterparts == and !=.

The reason for this is that == and != do type coercion which follows the rather obscure Abstract Equality Comparison Algorithm. For instance, the following statements are all considered true:

• [] == false
• [] == ![]
• 3 == "03"

If one of those occurs in an innocent-looking statement such as a == b the actual problem is very difficult to spot.

Rule Details

This rule is aimed at eliminating the type-unsafe equality operators.

Examples of incorrect code for this rule:

/*eslint eqeqeq: "error"*/

if (x == 42) { }

if ("" == text) { }

if (obj.getStuff() != undefined) { }

The --fix option on the command line automatically fixes some problems reported by this rule. A problem is only fixed if one of the operands is a typeof expression, or if both operands are literals with the same type.

Options

always

The "always" option (default) enforces the use of === and !== in every situation (except when you opt-in to more specific handling of null [see below]).

Examples of incorrect code for the "always" option:

/*eslint eqeqeq: ["error", "always"]*/

a == b
foo == true
bananas != 1
value == undefined
typeof foo == 'undefined'
'hello' != 'world'
0 == 0
true == true
foo == null

Examples of correct code for the "always" option:

/*eslint eqeqeq: ["error", "always"]*/

a === b
foo === true
bananas !== 1
value === undefined
typeof foo === 'undefined'
'hello' !== 'world'
0 === 0
true === true
foo === null

This rule optionally takes a second argument, which should be an object with the following supported properties:

• "null": Customize how this rule treats null literals. Possible values:
  • always (default) - Always use === or !==.
  • never - Never use === or !== with null.
  • ignore - Do not apply this rule to null.

smart

The "smart" option enforces the use of === and !== except for these cases:

• Comparing two literal values
• Evaluating the value of typeof
• Comparing against null

Examples of incorrect code for the "smart" option:

/*eslint eqeqeq: ["error", "smart"]*/

// comparing two variables requires ===
a == b

// only one side is a literal
foo == true
bananas != 1

// comparing to undefined requires ===
value == undefined

Examples of correct code for the "smart" option:

/*eslint eqeqeq: ["error", "smart"]*/

typeof foo == 'undefined'
'hello' != 'world'
0 == 0
true == true
foo == null

allow-null

Deprecated: Instead of using this option use "always" and pass a "null" option property with value "ignore". This will tell ESLint to always enforce strict equality except when comparing with the null literal.

["error", "always", {"null": "ignore"}]

When Not To Use It

If you don't want to enforce a style for using equality operators, then it's safe to disable this rule. Source: http://eslint.org/docs/rules/

Require === and !== (eqeqeq)

It is considered good practice to use the type-safe equality operators === and !== instead of their regular counterparts == and !=.

The reason for this is that == and != do type coercion which follows the rather obscure Abstract Equality Comparison Algorithm. For instance, the following statements are all considered true:

• [] == false
• [] == ![]
• 3 == "03"

If one of those occurs in an innocent-looking statement such as a == b the actual problem is very difficult to spot.

Rule Details

This rule is aimed at eliminating the type-unsafe equality operators.

Examples of incorrect code for this rule:

/*eslint eqeqeq: "error"*/

if (x == 42) { }

if ("" == text) { }

if (obj.getStuff() != undefined) { }

The --fix option on the command line automatically fixes some problems reported by this rule. A problem is only fixed if one of the operands is a typeof expression, or if both operands are literals with the same type.

Options

always

The "always" option (default) enforces the use of === and !== in every situation (except when you opt-in to more specific handling of null [see below]).

Examples of incorrect code for the "always" option:

/*eslint eqeqeq: ["error", "always"]*/

a == b
foo == true
bananas != 1
value == undefined
typeof foo == 'undefined'
'hello' != 'world'
0 == 0
true == true
foo == null

Examples of correct code for the "always" option:

/*eslint eqeqeq: ["error", "always"]*/

a === b
foo === true
bananas !== 1
value === undefined
typeof foo === 'undefined'
'hello' !== 'world'
0 === 0
true === true
foo === null

This rule optionally takes a second argument, which should be an object with the following supported properties:

• "null": Customize how this rule treats null literals. Possible values:
  • always (default) - Always use === or !==.
  • never - Never use === or !== with null.
  • ignore - Do not apply this rule to null.

smart

The "smart" option enforces the use of === and !== except for these cases:

• Comparing two literal values
• Evaluating the value of typeof
• Comparing against null

Examples of incorrect code for the "smart" option:

/*eslint eqeqeq: ["error", "smart"]*/

// comparing two variables requires ===
a == b

// only one side is a literal
foo == true
bananas != 1

// comparing to undefined requires ===
value == undefined

Examples of correct code for the "smart" option:

/*eslint eqeqeq: ["error", "smart"]*/

typeof foo == 'undefined'
'hello' != 'world'
0 == 0
true == true
foo == null

allow-null

Deprecated: Instead of using this option use "always" and pass a "null" option property with value "ignore". This will tell ESLint to always enforce strict equality except when comparing with the null literal.

["error", "always", {"null": "ignore"}]

When Not To Use It

If you don't want to enforce a style for using equality operators, then it's safe to disable this rule. Source: http://eslint.org/docs/rules/

Require === and !== (eqeqeq)

It is considered good practice to use the type-safe equality operators === and !== instead of their regular counterparts == and !=.

The reason for this is that == and != do type coercion which follows the rather obscure Abstract Equality Comparison Algorithm. For instance, the following statements are all considered true:

• [] == false
• [] == ![]
• 3 == "03"

If one of those occurs in an innocent-looking statement such as a == b the actual problem is very difficult to spot.

Rule Details

This rule is aimed at eliminating the type-unsafe equality operators.

Examples of incorrect code for this rule:

/*eslint eqeqeq: "error"*/

if (x == 42) { }

if ("" == text) { }

if (obj.getStuff() != undefined) { }

The --fix option on the command line automatically fixes some problems reported by this rule. A problem is only fixed if one of the operands is a typeof expression, or if both operands are literals with the same type.

Options

always

The "always" option (default) enforces the use of === and !== in every situation (except when you opt-in to more specific handling of null [see below]).

Examples of incorrect code for the "always" option:

/*eslint eqeqeq: ["error", "always"]*/

a == b
foo == true
bananas != 1
value == undefined
typeof foo == 'undefined'
'hello' != 'world'
0 == 0
true == true
foo == null

Examples of correct code for the "always" option:

/*eslint eqeqeq: ["error", "always"]*/

a === b
foo === true
bananas !== 1
value === undefined
typeof foo === 'undefined'
'hello' !== 'world'
0 === 0
true === true
foo === null

This rule optionally takes a second argument, which should be an object with the following supported properties:

• "null": Customize how this rule treats null literals. Possible values:
  • always (default) - Always use === or !==.
  • never - Never use === or !== with null.
  • ignore - Do not apply this rule to null.

smart

The "smart" option enforces the use of === and !== except for these cases:

• Comparing two literal values
• Evaluating the value of typeof
• Comparing against null

Examples of incorrect code for the "smart" option:

/*eslint eqeqeq: ["error", "smart"]*/

// comparing two variables requires ===
a == b

// only one side is a literal
foo == true
bananas != 1

// comparing to undefined requires ===
value == undefined

Examples of correct code for the "smart" option:

/*eslint eqeqeq: ["error", "smart"]*/

typeof foo == 'undefined'
'hello' != 'world'
0 == 0
true == true
foo == null

allow-null

Deprecated: Instead of using this option use "always" and pass a "null" option property with value "ignore". This will tell ESLint to always enforce strict equality except when comparing with the null literal.

["error", "always", {"null": "ignore"}]

When Not To Use It

If you don't want to enforce a style for using equality operators, then it's safe to disable this rule. Source: http://eslint.org/docs/rules/

Require === and !== (eqeqeq)

It is considered good practice to use the type-safe equality operators === and !== instead of their regular counterparts == and !=.

The reason for this is that == and != do type coercion which follows the rather obscure Abstract Equality Comparison Algorithm. For instance, the following statements are all considered true:

• [] == false
• [] == ![]
• 3 == "03"

If one of those occurs in an innocent-looking statement such as a == b the actual problem is very difficult to spot.

Rule Details

This rule is aimed at eliminating the type-unsafe equality operators.

Examples of incorrect code for this rule:

/*eslint eqeqeq: "error"*/

if (x == 42) { }

if ("" == text) { }

if (obj.getStuff() != undefined) { }

The --fix option on the command line automatically fixes some problems reported by this rule. A problem is only fixed if one of the operands is a typeof expression, or if both operands are literals with the same type.

Options

always

The "always" option (default) enforces the use of === and !== in every situation (except when you opt-in to more specific handling of null [see below]).

Examples of incorrect code for the "always" option:

/*eslint eqeqeq: ["error", "always"]*/

a == b
foo == true
bananas != 1
value == undefined
typeof foo == 'undefined'
'hello' != 'world'
0 == 0
true == true
foo == null

Examples of correct code for the "always" option:

/*eslint eqeqeq: ["error", "always"]*/

a === b
foo === true
bananas !== 1
value === undefined
typeof foo === 'undefined'
'hello' !== 'world'
0 === 0
true === true
foo === null

This rule optionally takes a second argument, which should be an object with the following supported properties:

• "null": Customize how this rule treats null literals. Possible values:
  • always (default) - Always use === or !==.
  • never - Never use === or !== with null.
  • ignore - Do not apply this rule to null.

smart

The "smart" option enforces the use of === and !== except for these cases:

• Comparing two literal values
• Evaluating the value of typeof
• Comparing against null

Examples of incorrect code for the "smart" option:

/*eslint eqeqeq: ["error", "smart"]*/

// comparing two variables requires ===
a == b

// only one side is a literal
foo == true
bananas != 1

// comparing to undefined requires ===
value == undefined

Examples of correct code for the "smart" option:

/*eslint eqeqeq: ["error", "smart"]*/

typeof foo == 'undefined'
'hello' != 'world'
0 == 0
true == true
foo == null

allow-null

Deprecated: Instead of using this option use "always" and pass a "null" option property with value "ignore". This will tell ESLint to always enforce strict equality except when comparing with the null literal.

["error", "always", {"null": "ignore"}]

When Not To Use It

If you don't want to enforce a style for using equality operators, then it's safe to disable this rule. Source: http://eslint.org/docs/rules/

Require === and !== (eqeqeq)

It is considered good practice to use the type-safe equality operators === and !== instead of their regular counterparts == and !=.

The reason for this is that == and != do type coercion which follows the rather obscure Abstract Equality Comparison Algorithm. For instance, the following statements are all considered true:

• [] == false
• [] == ![]
• 3 == "03"

If one of those occurs in an innocent-looking statement such as a == b the actual problem is very difficult to spot.

Rule Details

This rule is aimed at eliminating the type-unsafe equality operators.

Examples of incorrect code for this rule:

/*eslint eqeqeq: "error"*/

if (x == 42) { }

if ("" == text) { }

if (obj.getStuff() != undefined) { }

The --fix option on the command line automatically fixes some problems reported by this rule. A problem is only fixed if one of the operands is a typeof expression, or if both operands are literals with the same type.

Options

always

The "always" option (default) enforces the use of === and !== in every situation (except when you opt-in to more specific handling of null [see below]).

Examples of incorrect code for the "always" option:

/*eslint eqeqeq: ["error", "always"]*/

a == b
foo == true
bananas != 1
value == undefined
typeof foo == 'undefined'
'hello' != 'world'
0 == 0
true == true
foo == null

Examples of correct code for the "always" option:

/*eslint eqeqeq: ["error", "always"]*/

a === b
foo === true
bananas !== 1
value === undefined
typeof foo === 'undefined'
'hello' !== 'world'
0 === 0
true === true
foo === null

This rule optionally takes a second argument, which should be an object with the following supported properties:

• "null": Customize how this rule treats null literals. Possible values:
  • always (default) - Always use === or !==.
  • never - Never use === or !== with null.
  • ignore - Do not apply this rule to null.

smart

The "smart" option enforces the use of === and !== except for these cases:

• Comparing two literal values
• Evaluating the value of typeof
• Comparing against null

Examples of incorrect code for the "smart" option:

/*eslint eqeqeq: ["error", "smart"]*/

// comparing two variables requires ===
a == b

// only one side is a literal
foo == true
bananas != 1

// comparing to undefined requires ===
value == undefined

Examples of correct code for the "smart" option:

/*eslint eqeqeq: ["error", "smart"]*/

typeof foo == 'undefined'
'hello' != 'world'
0 == 0
true == true
foo == null

allow-null

Deprecated: Instead of using this option use "always" and pass a "null" option property with value "ignore". This will tell ESLint to always enforce strict equality except when comparing with the null literal.

["error", "always", {"null": "ignore"}]

When Not To Use It

If you don't want to enforce a style for using equality operators, then it's safe to disable this rule. Source: http://eslint.org/docs/rules/

Require === and !== (eqeqeq)

It is considered good practice to use the type-safe equality operators === and !== instead of their regular counterparts == and !=.

The reason for this is that == and != do type coercion which follows the rather obscure Abstract Equality Comparison Algorithm. For instance, the following statements are all considered true:

• [] == false
• [] == ![]
• 3 == "03"

If one of those occurs in an innocent-looking statement such as a == b the actual problem is very difficult to spot.

Rule Details

This rule is aimed at eliminating the type-unsafe equality operators.

Examples of incorrect code for this rule:

/*eslint eqeqeq: "error"*/

if (x == 42) { }

if ("" == text) { }

if (obj.getStuff() != undefined) { }

The --fix option on the command line automatically fixes some problems reported by this rule. A problem is only fixed if one of the operands is a typeof expression, or if both operands are literals with the same type.

Options

always

The "always" option (default) enforces the use of === and !== in every situation (except when you opt-in to more specific handling of null [see below]).

Examples of incorrect code for the "always" option:

/*eslint eqeqeq: ["error", "always"]*/

a == b
foo == true
bananas != 1
value == undefined
typeof foo == 'undefined'
'hello' != 'world'
0 == 0
true == true
foo == null

Examples of correct code for the "always" option:

/*eslint eqeqeq: ["error", "always"]*/

a === b
foo === true
bananas !== 1
value === undefined
typeof foo === 'undefined'
'hello' !== 'world'
0 === 0
true === true
foo === null

This rule optionally takes a second argument, which should be an object with the following supported properties:

• "null": Customize how this rule treats null literals. Possible values:
  • always (default) - Always use === or !==.
  • never - Never use === or !== with null.
  • ignore - Do not apply this rule to null.

smart

The "smart" option enforces the use of === and !== except for these cases:

• Comparing two literal values
• Evaluating the value of typeof
• Comparing against null

Examples of incorrect code for the "smart" option:

/*eslint eqeqeq: ["error", "smart"]*/

// comparing two variables requires ===
a == b

// only one side is a literal
foo == true
bananas != 1

// comparing to undefined requires ===
value == undefined

Examples of correct code for the "smart" option:

/*eslint eqeqeq: ["error", "smart"]*/

typeof foo == 'undefined'
'hello' != 'world'
0 == 0
true == true
foo == null

allow-null

Deprecated: Instead of using this option use "always" and pass a "null" option property with value "ignore". This will tell ESLint to always enforce strict equality except when comparing with the null literal.

["error", "always", {"null": "ignore"}]

When Not To Use It

If you don't want to enforce a style for using equality operators, then it's safe to disable this rule. Source: http://eslint.org/docs/rules/

Require === and !== (eqeqeq)

It is considered good practice to use the type-safe equality operators === and !== instead of their regular counterparts == and !=.

The reason for this is that == and != do type coercion which follows the rather obscure Abstract Equality Comparison Algorithm. For instance, the following statements are all considered true:

• [] == false
• [] == ![]
• 3 == "03"

If one of those occurs in an innocent-looking statement such as a == b the actual problem is very difficult to spot.

Rule Details

This rule is aimed at eliminating the type-unsafe equality operators.

Examples of incorrect code for this rule:

/*eslint eqeqeq: "error"*/

if (x == 42) { }

if ("" == text) { }

if (obj.getStuff() != undefined) { }

The --fix option on the command line automatically fixes some problems reported by this rule. A problem is only fixed if one of the operands is a typeof expression, or if both operands are literals with the same type.

Options

always

The "always" option (default) enforces the use of === and !== in every situation (except when you opt-in to more specific handling of null [see below]).

Examples of incorrect code for the "always" option:

/*eslint eqeqeq: ["error", "always"]*/

a == b
foo == true
bananas != 1
value == undefined
typeof foo == 'undefined'
'hello' != 'world'
0 == 0
true == true
foo == null

Examples of correct code for the "always" option:

/*eslint eqeqeq: ["error", "always"]*/

a === b
foo === true
bananas !== 1
value === undefined
typeof foo === 'undefined'
'hello' !== 'world'
0 === 0
true === true
foo === null

This rule optionally takes a second argument, which should be an object with the following supported properties:

• "null": Customize how this rule treats null literals. Possible values:
  • always (default) - Always use === or !==.
  • never - Never use === or !== with null.
  • ignore - Do not apply this rule to null.

smart

The "smart" option enforces the use of === and !== except for these cases:

• Comparing two literal values
• Evaluating the value of typeof
• Comparing against null

Examples of incorrect code for the "smart" option:

/*eslint eqeqeq: ["error", "smart"]*/

// comparing two variables requires ===
a == b

// only one side is a literal
foo == true
bananas != 1

// comparing to undefined requires ===
value == undefined

Examples of correct code for the "smart" option:

/*eslint eqeqeq: ["error", "smart"]*/

typeof foo == 'undefined'
'hello' != 'world'
0 == 0
true == true
foo == null

allow-null

Deprecated: Instead of using this option use "always" and pass a "null" option property with value "ignore". This will tell ESLint to always enforce strict equality except when comparing with the null literal.

["error", "always", {"null": "ignore"}]

When Not To Use It

If you don't want to enforce a style for using equality operators, then it's safe to disable this rule. Source: http://eslint.org/docs/rules/

Require === and !== (eqeqeq)

It is considered good practice to use the type-safe equality operators === and !== instead of their regular counterparts == and !=.

The reason for this is that == and != do type coercion which follows the rather obscure Abstract Equality Comparison Algorithm. For instance, the following statements are all considered true:

• [] == false
• [] == ![]
• 3 == "03"

If one of those occurs in an innocent-looking statement such as a == b the actual problem is very difficult to spot.

Rule Details

This rule is aimed at eliminating the type-unsafe equality operators.

Examples of incorrect code for this rule:

/*eslint eqeqeq: "error"*/

if (x == 42) { }

if ("" == text) { }

if (obj.getStuff() != undefined) { }

The --fix option on the command line automatically fixes some problems reported by this rule. A problem is only fixed if one of the operands is a typeof expression, or if both operands are literals with the same type.

Options

always

The "always" option (default) enforces the use of === and !== in every situation (except when you opt-in to more specific handling of null [see below]).

Examples of incorrect code for the "always" option:

/*eslint eqeqeq: ["error", "always"]*/

a == b
foo == true
bananas != 1
value == undefined
typeof foo == 'undefined'
'hello' != 'world'
0 == 0
true == true
foo == null

Examples of correct code for the "always" option:

/*eslint eqeqeq: ["error", "always"]*/

a === b
foo === true
bananas !== 1
value === undefined
typeof foo === 'undefined'
'hello' !== 'world'
0 === 0
true === true
foo === null

This rule optionally takes a second argument, which should be an object with the following supported properties:

• "null": Customize how this rule treats null literals. Possible values:
  • always (default) - Always use === or !==.
  • never - Never use === or !== with null.
  • ignore - Do not apply this rule to null.

smart

The "smart" option enforces the use of === and !== except for these cases:

• Comparing two literal values
• Evaluating the value of typeof
• Comparing against null

Examples of incorrect code for the "smart" option:

/*eslint eqeqeq: ["error", "smart"]*/

// comparing two variables requires ===
a == b

// only one side is a literal
foo == true
bananas != 1

// comparing to undefined requires ===
value == undefined

Examples of correct code for the "smart" option:

/*eslint eqeqeq: ["error", "smart"]*/

typeof foo == 'undefined'
'hello' != 'world'
0 == 0
true == true
foo == null

allow-null

Deprecated: Instead of using this option use "always" and pass a "null" option property with value "ignore". This will tell ESLint to always enforce strict equality except when comparing with the null literal.

["error", "always", {"null": "ignore"}]

When Not To Use It

If you don't want to enforce a style for using equality operators, then it's safe to disable this rule. Source: http://eslint.org/docs/rules/

Require === and !== (eqeqeq)

It is considered good practice to use the type-safe equality operators === and !== instead of their regular counterparts == and !=.

The reason for this is that == and != do type coercion which follows the rather obscure Abstract Equality Comparison Algorithm. For instance, the following statements are all considered true:

• [] == false
• [] == ![]
• 3 == "03"

If one of those occurs in an innocent-looking statement such as a == b the actual problem is very difficult to spot.

Rule Details

This rule is aimed at eliminating the type-unsafe equality operators.

Examples of incorrect code for this rule:

/*eslint eqeqeq: "error"*/

if (x == 42) { }

if ("" == text) { }

if (obj.getStuff() != undefined) { }

The --fix option on the command line automatically fixes some problems reported by this rule. A problem is only fixed if one of the operands is a typeof expression, or if both operands are literals with the same type.

Options

always

The "always" option (default) enforces the use of === and !== in every situation (except when you opt-in to more specific handling of null [see below]).

Examples of incorrect code for the "always" option:

/*eslint eqeqeq: ["error", "always"]*/

a == b
foo == true
bananas != 1
value == undefined
typeof foo == 'undefined'
'hello' != 'world'
0 == 0
true == true
foo == null

Examples of correct code for the "always" option:

/*eslint eqeqeq: ["error", "always"]*/

a === b
foo === true
bananas !== 1
value === undefined
typeof foo === 'undefined'
'hello' !== 'world'
0 === 0
true === true
foo === null

This rule optionally takes a second argument, which should be an object with the following supported properties:

• "null": Customize how this rule treats null literals. Possible values:
  • always (default) - Always use === or !==.
  • never - Never use === or !== with null.
  • ignore - Do not apply this rule to null.

smart

The "smart" option enforces the use of === and !== except for these cases:

• Comparing two literal values
• Evaluating the value of typeof
• Comparing against null

Examples of incorrect code for the "smart" option:

/*eslint eqeqeq: ["error", "smart"]*/

// comparing two variables requires ===
a == b

// only one side is a literal
foo == true
bananas != 1

// comparing to undefined requires ===
value == undefined

Examples of correct code for the "smart" option:

/*eslint eqeqeq: ["error", "smart"]*/

typeof foo == 'undefined'
'hello' != 'world'
0 == 0
true == true
foo == null

allow-null

Deprecated: Instead of using this option use "always" and pass a "null" option property with value "ignore". This will tell ESLint to always enforce strict equality except when comparing with the null literal.

["error", "always", {"null": "ignore"}]

When Not To Use It

If you don't want to enforce a style for using equality operators, then it's safe to disable this rule. Source: http://eslint.org/docs/rules/

Require === and !== (eqeqeq)

It is considered good practice to use the type-safe equality operators === and !== instead of their regular counterparts == and !=.

The reason for this is that == and != do type coercion which follows the rather obscure Abstract Equality Comparison Algorithm. For instance, the following statements are all considered true:

• [] == false
• [] == ![]
• 3 == "03"

If one of those occurs in an innocent-looking statement such as a == b the actual problem is very difficult to spot.

Rule Details

This rule is aimed at eliminating the type-unsafe equality operators.

Examples of incorrect code for this rule:

/*eslint eqeqeq: "error"*/

if (x == 42) { }

if ("" == text) { }

if (obj.getStuff() != undefined) { }

The --fix option on the command line automatically fixes some problems reported by this rule. A problem is only fixed if one of the operands is a typeof expression, or if both operands are literals with the same type.

Options

always

The "always" option (default) enforces the use of === and !== in every situation (except when you opt-in to more specific handling of null [see below]).

Examples of incorrect code for the "always" option:

/*eslint eqeqeq: ["error", "always"]*/

a == b
foo == true
bananas != 1
value == undefined
typeof foo == 'undefined'
'hello' != 'world'
0 == 0
true == true
foo == null

Examples of correct code for the "always" option:

/*eslint eqeqeq: ["error", "always"]*/

a === b
foo === true
bananas !== 1
value === undefined
typeof foo === 'undefined'
'hello' !== 'world'
0 === 0
true === true
foo === null

This rule optionally takes a second argument, which should be an object with the following supported properties:

• "null": Customize how this rule treats null literals. Possible values:
  • always (default) - Always use === or !==.
  • never - Never use === or !== with null.
  • ignore - Do not apply this rule to null.

smart

The "smart" option enforces the use of === and !== except for these cases:

• Comparing two literal values
• Evaluating the value of typeof
• Comparing against null

Examples of incorrect code for the "smart" option:

/*eslint eqeqeq: ["error", "smart"]*/

// comparing two variables requires ===
a == b

// only one side is a literal
foo == true
bananas != 1

// comparing to undefined requires ===
value == undefined

Examples of correct code for the "smart" option:

/*eslint eqeqeq: ["error", "smart"]*/

typeof foo == 'undefined'
'hello' != 'world'
0 == 0
true == true
foo == null

allow-null

Deprecated: Instead of using this option use "always" and pass a "null" option property with value "ignore". This will tell ESLint to always enforce strict equality except when comparing with the null literal.

["error", "always", {"null": "ignore"}]

When Not To Use It

If you don't want to enforce a style for using equality operators, then it's safe to disable this rule. Source: http://eslint.org/docs/rules/

Require === and !== (eqeqeq)

It is considered good practice to use the type-safe equality operators === and !== instead of their regular counterparts == and !=.

The reason for this is that == and != do type coercion which follows the rather obscure Abstract Equality Comparison Algorithm. For instance, the following statements are all considered true:

• [] == false
• [] == ![]
• 3 == "03"

If one of those occurs in an innocent-looking statement such as a == b the actual problem is very difficult to spot.

Rule Details

This rule is aimed at eliminating the type-unsafe equality operators.

Examples of incorrect code for this rule:

/*eslint eqeqeq: "error"*/

if (x == 42) { }

if ("" == text) { }

if (obj.getStuff() != undefined) { }

The --fix option on the command line automatically fixes some problems reported by this rule. A problem is only fixed if one of the operands is a typeof expression, or if both operands are literals with the same type.

Options

always

The "always" option (default) enforces the use of === and !== in every situation (except when you opt-in to more specific handling of null [see below]).

Examples of incorrect code for the "always" option:

/*eslint eqeqeq: ["error", "always"]*/

a == b
foo == true
bananas != 1
value == undefined
typeof foo == 'undefined'
'hello' != 'world'
0 == 0
true == true
foo == null

Examples of correct code for the "always" option:

/*eslint eqeqeq: ["error", "always"]*/

a === b
foo === true
bananas !== 1
value === undefined
typeof foo === 'undefined'
'hello' !== 'world'
0 === 0
true === true
foo === null

This rule optionally takes a second argument, which should be an object with the following supported properties:

• "null": Customize how this rule treats null literals. Possible values:
  • always (default) - Always use === or !==.
  • never - Never use === or !== with null.
  • ignore - Do not apply this rule to null.

smart

The "smart" option enforces the use of === and !== except for these cases:

• Comparing two literal values
• Evaluating the value of typeof
• Comparing against null

Examples of incorrect code for the "smart" option:

/*eslint eqeqeq: ["error", "smart"]*/

// comparing two variables requires ===
a == b

// only one side is a literal
foo == true
bananas != 1

// comparing to undefined requires ===
value == undefined

Examples of correct code for the "smart" option:

/*eslint eqeqeq: ["error", "smart"]*/

typeof foo == 'undefined'
'hello' != 'world'
0 == 0
true == true
foo == null

allow-null

Deprecated: Instead of using this option use "always" and pass a "null" option property with value "ignore". This will tell ESLint to always enforce strict equality except when comparing with the null literal.

["error", "always", {"null": "ignore"}]

When Not To Use It

If you don't want to enforce a style for using equality operators, then it's safe to disable this rule. Source: http://eslint.org/docs/rules/

Require === and !== (eqeqeq)

It is considered good practice to use the type-safe equality operators === and !== instead of their regular counterparts == and !=.

The reason for this is that == and != do type coercion which follows the rather obscure Abstract Equality Comparison Algorithm. For instance, the following statements are all considered true:

• [] == false
• [] == ![]
• 3 == "03"

If one of those occurs in an innocent-looking statement such as a == b the actual problem is very difficult to spot.

Rule Details

This rule is aimed at eliminating the type-unsafe equality operators.

Examples of incorrect code for this rule:

/*eslint eqeqeq: "error"*/

if (x == 42) { }

if ("" == text) { }

if (obj.getStuff() != undefined) { }

The --fix option on the command line automatically fixes some problems reported by this rule. A problem is only fixed if one of the operands is a typeof expression, or if both operands are literals with the same type.

Options

always

The "always" option (default) enforces the use of === and !== in every situation (except when you opt-in to more specific handling of null [see below]).

Examples of incorrect code for the "always" option:

/*eslint eqeqeq: ["error", "always"]*/

a == b
foo == true
bananas != 1
value == undefined
typeof foo == 'undefined'
'hello' != 'world'
0 == 0
true == true
foo == null

Examples of correct code for the "always" option:

/*eslint eqeqeq: ["error", "always"]*/

a === b
foo === true
bananas !== 1
value === undefined
typeof foo === 'undefined'
'hello' !== 'world'
0 === 0
true === true
foo === null

This rule optionally takes a second argument, which should be an object with the following supported properties:

• "null": Customize how this rule treats null literals. Possible values:
  • always (default) - Always use === or !==.
  • never - Never use === or !== with null.
  • ignore - Do not apply this rule to null.

smart

The "smart" option enforces the use of === and !== except for these cases:

• Comparing two literal values
• Evaluating the value of typeof
• Comparing against null

Examples of incorrect code for the "smart" option:

/*eslint eqeqeq: ["error", "smart"]*/

// comparing two variables requires ===
a == b

// only one side is a literal
foo == true
bananas != 1

// comparing to undefined requires ===
value == undefined

Examples of correct code for the "smart" option:

/*eslint eqeqeq: ["error", "smart"]*/

typeof foo == 'undefined'
'hello' != 'world'
0 == 0
true == true
foo == null

allow-null

Deprecated: Instead of using this option use "always" and pass a "null" option property with value "ignore". This will tell ESLint to always enforce strict equality except when comparing with the null literal.

["error", "always", {"null": "ignore"}]

When Not To Use It

If you don't want to enforce a style for using equality operators, then it's safe to disable this rule. Source: http://eslint.org/docs/rules/

Require === and !== (eqeqeq)

It is considered good practice to use the type-safe equality operators === and !== instead of their regular counterparts == and !=.

The reason for this is that == and != do type coercion which follows the rather obscure Abstract Equality Comparison Algorithm. For instance, the following statements are all considered true:

• [] == false
• [] == ![]
• 3 == "03"

If one of those occurs in an innocent-looking statement such as a == b the actual problem is very difficult to spot.

Rule Details

This rule is aimed at eliminating the type-unsafe equality operators.

Examples of incorrect code for this rule:

/*eslint eqeqeq: "error"*/

if (x == 42) { }

if ("" == text) { }

if (obj.getStuff() != undefined) { }

The --fix option on the command line automatically fixes some problems reported by this rule. A problem is only fixed if one of the operands is a typeof expression, or if both operands are literals with the same type.

Options

always

The "always" option (default) enforces the use of === and !== in every situation (except when you opt-in to more specific handling of null [see below]).

Examples of incorrect code for the "always" option:

/*eslint eqeqeq: ["error", "always"]*/

a == b
foo == true
bananas != 1
value == undefined
typeof foo == 'undefined'
'hello' != 'world'
0 == 0
true == true
foo == null

Examples of correct code for the "always" option:

/*eslint eqeqeq: ["error", "always"]*/

a === b
foo === true
bananas !== 1
value === undefined
typeof foo === 'undefined'
'hello' !== 'world'
0 === 0
true === true
foo === null

This rule optionally takes a second argument, which should be an object with the following supported properties:

• "null": Customize how this rule treats null literals. Possible values:
  • always (default) - Always use === or !==.
  • never - Never use === or !== with null.
  • ignore - Do not apply this rule to null.

smart

The "smart" option enforces the use of === and !== except for these cases:

• Comparing two literal values
• Evaluating the value of typeof
• Comparing against null

Examples of incorrect code for the "smart" option:

/*eslint eqeqeq: ["error", "smart"]*/

// comparing two variables requires ===
a == b

// only one side is a literal
foo == true
bananas != 1

// comparing to undefined requires ===
value == undefined

Examples of correct code for the "smart" option:

/*eslint eqeqeq: ["error", "smart"]*/

typeof foo == 'undefined'
'hello' != 'world'
0 == 0
true == true
foo == null

allow-null

Deprecated: Instead of using this option use "always" and pass a "null" option property with value "ignore". This will tell ESLint to always enforce strict equality except when comparing with the null literal.

["error", "always", {"null": "ignore"}]

When Not To Use It

If you don't want to enforce a style for using equality operators, then it's safe to disable this rule. Source: http://eslint.org/docs/rules/

Require === and !== (eqeqeq)

It is considered good practice to use the type-safe equality operators === and !== instead of their regular counterparts == and !=.

The reason for this is that == and != do type coercion which follows the rather obscure Abstract Equality Comparison Algorithm. For instance, the following statements are all considered true:

• [] == false
• [] == ![]
• 3 == "03"

If one of those occurs in an innocent-looking statement such as a == b the actual problem is very difficult to spot.

Rule Details

This rule is aimed at eliminating the type-unsafe equality operators.

Examples of incorrect code for this rule:

/*eslint eqeqeq: "error"*/

if (x == 42) { }

if ("" == text) { }

if (obj.getStuff() != undefined) { }

The --fix option on the command line automatically fixes some problems reported by this rule. A problem is only fixed if one of the operands is a typeof expression, or if both operands are literals with the same type.

Options

always

The "always" option (default) enforces the use of === and !== in every situation (except when you opt-in to more specific handling of null [see below]).

Examples of incorrect code for the "always" option:

/*eslint eqeqeq: ["error", "always"]*/

a == b
foo == true
bananas != 1
value == undefined
typeof foo == 'undefined'
'hello' != 'world'
0 == 0
true == true
foo == null

Examples of correct code for the "always" option:

/*eslint eqeqeq: ["error", "always"]*/

a === b
foo === true
bananas !== 1
value === undefined
typeof foo === 'undefined'
'hello' !== 'world'
0 === 0
true === true
foo === null

This rule optionally takes a second argument, which should be an object with the following supported properties:

• "null": Customize how this rule treats null literals. Possible values:
  • always (default) - Always use === or !==.
  • never - Never use === or !== with null.
  • ignore - Do not apply this rule to null.

smart

The "smart" option enforces the use of === and !== except for these cases:

• Comparing two literal values
• Evaluating the value of typeof
• Comparing against null

Examples of incorrect code for the "smart" option:

/*eslint eqeqeq: ["error", "smart"]*/

// comparing two variables requires ===
a == b

// only one side is a literal
foo == true
bananas != 1

// comparing to undefined requires ===
value == undefined

Examples of correct code for the "smart" option:

/*eslint eqeqeq: ["error", "smart"]*/

typeof foo == 'undefined'
'hello' != 'world'
0 == 0
true == true
foo == null

allow-null

Deprecated: Instead of using this option use "always" and pass a "null" option property with value "ignore". This will tell ESLint to always enforce strict equality except when comparing with the null literal.

["error", "always", {"null": "ignore"}]

When Not To Use It

If you don't want to enforce a style for using equality operators, then it's safe to disable this rule. Source: http://eslint.org/docs/rules/

Require === and !== (eqeqeq)

It is considered good practice to use the type-safe equality operators === and !== instead of their regular counterparts == and !=.

The reason for this is that == and != do type coercion which follows the rather obscure Abstract Equality Comparison Algorithm. For instance, the following statements are all considered true:

• [] == false
• [] == ![]
• 3 == "03"

If one of those occurs in an innocent-looking statement such as a == b the actual problem is very difficult to spot.

Rule Details

This rule is aimed at eliminating the type-unsafe equality operators.

Examples of incorrect code for this rule:

/*eslint eqeqeq: "error"*/

if (x == 42) { }

if ("" == text) { }

if (obj.getStuff() != undefined) { }

The --fix option on the command line automatically fixes some problems reported by this rule. A problem is only fixed if one of the operands is a typeof expression, or if both operands are literals with the same type.

Options

always

The "always" option (default) enforces the use of === and !== in every situation (except when you opt-in to more specific handling of null [see below]).

Examples of incorrect code for the "always" option:

/*eslint eqeqeq: ["error", "always"]*/

a == b
foo == true
bananas != 1
value == undefined
typeof foo == 'undefined'
'hello' != 'world'
0 == 0
true == true
foo == null

Examples of correct code for the "always" option:

/*eslint eqeqeq: ["error", "always"]*/

a === b
foo === true
bananas !== 1
value === undefined
typeof foo === 'undefined'
'hello' !== 'world'
0 === 0
true === true
foo === null

This rule optionally takes a second argument, which should be an object with the following supported properties:

• "null": Customize how this rule treats null literals. Possible values:
  • always (default) - Always use === or !==.
  • never - Never use === or !== with null.
  • ignore - Do not apply this rule to null.

smart

The "smart" option enforces the use of === and !== except for these cases:

• Comparing two literal values
• Evaluating the value of typeof
• Comparing against null

Examples of incorrect code for the "smart" option:

/*eslint eqeqeq: ["error", "smart"]*/

// comparing two variables requires ===
a == b

// only one side is a literal
foo == true
bananas != 1

// comparing to undefined requires ===
value == undefined

Examples of correct code for the "smart" option:

/*eslint eqeqeq: ["error", "smart"]*/

typeof foo == 'undefined'
'hello' != 'world'
0 == 0
true == true
foo == null

allow-null

Deprecated: Instead of using this option use "always" and pass a "null" option property with value "ignore". This will tell ESLint to always enforce strict equality except when comparing with the null literal.

["error", "always", {"null": "ignore"}]

When Not To Use It

If you don't want to enforce a style for using equality operators, then it's safe to disable this rule. Source: http://eslint.org/docs/rules/

Duplicated Code

Duplicated code can lead to software that is hard to understand and difficult to change. The Don't Repeat Yourself (DRY) principle states:

Every piece of knowledge must have a single, unambiguous, authoritative representation within a system.

When you violate DRY, bugs and maintenance problems are sure to follow. Duplicated code has a tendency to both continue to replicate and also to diverge (leaving bugs as two similar implementations differ in subtle ways).

Tuning

This issue has a mass of 251.

We set useful threshold defaults for the languages we support but you may want to adjust these settings based on your project guidelines.

The threshold configuration represents the minimum mass a code block must have to be analyzed for duplication. The lower the threshold, the more fine-grained the comparison.

If the engine is too easily reporting duplication, try raising the threshold. If you suspect that the engine isn't catching enough duplication, try lowering the threshold. The best setting tends to differ from language to language.

See codeclimate-duplication's documentation for more information about tuning the mass threshold in your .codeclimate.yml.

Refactorings

• Extract Method
• Extract Class
• Form Template Method
• Introduce Null Object
• Pull Up Method
• Pull Up Field
• Substitute Algorithm

Further Reading

• Don't Repeat Yourself on the C2 Wiki
• Duplicated Code on SourceMaking
• Refactoring: Improving the Design of Existing Code by Martin Fowler. Duplicated Code, p76

Duplicated Code

Duplicated code can lead to software that is hard to understand and difficult to change. The Don't Repeat Yourself (DRY) principle states:

Every piece of knowledge must have a single, unambiguous, authoritative representation within a system.

When you violate DRY, bugs and maintenance problems are sure to follow. Duplicated code has a tendency to both continue to replicate and also to diverge (leaving bugs as two similar implementations differ in subtle ways).

Tuning

This issue has a mass of 144.

We set useful threshold defaults for the languages we support but you may want to adjust these settings based on your project guidelines.

The threshold configuration represents the minimum mass a code block must have to be analyzed for duplication. The lower the threshold, the more fine-grained the comparison.

If the engine is too easily reporting duplication, try raising the threshold. If you suspect that the engine isn't catching enough duplication, try lowering the threshold. The best setting tends to differ from language to language.

See codeclimate-duplication's documentation for more information about tuning the mass threshold in your .codeclimate.yml.

Refactorings

• Extract Method
• Extract Class
• Form Template Method
• Introduce Null Object
• Pull Up Method
• Pull Up Field
• Substitute Algorithm

Further Reading

• Don't Repeat Yourself on the C2 Wiki
• Duplicated Code on SourceMaking
• Refactoring: Improving the Design of Existing Code by Martin Fowler. Duplicated Code, p76

Duplicated Code

Duplicated code can lead to software that is hard to understand and difficult to change. The Don't Repeat Yourself (DRY) principle states:

Every piece of knowledge must have a single, unambiguous, authoritative representation within a system.

When you violate DRY, bugs and maintenance problems are sure to follow. Duplicated code has a tendency to both continue to replicate and also to diverge (leaving bugs as two similar implementations differ in subtle ways).

Tuning

This issue has a mass of 144.

We set useful threshold defaults for the languages we support but you may want to adjust these settings based on your project guidelines.

The threshold configuration represents the minimum mass a code block must have to be analyzed for duplication. The lower the threshold, the more fine-grained the comparison.

If the engine is too easily reporting duplication, try raising the threshold. If you suspect that the engine isn't catching enough duplication, try lowering the threshold. The best setting tends to differ from language to language.

See codeclimate-duplication's documentation for more information about tuning the mass threshold in your .codeclimate.yml.

Refactorings

• Extract Method
• Extract Class
• Form Template Method
• Introduce Null Object
• Pull Up Method
• Pull Up Field
• Substitute Algorithm

Further Reading

• Don't Repeat Yourself on the C2 Wiki
• Duplicated Code on SourceMaking
• Refactoring: Improving the Design of Existing Code by Martin Fowler. Duplicated Code, p76

Duplicated Code

Duplicated code can lead to software that is hard to understand and difficult to change. The Don't Repeat Yourself (DRY) principle states:

Every piece of knowledge must have a single, unambiguous, authoritative representation within a system.

When you violate DRY, bugs and maintenance problems are sure to follow. Duplicated code has a tendency to both continue to replicate and also to diverge (leaving bugs as two similar implementations differ in subtle ways).

Tuning

This issue has a mass of 110.

We set useful threshold defaults for the languages we support but you may want to adjust these settings based on your project guidelines.

The threshold configuration represents the minimum mass a code block must have to be analyzed for duplication. The lower the threshold, the more fine-grained the comparison.

If the engine is too easily reporting duplication, try raising the threshold. If you suspect that the engine isn't catching enough duplication, try lowering the threshold. The best setting tends to differ from language to language.

See codeclimate-duplication's documentation for more information about tuning the mass threshold in your .codeclimate.yml.

Refactorings

• Extract Method
• Extract Class
• Form Template Method
• Introduce Null Object
• Pull Up Method
• Pull Up Field
• Substitute Algorithm

Further Reading

• Don't Repeat Yourself on the C2 Wiki
• Duplicated Code on SourceMaking
• Refactoring: Improving the Design of Existing Code by Martin Fowler. Duplicated Code, p76

Duplicated Code

Duplicated code can lead to software that is hard to understand and difficult to change. The Don't Repeat Yourself (DRY) principle states:

Every piece of knowledge must have a single, unambiguous, authoritative representation within a system.

When you violate DRY, bugs and maintenance problems are sure to follow. Duplicated code has a tendency to both continue to replicate and also to diverge (leaving bugs as two similar implementations differ in subtle ways).

Tuning

This issue has a mass of 107.

We set useful threshold defaults for the languages we support but you may want to adjust these settings based on your project guidelines.

The threshold configuration represents the minimum mass a code block must have to be analyzed for duplication. The lower the threshold, the more fine-grained the comparison.

If the engine is too easily reporting duplication, try raising the threshold. If you suspect that the engine isn't catching enough duplication, try lowering the threshold. The best setting tends to differ from language to language.

See codeclimate-duplication's documentation for more information about tuning the mass threshold in your .codeclimate.yml.

Refactorings

• Extract Method
• Extract Class
• Form Template Method
• Introduce Null Object
• Pull Up Method
• Pull Up Field
• Substitute Algorithm

Further Reading

• Don't Repeat Yourself on the C2 Wiki
• Duplicated Code on SourceMaking
• Refactoring: Improving the Design of Existing Code by Martin Fowler. Duplicated Code, p76

Duplicated Code

Duplicated code can lead to software that is hard to understand and difficult to change. The Don't Repeat Yourself (DRY) principle states:

Every piece of knowledge must have a single, unambiguous, authoritative representation within a system.

When you violate DRY, bugs and maintenance problems are sure to follow. Duplicated code has a tendency to both continue to replicate and also to diverge (leaving bugs as two similar implementations differ in subtle ways).

Tuning

This issue has a mass of 82.

We set useful threshold defaults for the languages we support but you may want to adjust these settings based on your project guidelines.

The threshold configuration represents the minimum mass a code block must have to be analyzed for duplication. The lower the threshold, the more fine-grained the comparison.

If the engine is too easily reporting duplication, try raising the threshold. If you suspect that the engine isn't catching enough duplication, try lowering the threshold. The best setting tends to differ from language to language.

See codeclimate-duplication's documentation for more information about tuning the mass threshold in your .codeclimate.yml.

Refactorings

• Extract Method
• Extract Class
• Form Template Method
• Introduce Null Object
• Pull Up Method
• Pull Up Field
• Substitute Algorithm

Further Reading

• Don't Repeat Yourself on the C2 Wiki
• Duplicated Code on SourceMaking
• Refactoring: Improving the Design of Existing Code by Martin Fowler. Duplicated Code, p76

Duplicated Code

Duplicated code can lead to software that is hard to understand and difficult to change. The Don't Repeat Yourself (DRY) principle states:

Every piece of knowledge must have a single, unambiguous, authoritative representation within a system.

When you violate DRY, bugs and maintenance problems are sure to follow. Duplicated code has a tendency to both continue to replicate and also to diverge (leaving bugs as two similar implementations differ in subtle ways).

Tuning

This issue has a mass of 45.

We set useful threshold defaults for the languages we support but you may want to adjust these settings based on your project guidelines.

The threshold configuration represents the minimum mass a code block must have to be analyzed for duplication. The lower the threshold, the more fine-grained the comparison.

If the engine is too easily reporting duplication, try raising the threshold. If you suspect that the engine isn't catching enough duplication, try lowering the threshold. The best setting tends to differ from language to language.

See codeclimate-duplication's documentation for more information about tuning the mass threshold in your .codeclimate.yml.

Refactorings

• Extract Method
• Extract Class
• Form Template Method
• Introduce Null Object
• Pull Up Method
• Pull Up Field
• Substitute Algorithm

Further Reading

• Don't Repeat Yourself on the C2 Wiki
• Duplicated Code on SourceMaking
• Refactoring: Improving the Design of Existing Code by Martin Fowler. Duplicated Code, p76

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