Showing 59 of 59 total issues
Unexpected function expression. Open
return new Promise(function (resolve, reject) {
- Read upRead up
- Exclude checks
Suggest using arrow functions as callbacks. (prefer-arrow-callback)
Arrow functions are suited to callbacks, because:
-
this
keywords in arrow functions bind to the upper scope's. - The notation of the arrow function is shorter than function expression's.
Rule Details
This rule is aimed to flag usage of function expressions in an argument list.
The following patterns are considered problems:
/*eslint prefer-arrow-callback: "error"*/
foo(function(a) { return a; });
foo(function() { return this.a; }.bind(this));
The following patterns are not considered problems:
/*eslint prefer-arrow-callback: "error"*/
/*eslint-env es6*/
foo(a => a);
foo(function*() { yield; });
// this is not a callback.
var foo = function foo(a) { return a; };
// using `this` without `.bind(this)`.
foo(function() { return this.a; });
// recursively.
foo(function bar(n) { return n && n + bar(n - 1); });
Options
This rule takes one optional argument, an object which is an options object.
allowNamedFunctions
This is a boolean
option and it is false
by default. When set to true
, the rule doesn't warn on named functions used as callbacks.
Examples of correct code for the { "allowNamedFunctions": true }
option:
/*eslint prefer-arrow-callback: ["error", { "allowNamedFunctions": true }]*/
foo(function bar() {});
allowUnboundThis
This is a boolean
option and it is true
by default. When set to false
, this option allows the use of this
without restriction and checks for dynamically assigned this
values such as when using Array.prototype.map
with a context
argument. Normally, the rule will flag the use of this
whenever a function does not use bind()
to specify the value of this
constantly.
Examples of incorrect code for the { "allowUnboundThis": false }
option:
/*eslint prefer-arrow-callback: ["error", { "allowUnboundThis": false }]*/
/*eslint-env es6*/
foo(function() { this.a; });
foo(function() { (() => this); });
someArray.map(function (itm) { return this.doSomething(itm); }, someObject);
When Not To Use It
This rule should not be used in ES3/5 environments.
In ES2015 (ES6) or later, if you don't want to be notified about function expressions in an argument list, you can safely disable this rule. Source: http://eslint.org/docs/rules/
Unexpected var, use let or const instead. Open
var subscribeParams = {
- Read upRead up
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require let
or const
instead of var
(no-var)
ECMAScript 6 allows programmers to create variables with block scope instead of function scope using the let
and const
keywords. Block scope is common in many other programming languages and helps programmers avoid mistakes
such as:
var count = people.length;
var enoughFood = count > sandwiches.length;
if (enoughFood) {
var count = sandwiches.length; // accidentally overriding the count variable
console.log("We have " + count + " sandwiches for everyone. Plenty for all!");
}
// our count variable is no longer accurate
console.log("We have " + count + " people and " + sandwiches.length + " sandwiches!");
Rule Details
This rule is aimed at discouraging the use of var
and encouraging the use of const
or let
instead.
Examples
Examples of incorrect code for this rule:
/*eslint no-var: "error"*/
var x = "y";
var CONFIG = {};
Examples of correct code for this rule:
/*eslint no-var: "error"*/
/*eslint-env es6*/
let x = "y";
const CONFIG = {};
When Not To Use It
In addition to non-ES6 environments, existing JavaScript projects that are beginning to introduce ES6 into their
codebase may not want to apply this rule if the cost of migrating from var
to let
is too costly.
Source: http://eslint.org/docs/rules/
Unexpected function expression. Open
_this3.sns.publish(smsParams, function (err, data) {
- Read upRead up
- Exclude checks
Suggest using arrow functions as callbacks. (prefer-arrow-callback)
Arrow functions are suited to callbacks, because:
-
this
keywords in arrow functions bind to the upper scope's. - The notation of the arrow function is shorter than function expression's.
Rule Details
This rule is aimed to flag usage of function expressions in an argument list.
The following patterns are considered problems:
/*eslint prefer-arrow-callback: "error"*/
foo(function(a) { return a; });
foo(function() { return this.a; }.bind(this));
The following patterns are not considered problems:
/*eslint prefer-arrow-callback: "error"*/
/*eslint-env es6*/
foo(a => a);
foo(function*() { yield; });
// this is not a callback.
var foo = function foo(a) { return a; };
// using `this` without `.bind(this)`.
foo(function() { return this.a; });
// recursively.
foo(function bar(n) { return n && n + bar(n - 1); });
Options
This rule takes one optional argument, an object which is an options object.
allowNamedFunctions
This is a boolean
option and it is false
by default. When set to true
, the rule doesn't warn on named functions used as callbacks.
Examples of correct code for the { "allowNamedFunctions": true }
option:
/*eslint prefer-arrow-callback: ["error", { "allowNamedFunctions": true }]*/
foo(function bar() {});
allowUnboundThis
This is a boolean
option and it is true
by default. When set to false
, this option allows the use of this
without restriction and checks for dynamically assigned this
values such as when using Array.prototype.map
with a context
argument. Normally, the rule will flag the use of this
whenever a function does not use bind()
to specify the value of this
constantly.
Examples of incorrect code for the { "allowUnboundThis": false }
option:
/*eslint prefer-arrow-callback: ["error", { "allowUnboundThis": false }]*/
/*eslint-env es6*/
foo(function() { this.a; });
foo(function() { (() => this); });
someArray.map(function (itm) { return this.doSomething(itm); }, someObject);
When Not To Use It
This rule should not be used in ES3/5 environments.
In ES2015 (ES6) or later, if you don't want to be notified about function expressions in an argument list, you can safely disable this rule. Source: http://eslint.org/docs/rules/
Missing trailing comma. Open
}
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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/
Unexpected var, use let or const instead. Open
var _createClass = function () { function defineProperties(target, props) { for (var i = 0; i < props.length; i++) { var descriptor = props[i]; descriptor.enumerable = descriptor.enumerable || false; descriptor.configurable = true; if ("value" in descriptor) descriptor.writable = true; Object.defineProperty(target, descriptor.key, descriptor); } } return function (Constructor, protoProps, staticProps) { if (protoProps) defineProperties(Constructor.prototype, protoProps); if (staticProps) defineProperties(Constructor, staticProps); return Constructor; }; }();
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require let
or const
instead of var
(no-var)
ECMAScript 6 allows programmers to create variables with block scope instead of function scope using the let
and const
keywords. Block scope is common in many other programming languages and helps programmers avoid mistakes
such as:
var count = people.length;
var enoughFood = count > sandwiches.length;
if (enoughFood) {
var count = sandwiches.length; // accidentally overriding the count variable
console.log("We have " + count + " sandwiches for everyone. Plenty for all!");
}
// our count variable is no longer accurate
console.log("We have " + count + " people and " + sandwiches.length + " sandwiches!");
Rule Details
This rule is aimed at discouraging the use of var
and encouraging the use of const
or let
instead.
Examples
Examples of incorrect code for this rule:
/*eslint no-var: "error"*/
var x = "y";
var CONFIG = {};
Examples of correct code for this rule:
/*eslint no-var: "error"*/
/*eslint-env es6*/
let x = "y";
const CONFIG = {};
When Not To Use It
In addition to non-ES6 environments, existing JavaScript projects that are beginning to introduce ES6 into their
codebase may not want to apply this rule if the cost of migrating from var
to let
is too costly.
Source: http://eslint.org/docs/rules/
Expected property shorthand. Open
Endpoint: Endpoint
- Read upRead up
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Require Object Literal Shorthand Syntax (object-shorthand)
EcmaScript 6 provides a concise form for defining object literal methods and properties. This syntax can make defining complex object literals much cleaner.
Here are a few common examples using the ES5 syntax:
// properties
var foo = {
x: x,
y: y,
z: z,
};
// methods
var foo = {
a: function() {},
b: function() {}
};
Now here are ES6 equivalents:
/*eslint-env es6*/
// properties
var foo = {x, y, z};
// methods
var foo = {
a() {},
b() {}
};
Rule Details
This rule enforces the use of the shorthand syntax. This applies to all methods (including generators) defined in object literals and any properties defined where the key name matches name of the assigned variable.
Each of the following properties would warn:
/*eslint object-shorthand: "error"*/
/*eslint-env es6*/
var foo = {
w: function() {},
x: function *() {},
[y]: function() {},
z: z
};
In that case the expected syntax would have been:
/*eslint object-shorthand: "error"*/
/*eslint-env es6*/
var foo = {
w() {},
*x() {},
[y]() {},
z
};
This rule does not flag arrow functions inside of object literals. The following will not warn:
/*eslint object-shorthand: "error"*/
/*eslint-env es6*/
var foo = {
x: (y) => y
};
Options
The rule takes an option which specifies when it should be applied. It can be set to one of the following values:
-
"always"
(default) expects that the shorthand will be used whenever possible. -
"methods"
ensures the method shorthand is used (also applies to generators). -
"properties"
ensures the property shorthand is used (where the key and variable name match). -
"never"
ensures that no property or method shorthand is used in any object literal. -
"consistent"
ensures that either all shorthand or all longform will be used in an object literal. -
"consistent-as-needed"
ensures that either all shorthand or all longform will be used in an object literal, but ensures all shorthand whenever possible.
You can set the option in configuration like this:
{
"object-shorthand": ["error", "always"]
}
Additionally, the rule takes an optional object configuration:
-
"avoidQuotes": true
indicates that longform syntax is preferred whenever the object key is a string literal (default:false
). Note that this option can only be enabled when the string option is set to"always"
,"methods"
, or"properties"
. -
"ignoreConstructors": true
can be used to prevent the rule from reporting errors for constructor functions. (By default, the rule treats constructors the same way as other functions.) Note that this option can only be enabled when the string option is set to"always"
or"methods"
. -
"avoidExplicitReturnArrows": true
indicates that methods are preferred over explicit-return arrow functions for function properties. (By default, the rule allows either of these.) Note that this option can only be enabled when the string option is set to"always"
or"methods"
.
avoidQuotes
{
"object-shorthand": ["error", "always", { "avoidQuotes": true }]
}
Example of incorrect code for this rule with the "always", { "avoidQuotes": true }
option:
/*eslint object-shorthand: ["error", "always", { "avoidQuotes": true }]*/
/*eslint-env es6*/
var foo = {
"bar-baz"() {}
};
Example of correct code for this rule with the "always", { "avoidQuotes": true }
option:
/*eslint object-shorthand: ["error", "always", { "avoidQuotes": true }]*/
/*eslint-env es6*/
var foo = {
"bar-baz": function() {},
"qux": qux
};
ignoreConstructors
{
"object-shorthand": ["error", "always", { "ignoreConstructors": true }]
}
Example of correct code for this rule with the "always", { "ignoreConstructors": true }
option:
/*eslint object-shorthand: ["error", "always", { "ignoreConstructors": true }]*/
/*eslint-env es6*/
var foo = {
ConstructorFunction: function() {}
};
avoidExplicitReturnArrows
{
"object-shorthand": ["error", "always", { "avoidExplicitReturnArrows": true }]
}
Example of incorrect code for this rule with the "always", { "avoidExplicitReturnArrows": true }
option:
/*eslint object-shorthand: ["error", "always", { "avoidExplicitReturnArrows": true }]*/
/*eslint-env es6*/
var foo = {
foo: (bar, baz) => {
return bar + baz;
},
qux: (foobar) => {
return foobar * 2;
}
};
Example of correct code for this rule with the "always", { "avoidExplicitReturnArrows": true }
option:
/*eslint object-shorthand: ["error", "always", { "avoidExplicitReturnArrows": true }]*/
/*eslint-env es6*/
var foo = {
foo(bar, baz) {
return bar + baz;
},
qux: foobar => foobar * 2
};
Example of incorrect code for this rule with the "consistent"
option:
/*eslint object-shorthand: [2, "consistent"]*/
/*eslint-env es6*/
var foo = {
a,
b: "foo",
};
Examples of correct code for this rule with the "consistent"
option:
/*eslint object-shorthand: [2, "consistent"]*/
/*eslint-env es6*/
var foo = {
a: a,
b: "foo"
};
var bar = {
a,
b,
};
Example of incorrect code with the "consistent-as-needed"
option, which is very similar to "consistent"
:
/*eslint object-shorthand: [2, "consistent-as-needed"]*/
/*eslint-env es6*/
var foo = {
a: a,
b: b,
};
When Not To Use It
Anyone not yet in an ES6 environment would not want to apply this rule. Others may find the terseness of the shorthand syntax harder to read and may not want to encourage it with this rule.
Further Reading
Object initializer - MDN Source: http://eslint.org/docs/rules/
Unexpected function expression. Open
_this2.sns.subscribe(subscribeParams, function (err, data) {
- Read upRead up
- Exclude checks
Suggest using arrow functions as callbacks. (prefer-arrow-callback)
Arrow functions are suited to callbacks, because:
-
this
keywords in arrow functions bind to the upper scope's. - The notation of the arrow function is shorter than function expression's.
Rule Details
This rule is aimed to flag usage of function expressions in an argument list.
The following patterns are considered problems:
/*eslint prefer-arrow-callback: "error"*/
foo(function(a) { return a; });
foo(function() { return this.a; }.bind(this));
The following patterns are not considered problems:
/*eslint prefer-arrow-callback: "error"*/
/*eslint-env es6*/
foo(a => a);
foo(function*() { yield; });
// this is not a callback.
var foo = function foo(a) { return a; };
// using `this` without `.bind(this)`.
foo(function() { return this.a; });
// recursively.
foo(function bar(n) { return n && n + bar(n - 1); });
Options
This rule takes one optional argument, an object which is an options object.
allowNamedFunctions
This is a boolean
option and it is false
by default. When set to true
, the rule doesn't warn on named functions used as callbacks.
Examples of correct code for the { "allowNamedFunctions": true }
option:
/*eslint prefer-arrow-callback: ["error", { "allowNamedFunctions": true }]*/
foo(function bar() {});
allowUnboundThis
This is a boolean
option and it is true
by default. When set to false
, this option allows the use of this
without restriction and checks for dynamically assigned this
values such as when using Array.prototype.map
with a context
argument. Normally, the rule will flag the use of this
whenever a function does not use bind()
to specify the value of this
constantly.
Examples of incorrect code for the { "allowUnboundThis": false }
option:
/*eslint prefer-arrow-callback: ["error", { "allowUnboundThis": false }]*/
/*eslint-env es6*/
foo(function() { this.a; });
foo(function() { (() => this); });
someArray.map(function (itm) { return this.doSomething(itm); }, someObject);
When Not To Use It
This rule should not be used in ES3/5 environments.
In ES2015 (ES6) or later, if you don't want to be notified about function expressions in an argument list, you can safely disable this rule. Source: http://eslint.org/docs/rules/
Unexpected dangling '_' in '_this3'. Open
var _this3 = this;
- Read upRead up
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disallow dangling underscores in identifiers (no-underscore-dangle)
As far as naming conventions for identifiers go, dangling underscores may be the most polarizing in JavaScript. Dangling underscores are underscores at either the beginning or end of an identifier, such as:
var _foo;
There is actually a long history of using dangling underscores to indicate "private" members of objects in JavaScript (though JavaScript doesn't have truly private members, this convention served as a warning). This began with SpiderMonkey adding nonstandard methods such as __defineGetter__()
. The intent with the underscores was to make it obvious that this method was special in some way. Since that time, using a single underscore prefix has become popular as a way to indicate "private" members of objects.
Whether or not you choose to allow dangling underscores in identifiers is purely a convention and has no effect on performance, readability, or complexity. It's purely a preference.
Rule Details
This rule disallows dangling underscores in identifiers.
Examples of incorrect code for this rule:
/*eslint no-underscore-dangle: "error"*/
var foo_;
var __proto__ = {};
foo._bar();
Examples of correct code for this rule:
/*eslint no-underscore-dangle: "error"*/
var _ = require('underscore');
var obj = _.contains(items, item);
obj.__proto__ = {};
var file = __filename;
Options
This rule has an object option:
-
"allow"
allows specified identifiers to have dangling underscores -
"allowAfterThis": false
(default) disallows dangling underscores in members of thethis
object -
"allowAfterSuper": false
(default) disallows dangling underscores in members of thesuper
object
allow
Examples of additional correct code for this rule with the { "allow": ["foo_", "_bar"] }
option:
/*eslint no-underscore-dangle: ["error", { "allow": ["foo_", "_bar"] }]*/
var foo_;
foo._bar();
allowAfterThis
Examples of correct code for this rule with the { "allowAfterThis": true }
option:
/*eslint no-underscore-dangle: ["error", { "allowAfterThis": true }]*/
var a = this.foo_;
this._bar();
allowAfterSuper
Examples of correct code for this rule with the { "allowAfterSuper": true }
option:
/*eslint no-underscore-dangle: ["error", { "allowAfterSuper": true }]*/
var a = super.foo_;
super._bar();
When Not To Use It
If you want to allow dangling underscores in identifiers, then you can safely turn this rule off. Source: http://eslint.org/docs/rules/
Expected property shorthand. Open
PhoneNumber: PhoneNumber,
- Read upRead up
- Exclude checks
Require Object Literal Shorthand Syntax (object-shorthand)
EcmaScript 6 provides a concise form for defining object literal methods and properties. This syntax can make defining complex object literals much cleaner.
Here are a few common examples using the ES5 syntax:
// properties
var foo = {
x: x,
y: y,
z: z,
};
// methods
var foo = {
a: function() {},
b: function() {}
};
Now here are ES6 equivalents:
/*eslint-env es6*/
// properties
var foo = {x, y, z};
// methods
var foo = {
a() {},
b() {}
};
Rule Details
This rule enforces the use of the shorthand syntax. This applies to all methods (including generators) defined in object literals and any properties defined where the key name matches name of the assigned variable.
Each of the following properties would warn:
/*eslint object-shorthand: "error"*/
/*eslint-env es6*/
var foo = {
w: function() {},
x: function *() {},
[y]: function() {},
z: z
};
In that case the expected syntax would have been:
/*eslint object-shorthand: "error"*/
/*eslint-env es6*/
var foo = {
w() {},
*x() {},
[y]() {},
z
};
This rule does not flag arrow functions inside of object literals. The following will not warn:
/*eslint object-shorthand: "error"*/
/*eslint-env es6*/
var foo = {
x: (y) => y
};
Options
The rule takes an option which specifies when it should be applied. It can be set to one of the following values:
-
"always"
(default) expects that the shorthand will be used whenever possible. -
"methods"
ensures the method shorthand is used (also applies to generators). -
"properties"
ensures the property shorthand is used (where the key and variable name match). -
"never"
ensures that no property or method shorthand is used in any object literal. -
"consistent"
ensures that either all shorthand or all longform will be used in an object literal. -
"consistent-as-needed"
ensures that either all shorthand or all longform will be used in an object literal, but ensures all shorthand whenever possible.
You can set the option in configuration like this:
{
"object-shorthand": ["error", "always"]
}
Additionally, the rule takes an optional object configuration:
-
"avoidQuotes": true
indicates that longform syntax is preferred whenever the object key is a string literal (default:false
). Note that this option can only be enabled when the string option is set to"always"
,"methods"
, or"properties"
. -
"ignoreConstructors": true
can be used to prevent the rule from reporting errors for constructor functions. (By default, the rule treats constructors the same way as other functions.) Note that this option can only be enabled when the string option is set to"always"
or"methods"
. -
"avoidExplicitReturnArrows": true
indicates that methods are preferred over explicit-return arrow functions for function properties. (By default, the rule allows either of these.) Note that this option can only be enabled when the string option is set to"always"
or"methods"
.
avoidQuotes
{
"object-shorthand": ["error", "always", { "avoidQuotes": true }]
}
Example of incorrect code for this rule with the "always", { "avoidQuotes": true }
option:
/*eslint object-shorthand: ["error", "always", { "avoidQuotes": true }]*/
/*eslint-env es6*/
var foo = {
"bar-baz"() {}
};
Example of correct code for this rule with the "always", { "avoidQuotes": true }
option:
/*eslint object-shorthand: ["error", "always", { "avoidQuotes": true }]*/
/*eslint-env es6*/
var foo = {
"bar-baz": function() {},
"qux": qux
};
ignoreConstructors
{
"object-shorthand": ["error", "always", { "ignoreConstructors": true }]
}
Example of correct code for this rule with the "always", { "ignoreConstructors": true }
option:
/*eslint object-shorthand: ["error", "always", { "ignoreConstructors": true }]*/
/*eslint-env es6*/
var foo = {
ConstructorFunction: function() {}
};
avoidExplicitReturnArrows
{
"object-shorthand": ["error", "always", { "avoidExplicitReturnArrows": true }]
}
Example of incorrect code for this rule with the "always", { "avoidExplicitReturnArrows": true }
option:
/*eslint object-shorthand: ["error", "always", { "avoidExplicitReturnArrows": true }]*/
/*eslint-env es6*/
var foo = {
foo: (bar, baz) => {
return bar + baz;
},
qux: (foobar) => {
return foobar * 2;
}
};
Example of correct code for this rule with the "always", { "avoidExplicitReturnArrows": true }
option:
/*eslint object-shorthand: ["error", "always", { "avoidExplicitReturnArrows": true }]*/
/*eslint-env es6*/
var foo = {
foo(bar, baz) {
return bar + baz;
},
qux: foobar => foobar * 2
};
Example of incorrect code for this rule with the "consistent"
option:
/*eslint object-shorthand: [2, "consistent"]*/
/*eslint-env es6*/
var foo = {
a,
b: "foo",
};
Examples of correct code for this rule with the "consistent"
option:
/*eslint object-shorthand: [2, "consistent"]*/
/*eslint-env es6*/
var foo = {
a: a,
b: "foo"
};
var bar = {
a,
b,
};
Example of incorrect code with the "consistent-as-needed"
option, which is very similar to "consistent"
:
/*eslint object-shorthand: [2, "consistent-as-needed"]*/
/*eslint-env es6*/
var foo = {
a: a,
b: b,
};
When Not To Use It
Anyone not yet in an ES6 environment would not want to apply this rule. Others may find the terseness of the shorthand syntax harder to read and may not want to encourage it with this rule.
Further Reading
Object initializer - MDN Source: http://eslint.org/docs/rules/
Unexpected var, use let or const instead. Open
var _this2 = this;
- Read upRead up
- Exclude checks
require let
or const
instead of var
(no-var)
ECMAScript 6 allows programmers to create variables with block scope instead of function scope using the let
and const
keywords. Block scope is common in many other programming languages and helps programmers avoid mistakes
such as:
var count = people.length;
var enoughFood = count > sandwiches.length;
if (enoughFood) {
var count = sandwiches.length; // accidentally overriding the count variable
console.log("We have " + count + " sandwiches for everyone. Plenty for all!");
}
// our count variable is no longer accurate
console.log("We have " + count + " people and " + sandwiches.length + " sandwiches!");
Rule Details
This rule is aimed at discouraging the use of var
and encouraging the use of const
or let
instead.
Examples
Examples of incorrect code for this rule:
/*eslint no-var: "error"*/
var x = "y";
var CONFIG = {};
Examples of correct code for this rule:
/*eslint no-var: "error"*/
/*eslint-env es6*/
let x = "y";
const CONFIG = {};
When Not To Use It
In addition to non-ES6 environments, existing JavaScript projects that are beginning to introduce ES6 into their
codebase may not want to apply this rule if the cost of migrating from var
to let
is too costly.
Source: http://eslint.org/docs/rules/
Unexpected unnamed function. Open
return new Promise(function (resolve, reject) {
- Read upRead up
- Exclude checks
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
Compatibility
- JSCS: requireAnonymousFunctions
- JSCS: disallowAnonymousFunctions Source: http://eslint.org/docs/rules/
Unexpected var, use let or const instead. Open
var smsParams = void 0;
- Read upRead up
- Exclude checks
require let
or const
instead of var
(no-var)
ECMAScript 6 allows programmers to create variables with block scope instead of function scope using the let
and const
keywords. Block scope is common in many other programming languages and helps programmers avoid mistakes
such as:
var count = people.length;
var enoughFood = count > sandwiches.length;
if (enoughFood) {
var count = sandwiches.length; // accidentally overriding the count variable
console.log("We have " + count + " sandwiches for everyone. Plenty for all!");
}
// our count variable is no longer accurate
console.log("We have " + count + " people and " + sandwiches.length + " sandwiches!");
Rule Details
This rule is aimed at discouraging the use of var
and encouraging the use of const
or let
instead.
Examples
Examples of incorrect code for this rule:
/*eslint no-var: "error"*/
var x = "y";
var CONFIG = {};
Examples of correct code for this rule:
/*eslint no-var: "error"*/
/*eslint-env es6*/
let x = "y";
const CONFIG = {};
When Not To Use It
In addition to non-ES6 environments, existing JavaScript projects that are beginning to introduce ES6 into their
codebase may not want to apply this rule if the cost of migrating from var
to let
is too costly.
Source: http://eslint.org/docs/rules/
Missing whitespace after semicolon. Open
if (err) reject(err, err.stack);else resolve(data);
- Read upRead up
- Exclude checks
Enforce spacing before and after semicolons (semi-spacing)
JavaScript allows you to place unnecessary spaces before or after a semicolon.
Disallowing or enforcing space around a semicolon can improve the readability of your program.
var a = "b" ;
var c = "d";var e = "f";
Rule Details
This rule aims to enforce spacing around a semicolon. This rule prevents the use of spaces before a semicolon in expressions.
This rule doesn't check spacing in the following cases:
The spacing after the semicolon if it is the first token in the line.
The spacing before the semicolon if it is after an opening parenthesis (
(
or{
), or the spacing after the semicolon if it is before a closing parenthesis ()
or}
). That spacing is checked byspace-in-parens
orblock-spacing
.The spacing around the semicolon in a for loop with an empty condition (
for(;;)
).
Options
The rule takes one option, an object, which has two keys before
and after
having boolean values true
or false
.
If before
is true
, space is enforced before semicolons and if it's false
, space is disallowed before semicolons.
If after
is true
, space is enforced after semicolons and if it's false
, space is disallowed after semicolons.
The after
option will be only applied if a semicolon is not at the end of line.
The default is {"before": false, "after": true}
.
"semi-spacing": ["error", {"before": false, "after": true}]
{"before": false, "after": true}
This is the default option. It enforces spacing after semicolons and disallows spacing before semicolons.
Examples of incorrect code for this rule:
/*eslint semi-spacing: "error"*/
var foo ;
var foo;var bar;
throw new Error("error") ;
while (a) { break ; }
for (i = 0 ; i < 10 ; i++) {}
for (i = 0;i < 10;i++) {}
Examples of correct code for this rule:
/*eslint semi-spacing: "error"*/
var foo;
var foo; var bar;
throw new Error("error");
while (a) { break; }
for (i = 0; i < 10; i++) {}
for (;;) {}
if (true) {;}
;foo();
{"before": true, "after": false}
This option enforces spacing before semicolons and disallows spacing after semicolons.
Examples of incorrect code for this rule with the {"before": true, "after": false}
option:
/*eslint semi-spacing: ["error", { "before": true, "after": false }]*/
var foo;
var foo ; var bar;
throw new Error("error");
while (a) { break; }
for (i = 0;i < 10;i++) {}
for (i = 0; i < 10; i++) {}
Examples of correct code for this rule with the {"before": true, "after": false}
option:
/*eslint semi-spacing: ["error", { "before": true, "after": false }]*/
var foo ;
var foo ;var bar ;
throw new Error("error") ;
while (a) {break ;}
for (i = 0 ;i < 10 ;i++) {}
When Not To Use It
You can turn this rule off if you are not concerned with the consistency of spacing before or after semicolons.
Related Rules
- [semi](semi.md)
- [no-extra-semi](no-extra-semi.md)
- [comma-spacing](comma-spacing.md)
- [block-spacing](block-spacing.md)
- [space-in-parens](space-in-parens.md) Source: http://eslint.org/docs/rules/
Unexpected unnamed function. Open
_this3.sns.publish(smsParams, function (err, data) {
- Read upRead up
- Exclude checks
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
Compatibility
- JSCS: requireAnonymousFunctions
- JSCS: disallowAnonymousFunctions Source: http://eslint.org/docs/rules/
Unexpected unnamed function. Open
var _createClass = function () { function defineProperties(target, props) { for (var i = 0; i < props.length; i++) { var descriptor = props[i]; descriptor.enumerable = descriptor.enumerable || false; descriptor.configurable = true; if ("value" in descriptor) descriptor.writable = true; Object.defineProperty(target, descriptor.key, descriptor); } } return function (Constructor, protoProps, staticProps) { if (protoProps) defineProperties(Constructor.prototype, protoProps); if (staticProps) defineProperties(Constructor, staticProps); return Constructor; }; }();
- Read upRead up
- Exclude checks
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
Compatibility
- JSCS: requireAnonymousFunctions
- JSCS: disallowAnonymousFunctions Source: http://eslint.org/docs/rules/
Strings must use singlequote. Open
function _classCallCheck(instance, Constructor) { if (!(instance instanceof Constructor)) { throw new TypeError("Cannot call a class as a function"); } }
- Read upRead up
- Exclude checks
enforce the consistent use of either backticks, double, or single quotes (quotes)
JavaScript allows you to define strings in one of three ways: double quotes, single quotes, and backticks (as of ECMAScript 6). For example:
/*eslint-env es6*/
var double = "double";
var single = 'single';
var backtick = `backtick`; // ES6 only
Each of these lines creates a string and, in some cases, can be used interchangeably. The choice of how to define strings in a codebase is a stylistic one outside of template literals (which allow embedded of expressions to be interpreted).
Many codebases require strings to be defined in a consistent manner.
Rule Details
This rule enforces the consistent use of either backticks, double, or single quotes.
Options
This rule has two options, a string option and an object option.
String option:
-
"double"
(default) requires the use of double quotes wherever possible -
"single"
requires the use of single quotes wherever possible -
"backtick"
requires the use of backticks wherever possible
Object option:
-
"avoidEscape": true
allows strings to use single-quotes or double-quotes so long as the string contains a quote that would have to be escaped otherwise -
"allowTemplateLiterals": true
allows strings to use backticks
Deprecated: The object property avoid-escape
is deprecated; please use the object property avoidEscape
instead.
double
Examples of incorrect code for this rule with the default "double"
option:
/*eslint quotes: ["error", "double"]*/
var single = 'single';
var unescaped = 'a string containing "double" quotes';
Examples of correct code for this rule with the default "double"
option:
/*eslint quotes: ["error", "double"]*/
/*eslint-env es6*/
var double = "double";
var backtick = `back\ntick`; // backticks are allowed due to newline
var backtick = tag`backtick`; // backticks are allowed due to tag
single
Examples of incorrect code for this rule with the "single"
option:
/*eslint quotes: ["error", "single"]*/
var double = "double";
var unescaped = "a string containing 'single' quotes";
Examples of correct code for this rule with the "single"
option:
/*eslint quotes: ["error", "single"]*/
/*eslint-env es6*/
var single = 'single';
var backtick = `back${x}tick`; // backticks are allowed due to substitution
backticks
Examples of incorrect code for this rule with the "backtick"
option:
/*eslint quotes: ["error", "backtick"]*/
var single = 'single';
var double = "double";
var unescaped = 'a string containing `backticks`';
Examples of correct code for this rule with the "backtick"
option:
/*eslint quotes: ["error", "backtick"]*/
/*eslint-env es6*/
var backtick = `backtick`;
avoidEscape
Examples of additional correct code for this rule with the "double", { "avoidEscape": true }
options:
/*eslint quotes: ["error", "double", { "avoidEscape": true }]*/
var single = 'a string containing "double" quotes';
Examples of additional correct code for this rule with the "single", { "avoidEscape": true }
options:
/*eslint quotes: ["error", "single", { "avoidEscape": true }]*/
var double = "a string containing 'single' quotes";
Examples of additional correct code for this rule with the "backtick", { "avoidEscape": true }
options:
/*eslint quotes: ["error", "backtick", { "avoidEscape": true }]*/
var double = "a string containing `backtick` quotes"
allowTemplateLiterals
Examples of additional correct code for this rule with the "double", { "allowTemplateLiterals": true }
options:
/*eslint quotes: ["error", "double", { "allowTemplateLiterals": true }]*/
var double = "double";
var double = `double`;
Examples of additional correct code for this rule with the "single", { "allowTemplateLiterals": true }
options:
/*eslint quotes: ["error", "single", { "allowTemplateLiterals": true }]*/
var single = 'single';
var single = `single`;
When Not To Use It
If you do not need consistency in your string styles, you can safely disable this rule. Source: http://eslint.org/docs/rules/
Missing trailing comma. Open
TopicArn: _this.topicArn
- Read upRead up
- Exclude checks
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/
Unexpected dangling '_' in '_this2'. Open
var _this2 = this;
- Read upRead up
- Exclude checks
disallow dangling underscores in identifiers (no-underscore-dangle)
As far as naming conventions for identifiers go, dangling underscores may be the most polarizing in JavaScript. Dangling underscores are underscores at either the beginning or end of an identifier, such as:
var _foo;
There is actually a long history of using dangling underscores to indicate "private" members of objects in JavaScript (though JavaScript doesn't have truly private members, this convention served as a warning). This began with SpiderMonkey adding nonstandard methods such as __defineGetter__()
. The intent with the underscores was to make it obvious that this method was special in some way. Since that time, using a single underscore prefix has become popular as a way to indicate "private" members of objects.
Whether or not you choose to allow dangling underscores in identifiers is purely a convention and has no effect on performance, readability, or complexity. It's purely a preference.
Rule Details
This rule disallows dangling underscores in identifiers.
Examples of incorrect code for this rule:
/*eslint no-underscore-dangle: "error"*/
var foo_;
var __proto__ = {};
foo._bar();
Examples of correct code for this rule:
/*eslint no-underscore-dangle: "error"*/
var _ = require('underscore');
var obj = _.contains(items, item);
obj.__proto__ = {};
var file = __filename;
Options
This rule has an object option:
-
"allow"
allows specified identifiers to have dangling underscores -
"allowAfterThis": false
(default) disallows dangling underscores in members of thethis
object -
"allowAfterSuper": false
(default) disallows dangling underscores in members of thesuper
object
allow
Examples of additional correct code for this rule with the { "allow": ["foo_", "_bar"] }
option:
/*eslint no-underscore-dangle: ["error", { "allow": ["foo_", "_bar"] }]*/
var foo_;
foo._bar();
allowAfterThis
Examples of correct code for this rule with the { "allowAfterThis": true }
option:
/*eslint no-underscore-dangle: ["error", { "allowAfterThis": true }]*/
var a = this.foo_;
this._bar();
allowAfterSuper
Examples of correct code for this rule with the { "allowAfterSuper": true }
option:
/*eslint no-underscore-dangle: ["error", { "allowAfterSuper": true }]*/
var a = super.foo_;
super._bar();
When Not To Use It
If you want to allow dangling underscores in identifiers, then you can safely turn this rule off. Source: http://eslint.org/docs/rules/
Expected 'undefined' and instead saw 'void'. Open
var smsParams = void 0;
- Read upRead up
- Exclude checks
Disallow use of the void operator. (no-void)
The void
operator takes an operand and returns undefined
: void expression
will evaluate expression
and return undefined
. It can be used to ignore any side effects expression
may produce:
The common case of using void
operator is to get a "pure" undefined
value as prior to ES5 the undefined
variable was mutable:
// will always return undefined
(function(){
return void 0;
})();
// will return 1 in ES3 and undefined in ES5+
(function(){
undefined = 1;
return undefined;
})();
// will throw TypeError in ES5+
(function(){
'use strict';
undefined = 1;
})();
Another common case is to minify code as void 0
is shorter than undefined
:
foo = void 0;
foo = undefined;
When used with IIFE (immediately-invoked function expression), void
can be used to force the function keyword to be treated as an expression instead of a declaration:
var foo = 1;
void function(){ foo = 1; }() // will assign foo a value of 1
+function(){ foo = 1; }() // same as above
function(){ foo = 1; }() // will throw SyntaxError
Some code styles prohibit void
operator, marking it as non-obvious and hard to read.
Rule Details
This rule aims to eliminate use of void operator.
Examples of incorrect code for this rule:
/*eslint no-void: "error"*/
void foo
var foo = void bar();
When Not To Use It
If you intentionally use the void
operator then you can disable this rule.
Further Reading
Related Rules
- [no-undef-init](no-undef-init.md)
- [no-undefined](no-undefined.md) Source: http://eslint.org/docs/rules/
Missing trailing comma. Open
Subject: Subject
- Read upRead up
- Exclude checks
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/