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 54.

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 47.

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 47.

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 47.

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 47.

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 46.

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 46.

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

require or disallow strict mode directives (strict)

A strict mode directive is a "use strict" literal at the beginning of a script or function body. It enables strict mode semantics.

When a directive occurs in global scope, strict mode applies to the entire script:

"use strict";

// strict mode

function foo() {
    // strict mode
}

When a directive occurs at the beginning of a function body, strict mode applies only to that function, including all contained functions:

function foo() {
    "use strict";
    // strict mode
}

function foo2() {
    // not strict mode
};

(function() {
    "use strict";
    function bar() {
        // strict mode
    }
}());

In the CommonJS module system, a hidden function wraps each module and limits the scope of a "global" strict mode directive.

In ECMAScript modules, which always have strict mode semantics, the directives are unnecessary.

Rule Details

This rule requires or disallows strict mode directives.

This rule disallows strict mode directives, no matter which option is specified, if ESLint configuration specifies either of the following as [parser options](../user-guide/configuring#specifying-parser-options):

• "sourceType": "module" that is, files are ECMAScript modules
• "impliedStrict": true property in the ecmaFeatures object

This rule disallows strict mode directives, no matter which option is specified, in functions with non-simple parameter lists (for example, parameter lists with default parameter values) because that is a syntax error in ECMAScript 2016 and later. See the examples of the function option.

Options

This rule has a string option:

• "safe" (default) corresponds either of the following options:
  • "global" if ESLint considers a file to be a CommonJS module
  • "function" otherwise
• "global" requires one strict mode directive in the global scope (and disallows any other strict mode directives)
• "function" requires one strict mode directive in each top-level function declaration or expression (and disallows any other strict mode directives)
• "never" disallows strict mode directives

safe

The "safe" option corresponds to the "global" option if ESLint considers a file to be a Node.js or CommonJS module because the configuration specifies either of the following:

• node or commonjs [environments](../user-guide/configuring#specifying-environments)
• "globalReturn": true property in the ecmaFeatures object of [parser options](../user-guide/configuring#specifying-parser-options)

Otherwise the "safe" option corresponds to the "function" option.

global

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

/*eslint strict: ["error", "global"]*/

function foo() {
}

/*eslint strict: ["error", "global"]*/

function foo() {
    "use strict";
}

/*eslint strict: ["error", "global"]*/

"use strict";

function foo() {
    "use strict";
}

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

/*eslint strict: ["error", "global"]*/

"use strict";

function foo() {
}

function

This option ensures that all function bodies are strict mode code, while global code is not. Particularly if a build step concatenates multiple scripts, a strict mode directive in global code of one script could unintentionally enable strict mode in another script that was not intended to be strict code.

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

/*eslint strict: ["error", "function"]*/

"use strict";

function foo() {
}

/*eslint strict: ["error", "function"]*/

function foo() {
}

(function() {
    function bar() {
        "use strict";
    }
}());

/*eslint strict: ["error", "function"]*/
/*eslint-env es6*/

// Illegal "use strict" directive in function with non-simple parameter list.
// This is a syntax error since ES2016.
function foo(a = 1) {
    "use strict";
}

// We cannot write "use strict" directive in this function.
// So we have to wrap this function with a function with "use strict" directive.
function foo(a = 1) {
}

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

/*eslint strict: ["error", "function"]*/

function foo() {
    "use strict";
}

(function() {
    "use strict";

    function bar() {
    }

    function baz(a = 1) {
    }
}());

var foo = (function() {
    "use strict";

    return function foo(a = 1) {
    };
}());

never

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

/*eslint strict: ["error", "never"]*/

"use strict";

function foo() {
}

/*eslint strict: ["error", "never"]*/

function foo() {
    "use strict";
}

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

/*eslint strict: ["error", "never"]*/

function foo() {
}

earlier default (removed)

(removed) The default option (that is, no string option specified) for this rule was removed in ESLint v1.0. The "function" option is most similar to the removed option.

This option ensures that all functions are executed in strict mode. A strict mode directive must be present in global code or in every top-level function declaration or expression. It does not concern itself with unnecessary strict mode directives in nested functions that are already strict, nor with multiple strict mode directives at the same level.

Examples of incorrect code for this rule with the earlier default option which has been removed:

// "strict": "error"

function foo() {
}

// "strict": "error"

(function() {
    function bar() {
        "use strict";
    }
}());

Examples of correct code for this rule with the earlier default option which has been removed:

// "strict": "error"

"use strict";

function foo() {
}

// "strict": "error"

function foo() {
    "use strict";
}

// "strict": "error"

(function() {
    "use strict";
    function bar() {
        "use strict";
    }
}());

When Not To Use It

In a codebase that has both strict and non-strict code, either turn this rule off, or selectively disable it where necessary. For example, functions referencing arguments.callee are invalid in strict mode. A full list of strict mode differences is available on MDN. Source: http://eslint.org/docs/rules/

Enforce require() on the top-level module scope (global-require)

In Node.js, module dependencies are included using the require() function, such as:

var fs = require("fs");

While require() may be called anywhere in code, some style guides prescribe that it should be called only in the top level of a module to make it easier to identify dependencies. For instance, it's arguably harder to identify dependencies when they are deeply nested inside of functions and other statements:

function foo() {

    if (condition) {
        var fs = require("fs");
    }
}

Since require() does a synchronous load, it can cause performance problems when used in other locations.

Further, ES6 modules mandate that import and export statements can only occur in the top level of the module's body.

Rule Details

This rule requires all calls to require() to be at the top level of the module, similar to ES6 import and export statements, which also can occur only at the top level.

Examples of incorrect code for this rule:

/*eslint global-require: "error"*/
/*eslint-env es6*/

// calling require() inside of a function is not allowed
function readFile(filename, callback) {
    var fs = require('fs');
    fs.readFile(filename, callback)
}

// conditional requires like this are also not allowed
if (DEBUG) { require('debug'); }

// a require() in a switch statement is also flagged
switch(x) { case '1': require('1'); break; }

// you may not require() inside an arrow function body
var getModule = (name) => require(name);

// you may not require() inside of a function body as well
function getModule(name) { return require(name); }

// you may not require() inside of a try/catch block
try {
    require(unsafeModule);
} catch(e) {
    console.log(e);
}

Examples of correct code for this rule:

/*eslint global-require: "error"*/

// all these variations of require() are ok
require('x');
var y = require('y');
var z;
z = require('z').initialize();

// requiring a module and using it in a function is ok
var fs = require('fs');
function readFile(filename, callback) {
    fs.readFile(filename, callback)
}

// you can use a ternary to determine which module to require
var logger = DEBUG ? require('dev-logger') : require('logger');

// if you want you can require() at the end of your module
function doSomethingA() {}
function doSomethingB() {}
var x = require("x"),
    z = require("z");

When Not To Use It

If you have a module that must be initialized with information that comes from the file-system or if a module is only used in very rare situations and will cause significant overhead to load it may make sense to disable the rule. If you need to require() an optional dependency inside of a try/catch, you can disable this rule for just that dependency using the // eslint-disable-line global-require comment. 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

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;
//•••••

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

require a newline after each call in a method chain (newline-per-chained-call)

Chained method calls on a single line without line breaks are harder to read, so some developers place a newline character after each method call in the chain to make it more readable and easy to maintain.

Let's look at the following perfectly valid (but single line) code.

d3.select("body").selectAll("p").data([4, 8, 15, 16, 23, 42 ]).enter().append("p").text(function(d) { return "I'm number " + d + "!"; });

However, with appropriate new lines, it becomes easy to read and understand. Look at the same code written below with line breaks after each call.

d3
    .select("body")
    .selectAll("p")
    .data([
        4,
        8,
        15,
        16,
        23,
        42
    ])
    .enter()
    .append("p")
    .text(function (d) {
        return "I'm number " + d + "!";
    });

Another argument in favor of this style is that it improves the clarity of diffs when something in the method chain is changed:

Less clear:

-d3.select("body").selectAll("p").style("color", "white");
+d3.select("body").selectAll("p").style("color", "blue");

More clear:

d3
    .select("body")
    .selectAll("p")
-    .style("color", "white");
+    .style("color", "blue");

Rule Details

This rule requires a newline after each call in a method chain or deep member access. Computed property accesses such as instance[something] are excluded.

Options

This rule has an object option:

• "ignoreChainWithDepth" (default: 2) allows chains up to a specified depth.

ignoreChainWithDepth

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

/*eslint newline-per-chained-call: ["error", { "ignoreChainWithDepth": 2 }]*/

_.chain({}).map(foo).filter(bar).value();

// Or
_.chain({}).map(foo).filter(bar);

// Or
_
  .chain({}).map(foo)
  .filter(bar);

// Or
obj.method().method2().method3();

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

/*eslint newline-per-chained-call: ["error", { "ignoreChainWithDepth": 2 }]*/

_
  .chain({})
  .map(foo)
  .filter(bar)
  .value();

// Or
_
  .chain({})
  .map(foo)
  .filter(bar);

// Or
_.chain({})
  .map(foo)
  .filter(bar);

// Or
obj
  .prop
  .method().prop;

// Or
obj
  .prop.method()
  .method2()
  .method3().prop;

When Not To Use It

If you have conflicting rules or when you are fine with chained calls on one line, you can safely turn this rule off. Source: http://eslint.org/docs/rules/

require a newline after each call in a method chain (newline-per-chained-call)

Chained method calls on a single line without line breaks are harder to read, so some developers place a newline character after each method call in the chain to make it more readable and easy to maintain.

Let's look at the following perfectly valid (but single line) code.

d3.select("body").selectAll("p").data([4, 8, 15, 16, 23, 42 ]).enter().append("p").text(function(d) { return "I'm number " + d + "!"; });

However, with appropriate new lines, it becomes easy to read and understand. Look at the same code written below with line breaks after each call.

d3
    .select("body")
    .selectAll("p")
    .data([
        4,
        8,
        15,
        16,
        23,
        42
    ])
    .enter()
    .append("p")
    .text(function (d) {
        return "I'm number " + d + "!";
    });

Another argument in favor of this style is that it improves the clarity of diffs when something in the method chain is changed:

Less clear:

-d3.select("body").selectAll("p").style("color", "white");
+d3.select("body").selectAll("p").style("color", "blue");

More clear:

d3
    .select("body")
    .selectAll("p")
-    .style("color", "white");
+    .style("color", "blue");

Rule Details

This rule requires a newline after each call in a method chain or deep member access. Computed property accesses such as instance[something] are excluded.

Options

This rule has an object option:

• "ignoreChainWithDepth" (default: 2) allows chains up to a specified depth.

ignoreChainWithDepth

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

/*eslint newline-per-chained-call: ["error", { "ignoreChainWithDepth": 2 }]*/

_.chain({}).map(foo).filter(bar).value();

// Or
_.chain({}).map(foo).filter(bar);

// Or
_
  .chain({}).map(foo)
  .filter(bar);

// Or
obj.method().method2().method3();

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

/*eslint newline-per-chained-call: ["error", { "ignoreChainWithDepth": 2 }]*/

_
  .chain({})
  .map(foo)
  .filter(bar)
  .value();

// Or
_
  .chain({})
  .map(foo)
  .filter(bar);

// Or
_.chain({})
  .map(foo)
  .filter(bar);

// Or
obj
  .prop
  .method().prop;

// Or
obj
  .prop.method()
  .method2()
  .method3().prop;

When Not To Use It

If you have conflicting rules or when you are fine with chained calls on one line, you can safely turn this rule off. Source: http://eslint.org/docs/rules/

Require or disallow named function expressions (func-names)

A pattern that's becoming more common is to give function expressions names to aid in debugging. For example:

Foo.prototype.bar = function bar() {};

Adding the second bar in the above example is optional. If you leave off the function name then when the function throws an exception you are likely to get something similar to anonymous function in the stack trace. If you provide the optional name for a function expression then you will get the name of the function expression in the stack trace.

Rule Details

This rule can enforce or disallow the use of named function expressions.

Options

This rule has a string option:

• "always" (default) requires function expressions to have a name
• "as-needed" requires function expressions to have a name, if the name cannot be assigned automatically in an ES6 environment
• "never" disallows named function expressions, except in recursive functions, where a name is needed

always

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

/*eslint func-names: ["error", "always"]*/

Foo.prototype.bar = function() {};

(function() {
    // ...
}())

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

/*eslint func-names: ["error", "always"]*/

Foo.prototype.bar = function bar() {};

(function bar() {
    // ...
}())

as-needed

ECMAScript 6 introduced a name property on all functions. The value of name is determined by evaluating the code around the function to see if a name can be inferred. For example, a function assigned to a variable will automatically have a name property equal to the name of the variable. The value of name is then used in stack traces for easier debugging.

Examples of incorrect code for this rule with the default "as-needed" option:

/*eslint func-names: ["error", "as-needed"]*/

Foo.prototype.bar = function() {};

(function() {
    // ...
}())

Examples of correct code for this rule with the default "as-needed" option:

/*eslint func-names: ["error", "as-needed"]*/

var bar = function() {};

(function bar() {
    // ...
}())

never

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

/*eslint func-names: ["error", "never"]*/

Foo.prototype.bar = function bar() {};

(function bar() {
    // ...
}())

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

/*eslint func-names: ["error", "never"]*/

Foo.prototype.bar = function() {};

(function() {
    // ...
}())

Further Reading

• Functions Explained
• Function Names in ES6

Compatibility

• JSCS: requireAnonymousFunctions
• JSCS: disallowAnonymousFunctions Source: http://eslint.org/docs/rules/

disallow the use of console (no-console)

In JavaScript that is designed to be executed in the browser, it's considered a best practice to avoid using methods on console. Such messages are considered to be for debugging purposes and therefore not suitable to ship to the client. In general, calls using console should be stripped before being pushed to production.

console.log("Made it here.");
console.error("That shouldn't have happened.");

Rule Details

This rule disallows calls to methods of the console object.

Examples of incorrect code for this rule:

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

console.log("Log a debug level message.");
console.warn("Log a warn level message.");
console.error("Log an error level message.");

Examples of correct code for this rule:

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

// custom console
Console.log("Hello world!");

Options

This rule has an object option for exceptions:

• "allow" has an array of strings which are allowed methods of the console object

Examples of additional correct code for this rule with a sample { "allow": ["warn", "error"] } option:

/*eslint no-console: ["error", { allow: ["warn", "error"] }] */

console.warn("Log a warn level message.");
console.error("Log an error level message.");

When Not To Use It

If you're using Node.js, however, console is used to output information to the user and so is not strictly used for debugging purposes. If you are developing for Node.js then you most likely do not want this rule enabled.

Related Rules

• [no-alert](no-alert.md)
• [no-debugger](no-debugger.md) Source: http://eslint.org/docs/rules/

disallow the use of console (no-console)

In JavaScript that is designed to be executed in the browser, it's considered a best practice to avoid using methods on console. Such messages are considered to be for debugging purposes and therefore not suitable to ship to the client. In general, calls using console should be stripped before being pushed to production.

console.log("Made it here.");
console.error("That shouldn't have happened.");

Rule Details

This rule disallows calls to methods of the console object.

Examples of incorrect code for this rule:

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

console.log("Log a debug level message.");
console.warn("Log a warn level message.");
console.error("Log an error level message.");

Examples of correct code for this rule:

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

// custom console
Console.log("Hello world!");

Options

This rule has an object option for exceptions:

• "allow" has an array of strings which are allowed methods of the console object

Examples of additional correct code for this rule with a sample { "allow": ["warn", "error"] } option:

/*eslint no-console: ["error", { allow: ["warn", "error"] }] */

console.warn("Log a warn level message.");
console.error("Log an error level message.");

When Not To Use It

If you're using Node.js, however, console is used to output information to the user and so is not strictly used for debugging purposes. If you are developing for Node.js then you most likely do not want this rule enabled.

Related Rules

• [no-alert](no-alert.md)
• [no-debugger](no-debugger.md) Source: http://eslint.org/docs/rules/

require a newline after each call in a method chain (newline-per-chained-call)

Chained method calls on a single line without line breaks are harder to read, so some developers place a newline character after each method call in the chain to make it more readable and easy to maintain.

Let's look at the following perfectly valid (but single line) code.

d3.select("body").selectAll("p").data([4, 8, 15, 16, 23, 42 ]).enter().append("p").text(function(d) { return "I'm number " + d + "!"; });

However, with appropriate new lines, it becomes easy to read and understand. Look at the same code written below with line breaks after each call.

d3
    .select("body")
    .selectAll("p")
    .data([
        4,
        8,
        15,
        16,
        23,
        42
    ])
    .enter()
    .append("p")
    .text(function (d) {
        return "I'm number " + d + "!";
    });

Another argument in favor of this style is that it improves the clarity of diffs when something in the method chain is changed:

Less clear:

-d3.select("body").selectAll("p").style("color", "white");
+d3.select("body").selectAll("p").style("color", "blue");

More clear:

d3
    .select("body")
    .selectAll("p")
-    .style("color", "white");
+    .style("color", "blue");

Rule Details

This rule requires a newline after each call in a method chain or deep member access. Computed property accesses such as instance[something] are excluded.

Options

This rule has an object option:

• "ignoreChainWithDepth" (default: 2) allows chains up to a specified depth.

ignoreChainWithDepth

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

/*eslint newline-per-chained-call: ["error", { "ignoreChainWithDepth": 2 }]*/

_.chain({}).map(foo).filter(bar).value();

// Or
_.chain({}).map(foo).filter(bar);

// Or
_
  .chain({}).map(foo)
  .filter(bar);

// Or
obj.method().method2().method3();

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

/*eslint newline-per-chained-call: ["error", { "ignoreChainWithDepth": 2 }]*/

_
  .chain({})
  .map(foo)
  .filter(bar)
  .value();

// Or
_
  .chain({})
  .map(foo)
  .filter(bar);

// Or
_.chain({})
  .map(foo)
  .filter(bar);

// Or
obj
  .prop
  .method().prop;

// Or
obj
  .prop.method()
  .method2()
  .method3().prop;

When Not To Use It

If you have conflicting rules or when you are fine with chained calls on one line, you can safely turn this rule off. Source: http://eslint.org/docs/rules/

require or disallow trailing commas (comma-dangle)

Trailing commas in object literals are valid according to the ECMAScript 5 (and ECMAScript 3!) spec. However, IE8 (when not in IE8 document mode) and below will throw an error when it encounters trailing commas in JavaScript.

var foo = {
    bar: "baz",
    qux: "quux",
};

Trailing commas simplify adding and removing items to objects and arrays, since only the lines you are modifying must be touched. Another argument in favor of trailing commas is that it improves the clarity of diffs when an item is added or removed from an object or array:

Less clear:

var foo = {
-    bar: "baz",
-    qux: "quux"
+    bar: "baz"
 };

More clear:

var foo = {
     bar: "baz",
-    qux: "quux",
 };

Rule Details

This rule enforces consistent use of trailing commas in object and array literals.

Options

This rule has a string option or an object option:

{
    "comma-dangle": ["error", "never"],
    // or
    "comma-dangle": ["error", {
        "arrays": "never",
        "objects": "never",
        "imports": "never",
        "exports": "never",
        "functions": "ignore",
    }]
}

• "never" (default) disallows trailing commas
• "always" requires trailing commas
• "always-multiline" requires trailing commas when the last element or property is in a different line than the closing ] or } and disallows trailing commas when the last element or property is on the same line as the closing ] or }
• "only-multiline" allows (but does not require) trailing commas when the last element or property is in a different line than the closing ] or } and disallows trailing commas when the last element or property is on the same line as the closing ] or }

Trailing commas in function declarations and function calls are valid syntax since ECMAScript 2017; however, the string option does not check these situations for backwards compatibility.

You can also use an object option to configure this rule for each type of syntax. Each of the following options can be set to "never", "always", "always-multiline", "only-multiline", or "ignore". The default for each option is "never" unless otherwise specified.

• arrays is for array literals and array patterns of destructuring. (e.g. let [a,] = [1,];)
• objects is for object literals and object patterns of destructuring. (e.g. let {a,} = {a: 1};)
• imports is for import declarations of ES Modules. (e.g. import {a,} from "foo";)
• exports is for export declarations of ES Modules. (e.g. export {a,};)
• functions is for function declarations and function calls. (e.g. (function(a,){ })(b,);)
  functions is set to "ignore" by default for consistency with the string option.

never

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

/*eslint comma-dangle: ["error", "never"]*/

var foo = {
    bar: "baz",
    qux: "quux",
};

var arr = [1,2,];

foo({
  bar: "baz",
  qux: "quux",
});

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

/*eslint comma-dangle: ["error", "never"]*/

var foo = {
    bar: "baz",
    qux: "quux"
};

var arr = [1,2];

foo({
  bar: "baz",
  qux: "quux"
});

always

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

/*eslint comma-dangle: ["error", "always"]*/

var foo = {
    bar: "baz",
    qux: "quux"
};

var arr = [1,2];

foo({
  bar: "baz",
  qux: "quux"
});

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

/*eslint comma-dangle: ["error", "always"]*/

var foo = {
    bar: "baz",
    qux: "quux",
};

var arr = [1,2,];

foo({
  bar: "baz",
  qux: "quux",
});

always-multiline

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

/*eslint comma-dangle: ["error", "always-multiline"]*/

var foo = {
    bar: "baz",
    qux: "quux"
};

var foo = { bar: "baz", qux: "quux", };

var arr = [1,2,];

var arr = [1,
    2,];

var arr = [
    1,
    2
];

foo({
  bar: "baz",
  qux: "quux"
});

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

/*eslint comma-dangle: ["error", "always-multiline"]*/

var foo = {
    bar: "baz",
    qux: "quux",
};

var foo = {bar: "baz", qux: "quux"};
var arr = [1,2];

var arr = [1,
    2];

var arr = [
    1,
    2,
];

foo({
  bar: "baz",
  qux: "quux",
});

only-multiline

Examples of incorrect code for this rule with the "only-multiline" option:

/*eslint comma-dangle: ["error", "only-multiline"]*/

var foo = { bar: "baz", qux: "quux", };

var arr = [1,2,];

var arr = [1,
    2,];

Examples of correct code for this rule with the "only-multiline" option:

/*eslint comma-dangle: ["error", "only-multiline"]*/

var foo = {
    bar: "baz",
    qux: "quux",
};

var foo = {
    bar: "baz",
    qux: "quux"
};

var foo = {bar: "baz", qux: "quux"};
var arr = [1,2];

var arr = [1,
    2];

var arr = [
    1,
    2,
];

var arr = [
    1,
    2
];

foo({
  bar: "baz",
  qux: "quux",
});

foo({
  bar: "baz",
  qux: "quux"
});

functions

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

/*eslint comma-dangle: ["error", {"functions": "never"}]*/

function foo(a, b,) {
}

foo(a, b,);
new foo(a, b,);

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

/*eslint comma-dangle: ["error", {"functions": "never"}]*/

function foo(a, b) {
}

foo(a, b);
new foo(a, b);

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

/*eslint comma-dangle: ["error", {"functions": "always"}]*/

function foo(a, b) {
}

foo(a, b);
new foo(a, b);

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

/*eslint comma-dangle: ["error", {"functions": "always"}]*/

function foo(a, b,) {
}

foo(a, b,);
new foo(a, b,);

When Not To Use It

You can turn this rule off if you are not concerned with dangling commas. Source: http://eslint.org/docs/rules/

require or disallow trailing commas (comma-dangle)

Trailing commas in object literals are valid according to the ECMAScript 5 (and ECMAScript 3!) spec. However, IE8 (when not in IE8 document mode) and below will throw an error when it encounters trailing commas in JavaScript.

var foo = {
    bar: "baz",
    qux: "quux",
};

Trailing commas simplify adding and removing items to objects and arrays, since only the lines you are modifying must be touched. Another argument in favor of trailing commas is that it improves the clarity of diffs when an item is added or removed from an object or array:

Less clear:

var foo = {
-    bar: "baz",
-    qux: "quux"
+    bar: "baz"
 };

More clear:

var foo = {
     bar: "baz",
-    qux: "quux",
 };

Rule Details

This rule enforces consistent use of trailing commas in object and array literals.

Options

This rule has a string option or an object option:

{
    "comma-dangle": ["error", "never"],
    // or
    "comma-dangle": ["error", {
        "arrays": "never",
        "objects": "never",
        "imports": "never",
        "exports": "never",
        "functions": "ignore",
    }]
}

• "never" (default) disallows trailing commas
• "always" requires trailing commas
• "always-multiline" requires trailing commas when the last element or property is in a different line than the closing ] or } and disallows trailing commas when the last element or property is on the same line as the closing ] or }
• "only-multiline" allows (but does not require) trailing commas when the last element or property is in a different line than the closing ] or } and disallows trailing commas when the last element or property is on the same line as the closing ] or }

Trailing commas in function declarations and function calls are valid syntax since ECMAScript 2017; however, the string option does not check these situations for backwards compatibility.

You can also use an object option to configure this rule for each type of syntax. Each of the following options can be set to "never", "always", "always-multiline", "only-multiline", or "ignore". The default for each option is "never" unless otherwise specified.

• arrays is for array literals and array patterns of destructuring. (e.g. let [a,] = [1,];)
• objects is for object literals and object patterns of destructuring. (e.g. let {a,} = {a: 1};)
• imports is for import declarations of ES Modules. (e.g. import {a,} from "foo";)
• exports is for export declarations of ES Modules. (e.g. export {a,};)
• functions is for function declarations and function calls. (e.g. (function(a,){ })(b,);)
  functions is set to "ignore" by default for consistency with the string option.

never

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

/*eslint comma-dangle: ["error", "never"]*/

var foo = {
    bar: "baz",
    qux: "quux",
};

var arr = [1,2,];

foo({
  bar: "baz",
  qux: "quux",
});

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

/*eslint comma-dangle: ["error", "never"]*/

var foo = {
    bar: "baz",
    qux: "quux"
};

var arr = [1,2];

foo({
  bar: "baz",
  qux: "quux"
});

always

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

/*eslint comma-dangle: ["error", "always"]*/

var foo = {
    bar: "baz",
    qux: "quux"
};

var arr = [1,2];

foo({
  bar: "baz",
  qux: "quux"
});

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

/*eslint comma-dangle: ["error", "always"]*/

var foo = {
    bar: "baz",
    qux: "quux",
};

var arr = [1,2,];

foo({
  bar: "baz",
  qux: "quux",
});

always-multiline

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

/*eslint comma-dangle: ["error", "always-multiline"]*/

var foo = {
    bar: "baz",
    qux: "quux"
};

var foo = { bar: "baz", qux: "quux", };

var arr = [1,2,];

var arr = [1,
    2,];

var arr = [
    1,
    2
];

foo({
  bar: "baz",
  qux: "quux"
});

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

/*eslint comma-dangle: ["error", "always-multiline"]*/

var foo = {
    bar: "baz",
    qux: "quux",
};

var foo = {bar: "baz", qux: "quux"};
var arr = [1,2];

var arr = [1,
    2];

var arr = [
    1,
    2,
];

foo({
  bar: "baz",
  qux: "quux",
});

only-multiline

Examples of incorrect code for this rule with the "only-multiline" option:

/*eslint comma-dangle: ["error", "only-multiline"]*/

var foo = { bar: "baz", qux: "quux", };

var arr = [1,2,];

var arr = [1,
    2,];

Examples of correct code for this rule with the "only-multiline" option:

/*eslint comma-dangle: ["error", "only-multiline"]*/

var foo = {
    bar: "baz",
    qux: "quux",
};

var foo = {
    bar: "baz",
    qux: "quux"
};

var foo = {bar: "baz", qux: "quux"};
var arr = [1,2];

var arr = [1,
    2];

var arr = [
    1,
    2,
];

var arr = [
    1,
    2
];

foo({
  bar: "baz",
  qux: "quux",
});

foo({
  bar: "baz",
  qux: "quux"
});

functions

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

/*eslint comma-dangle: ["error", {"functions": "never"}]*/

function foo(a, b,) {
}

foo(a, b,);
new foo(a, b,);

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

/*eslint comma-dangle: ["error", {"functions": "never"}]*/

function foo(a, b) {
}

foo(a, b);
new foo(a, b);

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

/*eslint comma-dangle: ["error", {"functions": "always"}]*/

function foo(a, b) {
}

foo(a, b);
new foo(a, b);

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

/*eslint comma-dangle: ["error", {"functions": "always"}]*/

function foo(a, b,) {
}

foo(a, b,);
new foo(a, b,);

When Not To Use It

You can turn this rule off if you are not concerned with dangling commas. Source: http://eslint.org/docs/rules/

For more information visit Source: http://eslint.org/docs/rules/

For more information visit Source: http://eslint.org/docs/rules/