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/

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

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

Rule Details

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

Options

This rule has either a string option:

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

Or an object option:

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

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

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

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

always

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

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

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

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

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

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

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

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

never

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

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

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

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

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

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

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

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

multiline

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

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

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

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

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

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

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

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

minProperties

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

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

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

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

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

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

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

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

consistent

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

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

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

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

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

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

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

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

ObjectExpression and ObjectPattern

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

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

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

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

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

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

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

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

ImportDeclaration and ExportDeclaration

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

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

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

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

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

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

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

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

Compatibility

• JSCS: requirePaddingNewLinesInObjects
• JSCS: disallowPaddingNewLinesInObjects

When Not To Use It

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

Related Rules

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

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

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

Rule Details

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

Options

This rule has either a string option:

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

Or an object option:

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

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

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

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

always

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

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

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

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

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

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

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

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

never

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

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

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

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

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

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

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

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

multiline

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

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

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

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

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

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

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

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

minProperties

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

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

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

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

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

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

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

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

consistent

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

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

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

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

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

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

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

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

ObjectExpression and ObjectPattern

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

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

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

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

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

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

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

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

ImportDeclaration and ExportDeclaration

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

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

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

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

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

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

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

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

Compatibility

• JSCS: requirePaddingNewLinesInObjects
• JSCS: disallowPaddingNewLinesInObjects

When Not To Use It

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

Related Rules

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

Require 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/

Require or disallow spaces before/after unary operators (space-unary-ops)

Some style guides require or disallow spaces before or after unary operators. This is mainly a stylistic issue, however, some JavaScript expressions can be written without spacing which makes it harder to read and maintain.

Rule Details

This rule enforces consistency regarding the spaces after words unary operators and after/before nonwords unary operators.

Examples of unary words operators:

// new
var joe = new Person();

// delete
var obj = {
    foo: 'bar'
};
delete obj.foo;

// typeof
typeof {} // object

// void
void 0 // undefined

Examples of unary nonwords operators:

if ([1,2,3].indexOf(1) !== -1) {};
foo = --foo;
bar = bar++;
baz = !foo;
qux = !!baz;

Options

This rule has three options:

• words - applies to unary word operators such as: new, delete, typeof, void, yield
• nonwords - applies to unary operators such as: -, +, --, ++, !, !!
• overrides - specifies overwriting usage of spacing for each operator, word or non word. This is empty by default, but can be used to enforce or disallow spacing around operators. For example:

"space-unary-ops": [
        2, {
          "words": true,
          "nonwords": false,
          "overrides": {
            "new": false,
            "++": true
          }
    }]

In this case, spacing will be disallowed after a new operator and required before/after a ++ operator.

Examples of incorrect code for this rule with the default {"words": true, "nonwords": false} option:

/*eslint space-unary-ops: "error"*/

typeof!foo;

void{foo:0};

new[foo][0];

delete(foo.bar);

++ foo;

foo --;

- foo;

+ "3";

/*eslint space-unary-ops: "error"*/
/*eslint-env es6*/

function *foo() {
    yield(0)
}

/*eslint space-unary-ops: "error"*/

async function foo() {
    await(bar);
}

Examples of correct code for this rule with the {"words": true, "nonwords": false} option:

/*eslint space-unary-ops: "error"*/

// Word unary operator "delete" is followed by a whitespace.
delete foo.bar;

// Word unary operator "new" is followed by a whitespace.
new Foo;

// Word unary operator "void" is followed by a whitespace.
void 0;

// Unary operator "++" is not followed by whitespace.
++foo;

// Unary operator "--" is not preceded by whitespace.
foo--;

// Unary operator "-" is not followed by whitespace.
-foo;

// Unary operator "+" is not followed by whitespace.
+"3";

/*eslint space-unary-ops: "error"*/
/*eslint-env es6*/

function *foo() {
    yield (0)
}

/*eslint space-unary-ops: "error"*/

async function foo() {
    await (bar);
}

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

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

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

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

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

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

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

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

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

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

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

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