const result = self[event.method].apply(self, event.args);

Close

Suggest using the spread operator instead of .apply(). (prefer-spread)

Before ES2015, one must use Function.prototype.apply() to call variadic functions.

var args = [1, 2, 3, 4];
Math.max.apply(Math, args);

In ES2015, one can use the spread operator to call variadic functions.

/*eslint-env es6*/

var args = [1, 2, 3, 4];
Math.max(...args);

Rule Details

This rule is aimed to flag usage of Function.prototype.apply() in situations where the spread operator could be used instead.

Examples

Examples of incorrect code for this rule:

/*eslint prefer-spread: "error"*/

foo.apply(undefined, args);
foo.apply(null, args);
obj.foo.apply(obj, args);

Examples of correct code for this rule:

/*eslint prefer-spread: "error"*/

// Using the spread operator
foo(...args);
obj.foo(...args);

// The `this` binding is different.
foo.apply(obj, args);
obj.foo.apply(null, args);
obj.foo.apply(otherObj, args);

// The argument list is not variadic.
// Those are warned by the `no-useless-call` rule.
foo.apply(undefined, [1, 2, 3]);
foo.apply(null, [1, 2, 3]);
obj.foo.apply(obj, [1, 2, 3]);

Known limitations:

This rule analyzes code statically to check whether or not the this argument is changed. So, if the this argument is computed in a dynamic expression, this rule cannot detect a violation.

/*eslint prefer-spread: "error"*/

// This warns.
a[i++].foo.apply(a[i++], args);

// This does not warn.
a[++i].foo.apply(a[i], args);

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.prototype.apply() callings, you can safely disable this rule.

Related Rules

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

  saveObject(obj) { // eslint-disable-line no-unused-vars

Close

Enforce that class methods utilize this (class-methods-use-this)

If a class method does not use this, it can safely be made a static function.

It's possible to have a class method which doesn't use this, such as:

class A {
    constructor() {
        this.a = "hi";
    }

    print() {
        console.log(this.a);
    }

    sayHi() {
        console.log("hi");
    }
}

let a = new A();
a.sayHi(); // => "hi"

In the example above, the sayHi method doesn't use this, so we can make it a static method:

class A {
    constructor() {
        this.a = "hi";
    }

    print() {
        console.log(this.a);
    }

    static sayHi() {
        console.log("hi");
    }
}

A.sayHi(); // => "hi"

Also note in the above examples that the code calling the function on an instance of the class (let a = new A(); a.sayHi();) changes to calling it on the class itself (A.sayHi();).

Rule Details

This rule is aimed to flag class methods that do not use this.

Examples of incorrect code for this rule:

/*eslint class-methods-use-this: "error"*/
/*eslint-env es6*/

class A {
    foo() {
        console.log("Hello World");     /*error Expected 'this' to be used by class method 'foo'.*/
    }
}

Examples of correct code for this rule:

/*eslint class-methods-use-this: "error"*/
/*eslint-env es6*/
class A {
    foo() {
        this.bar = "Hello World"; // OK, this is used
    }
}

class A {
    constructor() {
        // OK. constructor is exempt
    }
}

class A {
    static foo() {
        // OK. static methods aren't expected to use this.
    }
}

Options

Exceptions

"class-methods-use-this": [<enabled>, { "exceptMethods": [&lt;...exceptions&gt;] }]</enabled>

The exceptMethods option allows you to pass an array of method names for which you would like to ignore warnings.

Examples of incorrect code for this rule when used without exceptMethods:

/*eslint class-methods-use-this: "error"*/

class A {
    foo() {
    }
}

Examples of correct code for this rule when used with exceptMethods:

/*eslint class-methods-use-this: ["error", { "exceptMethods": ["foo"] }] */

class A {
    foo() {
    }
}

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

  connect() {

Close

Enforce that class methods utilize this (class-methods-use-this)

If a class method does not use this, it can safely be made a static function.

It's possible to have a class method which doesn't use this, such as:

class A {
    constructor() {
        this.a = "hi";
    }

    print() {
        console.log(this.a);
    }

    sayHi() {
        console.log("hi");
    }
}

let a = new A();
a.sayHi(); // => "hi"

In the example above, the sayHi method doesn't use this, so we can make it a static method:

class A {
    constructor() {
        this.a = "hi";
    }

    print() {
        console.log(this.a);
    }

    static sayHi() {
        console.log("hi");
    }
}

A.sayHi(); // => "hi"

Also note in the above examples that the code calling the function on an instance of the class (let a = new A(); a.sayHi();) changes to calling it on the class itself (A.sayHi();).

Rule Details

This rule is aimed to flag class methods that do not use this.

Examples of incorrect code for this rule:

/*eslint class-methods-use-this: "error"*/
/*eslint-env es6*/

class A {
    foo() {
        console.log("Hello World");     /*error Expected 'this' to be used by class method 'foo'.*/
    }
}

Examples of correct code for this rule:

/*eslint class-methods-use-this: "error"*/
/*eslint-env es6*/
class A {
    foo() {
        this.bar = "Hello World"; // OK, this is used
    }
}

class A {
    constructor() {
        // OK. constructor is exempt
    }
}

class A {
    static foo() {
        // OK. static methods aren't expected to use this.
    }
}

Options

Exceptions

"class-methods-use-this": [<enabled>, { "exceptMethods": [&lt;...exceptions&gt;] }]</enabled>

The exceptMethods option allows you to pass an array of method names for which you would like to ignore warnings.

Examples of incorrect code for this rule when used without exceptMethods:

/*eslint class-methods-use-this: "error"*/

class A {
    foo() {
    }
}

Examples of correct code for this rule when used with exceptMethods:

/*eslint class-methods-use-this: ["error", { "exceptMethods": ["foo"] }] */

class A {
    foo() {
    }
}

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

      func.apply(null, args);

Close

Suggest using the spread operator instead of .apply(). (prefer-spread)

Before ES2015, one must use Function.prototype.apply() to call variadic functions.

var args = [1, 2, 3, 4];
Math.max.apply(Math, args);

In ES2015, one can use the spread operator to call variadic functions.

/*eslint-env es6*/

var args = [1, 2, 3, 4];
Math.max(...args);

Rule Details

This rule is aimed to flag usage of Function.prototype.apply() in situations where the spread operator could be used instead.

Examples

Examples of incorrect code for this rule:

/*eslint prefer-spread: "error"*/

foo.apply(undefined, args);
foo.apply(null, args);
obj.foo.apply(obj, args);

Examples of correct code for this rule:

/*eslint prefer-spread: "error"*/

// Using the spread operator
foo(...args);
obj.foo(...args);

// The `this` binding is different.
foo.apply(obj, args);
obj.foo.apply(null, args);
obj.foo.apply(otherObj, args);

// The argument list is not variadic.
// Those are warned by the `no-useless-call` rule.
foo.apply(undefined, [1, 2, 3]);
foo.apply(null, [1, 2, 3]);
obj.foo.apply(obj, [1, 2, 3]);

Known limitations:

This rule analyzes code statically to check whether or not the this argument is changed. So, if the this argument is computed in a dynamic expression, this rule cannot detect a violation.

/*eslint prefer-spread: "error"*/

// This warns.
a[i++].foo.apply(a[i++], args);

// This does not warn.
a[++i].foo.apply(a[i], args);

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.prototype.apply() callings, you can safely disable this rule.

Related Rules

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

      this.getObjectCollectionName(obj)

Close

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/

    return new Promise(resolve => {

Close

Require parens in arrow function arguments (arrow-parens)

Arrow functions can omit parentheses when they have exactly one parameter. In all other cases the parameter(s) must be wrapped in parentheses. This rule enforces the consistent use of parentheses in arrow functions.

Rule Details

This rule enforces parentheses around arrow function parameters regardless of arity. For example:

/*eslint-env es6*/

// Bad
a => {}

// Good
(a) => {}

Following this style will help you find arrow functions (=>) which may be mistakenly included in a condition when a comparison such as >= was the intent.

/*eslint-env es6*/

// Bad
if (a => 2) {
}

// Good
if (a >= 2) {
}

The rule can also be configured to discourage the use of parens when they are not required:

/*eslint-env es6*/

// Bad
(a) => {}

// Good
a => {}

Options

This rule has a string option and an object one.

String options are:

• "always" (default) requires parens around arguments in all cases.
• "as-needed" allows omitting parens when there is only one argument.

Object properties for variants of the "as-needed" option:

• "requireForBlockBody": true modifies the as-needed rule in order to require parens if the function body is in an instructions block (surrounded by braces).

always

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

/*eslint arrow-parens: ["error", "always"]*/
/*eslint-env es6*/

a => {};
a => a;
a => {'\n'};
a.then(foo => {});
a.then(foo => a);
a(foo => { if (true) {} });

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

/*eslint arrow-parens: ["error", "always"]*/
/*eslint-env es6*/

() => {};
(a) => {};
(a) => a;
(a) => {'\n'}
a.then((foo) => {});
a.then((foo) => { if (true) {} });

If Statements

One of benefits of this option is that it prevents the incorrect use of arrow functions in conditionals:

/*eslint-env es6*/

var a = 1;
var b = 2;
// ...
if (a => b) {
 console.log('bigger');
} else {
 console.log('smaller');
}
// outputs 'bigger', not smaller as expected

The contents of the if statement is an arrow function, not a comparison.

If the arrow function is intentional, it should be wrapped in parens to remove ambiguity.

/*eslint-env es6*/

var a = 1;
var b = 0;
// ...
if ((a) => b) {
 console.log('truthy value returned');
} else {
 console.log('falsey value returned');
}
// outputs 'truthy value returned'

The following is another example of this behavior:

/*eslint-env es6*/

var a = 1, b = 2, c = 3, d = 4;
var f = a => b ? c: d;
// f = ?

f is an arrow function which takes a as an argument and returns the result of b ? c: d.

This should be rewritten like so:

/*eslint-env es6*/

var a = 1, b = 2, c = 3, d = 4;
var f = (a) => b ? c: d;

as-needed

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

/*eslint arrow-parens: ["error", "as-needed"]*/
/*eslint-env es6*/

(a) => {};
(a) => a;
(a) => {'\n'};
a.then((foo) => {});
a.then((foo) => a);
a((foo) => { if (true) {} });

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

/*eslint arrow-parens: ["error", "as-needed"]*/
/*eslint-env es6*/

() => {};
a => {};
a => a;
a => {'\n'};
a.then(foo => {});
a.then(foo => { if (true) {} });
(a, b, c) => a;
(a = 10) => a;
([a, b]) => a;
({a, b}) => a;

requireForBlockBody

Examples of incorrect code for the { "requireForBlockBody": true } option:

/*eslint arrow-parens: [2, "as-needed", { "requireForBlockBody": true }]*/
/*eslint-env es6*/

(a) => a;
a => {};
a => {'\n'};
a.map((x) => x * x);
a.map(x => {
  return x * x;
});
a.then(foo => {});

Examples of correct code for the { "requireForBlockBody": true } option:

/*eslint arrow-parens: [2, "as-needed", { "requireForBlockBody": true }]*/
/*eslint-env es6*/

(a) => {};
(a) => {'\n'};
a => ({});
() => {};
a => a;
a.then((foo) => {});
a.then((foo) => { if (true) {} });
a((foo) => { if (true) {} });
(a, b, c) => a;
(a = 10) => a;
([a, b]) => a;
({a, b}) => a;

Further Reading

• The "as-needed", { "requireForBlockBody": true } rule is directly inspired by the Airbnb JS Style Guide. Source: http://eslint.org/docs/rules/

  deleteObject() {

Close

Enforce that class methods utilize this (class-methods-use-this)

If a class method does not use this, it can safely be made a static function.

It's possible to have a class method which doesn't use this, such as:

class A {
    constructor() {
        this.a = "hi";
    }

    print() {
        console.log(this.a);
    }

    sayHi() {
        console.log("hi");
    }
}

let a = new A();
a.sayHi(); // => "hi"

In the example above, the sayHi method doesn't use this, so we can make it a static method:

class A {
    constructor() {
        this.a = "hi";
    }

    print() {
        console.log(this.a);
    }

    static sayHi() {
        console.log("hi");
    }
}

A.sayHi(); // => "hi"

Also note in the above examples that the code calling the function on an instance of the class (let a = new A(); a.sayHi();) changes to calling it on the class itself (A.sayHi();).

Rule Details

This rule is aimed to flag class methods that do not use this.

Examples of incorrect code for this rule:

/*eslint class-methods-use-this: "error"*/
/*eslint-env es6*/

class A {
    foo() {
        console.log("Hello World");     /*error Expected 'this' to be used by class method 'foo'.*/
    }
}

Examples of correct code for this rule:

/*eslint class-methods-use-this: "error"*/
/*eslint-env es6*/
class A {
    foo() {
        this.bar = "Hello World"; // OK, this is used
    }
}

class A {
    constructor() {
        // OK. constructor is exempt
    }
}

class A {
    static foo() {
        // OK. static methods aren't expected to use this.
    }
}

Options

Exceptions

"class-methods-use-this": [<enabled>, { "exceptMethods": [&lt;...exceptions&gt;] }]</enabled>

The exceptMethods option allows you to pass an array of method names for which you would like to ignore warnings.

Examples of incorrect code for this rule when used without exceptMethods:

/*eslint class-methods-use-this: "error"*/

class A {
    foo() {
    }
}

Examples of correct code for this rule when used with exceptMethods:

/*eslint class-methods-use-this: ["error", { "exceptMethods": ["foo"] }] */

class A {
    foo() {
    }
}

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

      func.apply(null, args);

Close

Suggest using the spread operator instead of .apply(). (prefer-spread)

Before ES2015, one must use Function.prototype.apply() to call variadic functions.

var args = [1, 2, 3, 4];
Math.max.apply(Math, args);

In ES2015, one can use the spread operator to call variadic functions.

/*eslint-env es6*/

var args = [1, 2, 3, 4];
Math.max(...args);

Rule Details

This rule is aimed to flag usage of Function.prototype.apply() in situations where the spread operator could be used instead.

Examples

Examples of incorrect code for this rule:

/*eslint prefer-spread: "error"*/

foo.apply(undefined, args);
foo.apply(null, args);
obj.foo.apply(obj, args);

Examples of correct code for this rule:

/*eslint prefer-spread: "error"*/

// Using the spread operator
foo(...args);
obj.foo(...args);

// The `this` binding is different.
foo.apply(obj, args);
obj.foo.apply(null, args);
obj.foo.apply(otherObj, args);

// The argument list is not variadic.
// Those are warned by the `no-useless-call` rule.
foo.apply(undefined, [1, 2, 3]);
foo.apply(null, [1, 2, 3]);
obj.foo.apply(obj, [1, 2, 3]);

Known limitations:

This rule analyzes code statically to check whether or not the this argument is changed. So, if the this argument is computed in a dynamic expression, this rule cannot detect a violation.

/*eslint prefer-spread: "error"*/

// This warns.
a[i++].foo.apply(a[i++], args);

// This does not warn.
a[++i].foo.apply(a[i], args);

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.prototype.apply() callings, you can safely disable this rule.

Related Rules

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

  accessor() {

Close

Enforce that class methods utilize this (class-methods-use-this)

If a class method does not use this, it can safely be made a static function.

It's possible to have a class method which doesn't use this, such as:

class A {
    constructor() {
        this.a = "hi";
    }

    print() {
        console.log(this.a);
    }

    sayHi() {
        console.log("hi");
    }
}

let a = new A();
a.sayHi(); // => "hi"

In the example above, the sayHi method doesn't use this, so we can make it a static method:

class A {
    constructor() {
        this.a = "hi";
    }

    print() {
        console.log(this.a);
    }

    static sayHi() {
        console.log("hi");
    }
}

A.sayHi(); // => "hi"

Also note in the above examples that the code calling the function on an instance of the class (let a = new A(); a.sayHi();) changes to calling it on the class itself (A.sayHi();).

Rule Details

This rule is aimed to flag class methods that do not use this.

Examples of incorrect code for this rule:

/*eslint class-methods-use-this: "error"*/
/*eslint-env es6*/

class A {
    foo() {
        console.log("Hello World");     /*error Expected 'this' to be used by class method 'foo'.*/
    }
}

Examples of correct code for this rule:

/*eslint class-methods-use-this: "error"*/
/*eslint-env es6*/
class A {
    foo() {
        this.bar = "Hello World"; // OK, this is used
    }
}

class A {
    constructor() {
        // OK. constructor is exempt
    }
}

class A {
    static foo() {
        // OK. static methods aren't expected to use this.
    }
}

Options

Exceptions

"class-methods-use-this": [<enabled>, { "exceptMethods": [&lt;...exceptions&gt;] }]</enabled>

The exceptMethods option allows you to pass an array of method names for which you would like to ignore warnings.

Examples of incorrect code for this rule when used without exceptMethods:

/*eslint class-methods-use-this: "error"*/

class A {
    foo() {
    }
}

Examples of correct code for this rule when used with exceptMethods:

/*eslint class-methods-use-this: ["error", { "exceptMethods": ["foo"] }] */

class A {
    foo() {
    }
}

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

      for (const event of self._sourcedEvents) {

Close

disallow specified syntax (no-restricted-syntax)

JavaScript has a lot of language features, and not everyone likes all of them. As a result, some projects choose to disallow the use of certain language features altogether. For instance, you might decide to disallow the use of try-catch or class, or you might decide to disallow the use of the in operator.

Rather than creating separate rules for every language feature you want to turn off, this rule allows you to configure the syntax elements you want to restrict use of. These elements are represented by their ESTree node types. For example, a function declaration is represented by FunctionDeclaration and the with statement is represented by WithStatement. You may find the full list of AST node names you can use on GitHub and use the online parser to see what type of nodes your code consists of.

You can also specify [AST selectors](../developer-guide/selectors) to restrict, allowing much more precise control over syntax patterns.

Rule Details

This rule disallows specified (that is, user-defined) syntax.

Options

This rule takes a list of strings, where each string is an AST selector:

{
    "rules": {
        "no-restricted-syntax": ["error", "FunctionExpression", "WithStatement", "BinaryExpression[operator='in']"]
    }
}

Alternatively, the rule also accepts objects, where the selector and an optional custom message are specified:

{
    "rules": {
        "no-restricted-syntax": [
            "error",
            {
                "selector": "FunctionExpression",
                "message": "Function expressions are not allowed."
            },
            {
                "selector": "CallExpression[callee.name='setTimeout'][arguments.length!=2]",
                "message": "setTimeout must always be invoked with two arguments."
            }
        ]
    }
}

If a custom message is specified with the message property, ESLint will use that message when reporting occurrences of the syntax specified in the selector property.

The string and object formats can be freely mixed in the configuration as needed.

Examples of incorrect code for this rule with the "FunctionExpression", "WithStatement", BinaryExpression[operator='in'] options:

/* eslint no-restricted-syntax: ["error", "FunctionExpression", "WithStatement", "BinaryExpression[operator='in']"] */

with (me) {
    dontMess();
}

var doSomething = function () {};

foo in bar;

Examples of correct code for this rule with the "FunctionExpression", "WithStatement", BinaryExpression[operator='in'] options:

/* eslint no-restricted-syntax: ["error", "FunctionExpression", "WithStatement", "BinaryExpression[operator='in']"] */

me.dontMess();

function doSomething() {};

foo instanceof bar;

When Not To Use It

If you don't want to restrict your code from using any JavaScript features or syntax, you should not use this rule.

Related Rules

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

  getObjectCollectionName(obj) {

Close

Enforce that class methods utilize this (class-methods-use-this)

If a class method does not use this, it can safely be made a static function.

It's possible to have a class method which doesn't use this, such as:

class A {
    constructor() {
        this.a = "hi";
    }

    print() {
        console.log(this.a);
    }

    sayHi() {
        console.log("hi");
    }
}

let a = new A();
a.sayHi(); // => "hi"

In the example above, the sayHi method doesn't use this, so we can make it a static method:

class A {
    constructor() {
        this.a = "hi";
    }

    print() {
        console.log(this.a);
    }

    static sayHi() {
        console.log("hi");
    }
}

A.sayHi(); // => "hi"

Also note in the above examples that the code calling the function on an instance of the class (let a = new A(); a.sayHi();) changes to calling it on the class itself (A.sayHi();).

Rule Details

This rule is aimed to flag class methods that do not use this.

Examples of incorrect code for this rule:

/*eslint class-methods-use-this: "error"*/
/*eslint-env es6*/

class A {
    foo() {
        console.log("Hello World");     /*error Expected 'this' to be used by class method 'foo'.*/
    }
}

Examples of correct code for this rule:

/*eslint class-methods-use-this: "error"*/
/*eslint-env es6*/
class A {
    foo() {
        this.bar = "Hello World"; // OK, this is used
    }
}

class A {
    constructor() {
        // OK. constructor is exempt
    }
}

class A {
    static foo() {
        // OK. static methods aren't expected to use this.
    }
}

Options

Exceptions

"class-methods-use-this": [<enabled>, { "exceptMethods": [&lt;...exceptions&gt;] }]</enabled>

The exceptMethods option allows you to pass an array of method names for which you would like to ignore warnings.

Examples of incorrect code for this rule when used without exceptMethods:

/*eslint class-methods-use-this: "error"*/

class A {
    foo() {
    }
}

Examples of correct code for this rule when used with exceptMethods:

/*eslint class-methods-use-this: ["error", { "exceptMethods": ["foo"] }] */

class A {
    foo() {
    }
}

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

    const cb = co.wrap(function* reconnect() {

Close

Disallow generator functions that do not have yield (require-yield)

Rule Details

This rule generates warnings for generator functions that do not have the yield keyword.

Examples

Examples of incorrect code for this rule:

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

function* foo() {
  return 10;
}

Examples of correct code for this rule:

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

function* foo() {
  yield 5;
  return 10;
}

function foo() {
  return 10;
}

// This rule does not warn on empty generator functions.
function* foo() { }

When Not To Use It

If you don't want to notify generator functions that have no yield expression, then it's safe to disable this rule.

Related Rules

• [require-await](require-await.md) Source: http://eslint.org/docs/rules/

  getClassName(obj) {

Close

Enforce that class methods utilize this (class-methods-use-this)

If a class method does not use this, it can safely be made a static function.

It's possible to have a class method which doesn't use this, such as:

class A {
    constructor() {
        this.a = "hi";
    }

    print() {
        console.log(this.a);
    }

    sayHi() {
        console.log("hi");
    }
}

let a = new A();
a.sayHi(); // => "hi"

In the example above, the sayHi method doesn't use this, so we can make it a static method:

class A {
    constructor() {
        this.a = "hi";
    }

    print() {
        console.log(this.a);
    }

    static sayHi() {
        console.log("hi");
    }
}

A.sayHi(); // => "hi"

Also note in the above examples that the code calling the function on an instance of the class (let a = new A(); a.sayHi();) changes to calling it on the class itself (A.sayHi();).

Rule Details

This rule is aimed to flag class methods that do not use this.

Examples of incorrect code for this rule:

/*eslint class-methods-use-this: "error"*/
/*eslint-env es6*/

class A {
    foo() {
        console.log("Hello World");     /*error Expected 'this' to be used by class method 'foo'.*/
    }
}

Examples of correct code for this rule:

/*eslint class-methods-use-this: "error"*/
/*eslint-env es6*/
class A {
    foo() {
        this.bar = "Hello World"; // OK, this is used
    }
}

class A {
    constructor() {
        // OK. constructor is exempt
    }
}

class A {
    static foo() {
        // OK. static methods aren't expected to use this.
    }
}

Options

Exceptions

"class-methods-use-this": [<enabled>, { "exceptMethods": [&lt;...exceptions&gt;] }]</enabled>

The exceptMethods option allows you to pass an array of method names for which you would like to ignore warnings.

Examples of incorrect code for this rule when used without exceptMethods:

/*eslint class-methods-use-this: "error"*/

class A {
    foo() {
    }
}

Examples of correct code for this rule when used with exceptMethods:

/*eslint class-methods-use-this: ["error", { "exceptMethods": ["foo"] }] */

class A {
    foo() {
    }
}

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

    this.index++;

Close

disallow the unary operators ++ and -- (no-plusplus)

Because the unary ++ and -- operators are subject to automatic semicolon insertion, differences in whitespace can change semantics of source code.

var i = 10;
var j = 20;

i ++
j
// i = 11, j = 20

var i = 10;
var j = 20;

i
++
j
// i = 10, j = 21

Rule Details

This rule disallows the unary operators ++ and --.

Examples of incorrect code for this rule:

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

var foo = 0;
foo++;

var bar = 42;
bar--;

for (i = 0; i < l; i++) {
    return;
}

Examples of correct code for this rule:

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

var foo = 0;
foo += 1;

var bar = 42;
bar -= 1;

for (i = 0; i < l; i += 1) {
    return;
}

Options

This rule has an object option.

• "allowForLoopAfterthoughts": true allows unary operators ++ and -- in the afterthought (final expression) of a for loop.

allowForLoopAfterthoughts

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

/*eslint no-plusplus: ["error", { "allowForLoopAfterthoughts": true }]*/

for (i = 0; i < l; i++) {
    return;
}

for (i = 0; i < l; i--) {
    return;
}

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

        if (!constructor.prototype.hasOwnProperty(key)) {

Close

Disallow use of Object.prototypes builtins directly (no-prototype-builtins)

In ECMAScript 5.1, Object.create was added, which enables the creation of objects with a specified [[Prototype]]. Object.create(null) is a common pattern used to create objects that will be used as a Map. This can lead to errors when it is assumed that objects will have properties from Object.prototype. This rule prevents calling Object.prototype methods directly from an object.

Rule Details

This rule disallows calling some Object.prototype methods directly on object instances.

Examples of incorrect code for this rule:

/*eslint no-prototype-builtins: "error"*/

var hasBarProperty = foo.hasOwnProperty("bar");

var isPrototypeOfBar = foo.isPrototypeOf(bar);

var barIsEnumerable = foo.propertyIsEnumerable("bar");

Examples of correct code for this rule:

/*eslint no-prototype-builtins: "error"*/

var hasBarProperty = Object.prototype.hasOwnProperty.call(foo, "bar");

var isPrototypeOfBar = Object.prototype.isPrototypeOf.call(foo, bar);

var barIsEnumerable = {}.propertyIsEnumerable.call(foo, "bar");

When Not To Use It

You may want to turn this rule off if you will never use an object that shadows an Object.prototype method or which does not inherit from Object.prototype. Source: http://eslint.org/docs/rules/

    return new Promise(resolve => {

Close

Require parens in arrow function arguments (arrow-parens)

Arrow functions can omit parentheses when they have exactly one parameter. In all other cases the parameter(s) must be wrapped in parentheses. This rule enforces the consistent use of parentheses in arrow functions.

Rule Details

This rule enforces parentheses around arrow function parameters regardless of arity. For example:

/*eslint-env es6*/

// Bad
a => {}

// Good
(a) => {}

Following this style will help you find arrow functions (=>) which may be mistakenly included in a condition when a comparison such as >= was the intent.

/*eslint-env es6*/

// Bad
if (a => 2) {
}

// Good
if (a >= 2) {
}

The rule can also be configured to discourage the use of parens when they are not required:

/*eslint-env es6*/

// Bad
(a) => {}

// Good
a => {}

Options

This rule has a string option and an object one.

String options are:

• "always" (default) requires parens around arguments in all cases.
• "as-needed" allows omitting parens when there is only one argument.

Object properties for variants of the "as-needed" option:

• "requireForBlockBody": true modifies the as-needed rule in order to require parens if the function body is in an instructions block (surrounded by braces).

always

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

/*eslint arrow-parens: ["error", "always"]*/
/*eslint-env es6*/

a => {};
a => a;
a => {'\n'};
a.then(foo => {});
a.then(foo => a);
a(foo => { if (true) {} });

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

/*eslint arrow-parens: ["error", "always"]*/
/*eslint-env es6*/

() => {};
(a) => {};
(a) => a;
(a) => {'\n'}
a.then((foo) => {});
a.then((foo) => { if (true) {} });

If Statements

One of benefits of this option is that it prevents the incorrect use of arrow functions in conditionals:

/*eslint-env es6*/

var a = 1;
var b = 2;
// ...
if (a => b) {
 console.log('bigger');
} else {
 console.log('smaller');
}
// outputs 'bigger', not smaller as expected

The contents of the if statement is an arrow function, not a comparison.

If the arrow function is intentional, it should be wrapped in parens to remove ambiguity.

/*eslint-env es6*/

var a = 1;
var b = 0;
// ...
if ((a) => b) {
 console.log('truthy value returned');
} else {
 console.log('falsey value returned');
}
// outputs 'truthy value returned'

The following is another example of this behavior:

/*eslint-env es6*/

var a = 1, b = 2, c = 3, d = 4;
var f = a => b ? c: d;
// f = ?

f is an arrow function which takes a as an argument and returns the result of b ? c: d.

This should be rewritten like so:

/*eslint-env es6*/

var a = 1, b = 2, c = 3, d = 4;
var f = (a) => b ? c: d;

as-needed

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

/*eslint arrow-parens: ["error", "as-needed"]*/
/*eslint-env es6*/

(a) => {};
(a) => a;
(a) => {'\n'};
a.then((foo) => {});
a.then((foo) => a);
a((foo) => { if (true) {} });

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

/*eslint arrow-parens: ["error", "as-needed"]*/
/*eslint-env es6*/

() => {};
a => {};
a => a;
a => {'\n'};
a.then(foo => {});
a.then(foo => { if (true) {} });
(a, b, c) => a;
(a = 10) => a;
([a, b]) => a;
({a, b}) => a;

requireForBlockBody

Examples of incorrect code for the { "requireForBlockBody": true } option:

/*eslint arrow-parens: [2, "as-needed", { "requireForBlockBody": true }]*/
/*eslint-env es6*/

(a) => a;
a => {};
a => {'\n'};
a.map((x) => x * x);
a.map(x => {
  return x * x;
});
a.then(foo => {});

Examples of correct code for the { "requireForBlockBody": true } option:

/*eslint arrow-parens: [2, "as-needed", { "requireForBlockBody": true }]*/
/*eslint-env es6*/

(a) => {};
(a) => {'\n'};
a => ({});
() => {};
a => a;
a.then((foo) => {});
a.then((foo) => { if (true) {} });
a((foo) => { if (true) {} });
(a, b, c) => a;
(a = 10) => a;
([a, b]) => a;
({a, b}) => a;

Further Reading

• The "as-needed", { "requireForBlockBody": true } rule is directly inspired by the Airbnb JS Style Guide. Source: http://eslint.org/docs/rules/

      self._attempts++;

Close

disallow the unary operators ++ and -- (no-plusplus)

Because the unary ++ and -- operators are subject to automatic semicolon insertion, differences in whitespace can change semantics of source code.

var i = 10;
var j = 20;

i ++
j
// i = 11, j = 20

var i = 10;
var j = 20;

i
++
j
// i = 10, j = 21

Rule Details

This rule disallows the unary operators ++ and --.

Examples of incorrect code for this rule:

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

var foo = 0;
foo++;

var bar = 42;
bar--;

for (i = 0; i < l; i++) {
    return;
}

Examples of correct code for this rule:

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

var foo = 0;
foo += 1;

var bar = 42;
bar -= 1;

for (i = 0; i < l; i += 1) {
    return;
}

Options

This rule has an object option.

• "allowForLoopAfterthoughts": true allows unary operators ++ and -- in the afterthought (final expression) of a for loop.

allowForLoopAfterthoughts

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

/*eslint no-plusplus: ["error", { "allowForLoopAfterthoughts": true }]*/

for (i = 0; i < l; i++) {
    return;
}

for (i = 0; i < l; i--) {
    return;
}

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

    server.on('error', error => {

Close

Require parens in arrow function arguments (arrow-parens)

Arrow functions can omit parentheses when they have exactly one parameter. In all other cases the parameter(s) must be wrapped in parentheses. This rule enforces the consistent use of parentheses in arrow functions.

Rule Details

This rule enforces parentheses around arrow function parameters regardless of arity. For example:

/*eslint-env es6*/

// Bad
a => {}

// Good
(a) => {}

Following this style will help you find arrow functions (=>) which may be mistakenly included in a condition when a comparison such as >= was the intent.

/*eslint-env es6*/

// Bad
if (a => 2) {
}

// Good
if (a >= 2) {
}

The rule can also be configured to discourage the use of parens when they are not required:

/*eslint-env es6*/

// Bad
(a) => {}

// Good
a => {}

Options

This rule has a string option and an object one.

String options are:

• "always" (default) requires parens around arguments in all cases.
• "as-needed" allows omitting parens when there is only one argument.

Object properties for variants of the "as-needed" option:

• "requireForBlockBody": true modifies the as-needed rule in order to require parens if the function body is in an instructions block (surrounded by braces).

always

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

/*eslint arrow-parens: ["error", "always"]*/
/*eslint-env es6*/

a => {};
a => a;
a => {'\n'};
a.then(foo => {});
a.then(foo => a);
a(foo => { if (true) {} });

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

/*eslint arrow-parens: ["error", "always"]*/
/*eslint-env es6*/

() => {};
(a) => {};
(a) => a;
(a) => {'\n'}
a.then((foo) => {});
a.then((foo) => { if (true) {} });

If Statements

One of benefits of this option is that it prevents the incorrect use of arrow functions in conditionals:

/*eslint-env es6*/

var a = 1;
var b = 2;
// ...
if (a => b) {
 console.log('bigger');
} else {
 console.log('smaller');
}
// outputs 'bigger', not smaller as expected

The contents of the if statement is an arrow function, not a comparison.

If the arrow function is intentional, it should be wrapped in parens to remove ambiguity.

/*eslint-env es6*/

var a = 1;
var b = 0;
// ...
if ((a) => b) {
 console.log('truthy value returned');
} else {
 console.log('falsey value returned');
}
// outputs 'truthy value returned'

The following is another example of this behavior:

/*eslint-env es6*/

var a = 1, b = 2, c = 3, d = 4;
var f = a => b ? c: d;
// f = ?

f is an arrow function which takes a as an argument and returns the result of b ? c: d.

This should be rewritten like so:

/*eslint-env es6*/

var a = 1, b = 2, c = 3, d = 4;
var f = (a) => b ? c: d;

as-needed

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

/*eslint arrow-parens: ["error", "as-needed"]*/
/*eslint-env es6*/

(a) => {};
(a) => a;
(a) => {'\n'};
a.then((foo) => {});
a.then((foo) => a);
a((foo) => { if (true) {} });

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

/*eslint arrow-parens: ["error", "as-needed"]*/
/*eslint-env es6*/

() => {};
a => {};
a => a;
a => {'\n'};
a.then(foo => {});
a.then(foo => { if (true) {} });
(a, b, c) => a;
(a = 10) => a;
([a, b]) => a;
({a, b}) => a;

requireForBlockBody

Examples of incorrect code for the { "requireForBlockBody": true } option:

/*eslint arrow-parens: [2, "as-needed", { "requireForBlockBody": true }]*/
/*eslint-env es6*/

(a) => a;
a => {};
a => {'\n'};
a.map((x) => x * x);
a.map(x => {
  return x * x;
});
a.then(foo => {});

Examples of correct code for the { "requireForBlockBody": true } option:

/*eslint arrow-parens: [2, "as-needed", { "requireForBlockBody": true }]*/
/*eslint-env es6*/

(a) => {};
(a) => {'\n'};
a => ({});
() => {};
a => a;
a.then((foo) => {});
a.then((foo) => { if (true) {} });
a((foo) => { if (true) {} });
(a, b, c) => a;
(a = 10) => a;
([a, b]) => a;
({a, b}) => a;

Further Reading

• The "as-needed", { "requireForBlockBody": true } rule is directly inspired by the Airbnb JS Style Guide. Source: http://eslint.org/docs/rules/

        version: ++this._version,

Close

disallow the unary operators ++ and -- (no-plusplus)

Because the unary ++ and -- operators are subject to automatic semicolon insertion, differences in whitespace can change semantics of source code.

var i = 10;
var j = 20;

i ++
j
// i = 11, j = 20

var i = 10;
var j = 20;

i
++
j
// i = 10, j = 21

Rule Details

This rule disallows the unary operators ++ and --.

Examples of incorrect code for this rule:

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

var foo = 0;
foo++;

var bar = 42;
bar--;

for (i = 0; i < l; i++) {
    return;
}

Examples of correct code for this rule:

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

var foo = 0;
foo += 1;

var bar = 42;
bar -= 1;

for (i = 0; i < l; i += 1) {
    return;
}

Options

This rule has an object option.

• "allowForLoopAfterthoughts": true allows unary operators ++ and -- in the afterthought (final expression) of a for loop.

allowForLoopAfterthoughts

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

/*eslint no-plusplus: ["error", { "allowForLoopAfterthoughts": true }]*/

for (i = 0; i < l; i++) {
    return;
}

for (i = 0; i < l; i--) {
    return;
}

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

    this.rxChannel.consume(this.queueName, msg => {

Close

Require parens in arrow function arguments (arrow-parens)

Arrow functions can omit parentheses when they have exactly one parameter. In all other cases the parameter(s) must be wrapped in parentheses. This rule enforces the consistent use of parentheses in arrow functions.

Rule Details

This rule enforces parentheses around arrow function parameters regardless of arity. For example:

/*eslint-env es6*/

// Bad
a => {}

// Good
(a) => {}

Following this style will help you find arrow functions (=>) which may be mistakenly included in a condition when a comparison such as >= was the intent.

/*eslint-env es6*/

// Bad
if (a => 2) {
}

// Good
if (a >= 2) {
}

The rule can also be configured to discourage the use of parens when they are not required:

/*eslint-env es6*/

// Bad
(a) => {}

// Good
a => {}

Options

This rule has a string option and an object one.

String options are:

• "always" (default) requires parens around arguments in all cases.
• "as-needed" allows omitting parens when there is only one argument.

Object properties for variants of the "as-needed" option:

• "requireForBlockBody": true modifies the as-needed rule in order to require parens if the function body is in an instructions block (surrounded by braces).

always

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

/*eslint arrow-parens: ["error", "always"]*/
/*eslint-env es6*/

a => {};
a => a;
a => {'\n'};
a.then(foo => {});
a.then(foo => a);
a(foo => { if (true) {} });

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

/*eslint arrow-parens: ["error", "always"]*/
/*eslint-env es6*/

() => {};
(a) => {};
(a) => a;
(a) => {'\n'}
a.then((foo) => {});
a.then((foo) => { if (true) {} });

If Statements

One of benefits of this option is that it prevents the incorrect use of arrow functions in conditionals:

/*eslint-env es6*/

var a = 1;
var b = 2;
// ...
if (a => b) {
 console.log('bigger');
} else {
 console.log('smaller');
}
// outputs 'bigger', not smaller as expected

The contents of the if statement is an arrow function, not a comparison.

If the arrow function is intentional, it should be wrapped in parens to remove ambiguity.

/*eslint-env es6*/

var a = 1;
var b = 0;
// ...
if ((a) => b) {
 console.log('truthy value returned');
} else {
 console.log('falsey value returned');
}
// outputs 'truthy value returned'

The following is another example of this behavior:

/*eslint-env es6*/

var a = 1, b = 2, c = 3, d = 4;
var f = a => b ? c: d;
// f = ?

f is an arrow function which takes a as an argument and returns the result of b ? c: d.

This should be rewritten like so:

/*eslint-env es6*/

var a = 1, b = 2, c = 3, d = 4;
var f = (a) => b ? c: d;

as-needed

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

/*eslint arrow-parens: ["error", "as-needed"]*/
/*eslint-env es6*/

(a) => {};
(a) => a;
(a) => {'\n'};
a.then((foo) => {});
a.then((foo) => a);
a((foo) => { if (true) {} });

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

/*eslint arrow-parens: ["error", "as-needed"]*/
/*eslint-env es6*/

() => {};
a => {};
a => a;
a => {'\n'};
a.then(foo => {});
a.then(foo => { if (true) {} });
(a, b, c) => a;
(a = 10) => a;
([a, b]) => a;
({a, b}) => a;

requireForBlockBody

Examples of incorrect code for the { "requireForBlockBody": true } option:

/*eslint arrow-parens: [2, "as-needed", { "requireForBlockBody": true }]*/
/*eslint-env es6*/

(a) => a;
a => {};
a => {'\n'};
a.map((x) => x * x);
a.map(x => {
  return x * x;
});
a.then(foo => {});

Examples of correct code for the { "requireForBlockBody": true } option:

/*eslint arrow-parens: [2, "as-needed", { "requireForBlockBody": true }]*/
/*eslint-env es6*/

(a) => {};
(a) => {'\n'};
a => ({});
() => {};
a => a;
a.then((foo) => {});
a.then((foo) => { if (true) {} });
a((foo) => { if (true) {} });
(a, b, c) => a;
(a = 10) => a;
([a, b]) => a;
({a, b}) => a;

Further Reading

• The "as-needed", { "requireForBlockBody": true } rule is directly inspired by the Airbnb JS Style Guide. Source: http://eslint.org/docs/rules/