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disallow unnecessary parentheses (no-extra-parens)

The --fix option on the command line can automatically fix some of the problems reported by this rule.

This rule restricts the use of parentheses to only where they are necessary.

Rule Details

This rule always ignores extra parentheses around the following:

• RegExp literals such as (/abc/).test(var) to avoid conflicts with the wrap-regex rule
• immediately-invoked function expressions (also known as IIFEs) such as var x = (function () {})(); and var x = (function () {}()); to avoid conflicts with the wrap-iife rule
• arrow function arguments to avoid conflicts with the arrow-parens rule

Options

This rule has a string option:

• "all" (default) disallows unnecessary parentheses around any expression
• "functions" disallows unnecessary parentheses only around function expressions

This rule has an object option for exceptions to the "all" option:

• "conditionalAssign": false allows extra parentheses around assignments in conditional test expressions
• "returnAssign": false allows extra parentheses around assignments in return statements
• "nestedBinaryExpressions": false allows extra parentheses in nested binary expressions
• "ignoreJSX": "none|all|multi-line|single-line" allows extra parentheses around no/all/multi-line/single-line JSX components. Defaults to none.
• "enforceForArrowConditionals": false allows extra parentheses around ternary expressions which are the body of an arrow function
• "enforceForSequenceExpressions": false allows extra parentheses around sequence expressions
• "enforceForNewInMemberExpressions": false allows extra parentheses around new expressions in member expressions
• "enforceForFunctionPrototypeMethods": false allows extra parentheses around immediate .call and .apply method calls on function expressions and around function expressions in the same context.

all

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

/* eslint no-extra-parens: "error" */

a = (b * c);

(a * b) + c;

for (a in (b, c));

for (a in (b));

for (a of (b));

typeof (a);

(function(){} ? a() : b());

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

/* eslint no-extra-parens: "error" */

(0).toString();

(Object.prototype.toString.call());

({}.toString.call());

(function(){}) ? a() : b();

(/^a$/).test(x);

for (a of (b, c));

for (a of b);

for (a in b, c);

for (a in b);

conditionalAssign

Examples of correct code for this rule with the "all" and { "conditionalAssign": false } options:

/* eslint no-extra-parens: ["error", "all", { "conditionalAssign": false }] */

while ((foo = bar())) {}

if ((foo = bar())) {}

do; while ((foo = bar()))

for (;(a = b););

returnAssign

Examples of correct code for this rule with the "all" and { "returnAssign": false } options:

/* eslint no-extra-parens: ["error", "all", { "returnAssign": false }] */

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

function a(b) {
  return b ? (c = d) : (c = e);
}

b => (b = 1);

b => b ? (c = d) : (c = e);

nestedBinaryExpressions

Examples of correct code for this rule with the "all" and { "nestedBinaryExpressions": false } options:

/* eslint no-extra-parens: ["error", "all", { "nestedBinaryExpressions": false }] */

x = a || (b && c);
x = a + (b * c);
x = (a * b) / c;

ignoreJSX

Examples of correct code for this rule with the all and { "ignoreJSX": "all" } options:

/* eslint no-extra-parens: ["error", "all", { ignoreJSX: "all" }] */
const Component = (<div />)
const Component = (
    <div
        prop={true}
    />
)

Examples of incorrect code for this rule with the all and { "ignoreJSX": "multi-line" } options:

/* eslint no-extra-parens: ["error", "all", { ignoreJSX: "multi-line" }] */
const Component = (<div />)
const Component = (<div><p /></div>)

Examples of correct code for this rule with the all and { "ignoreJSX": "multi-line" } options:

/* eslint no-extra-parens: ["error", "all", { ignoreJSX: "multi-line" }] */
const Component = (
    <div>
        <p />
    </div>
)
const Component = (
    <div
        prop={true}
    />
)

Examples of incorrect code for this rule with the all and { "ignoreJSX": "single-line" } options:

/* eslint no-extra-parens: ["error", "all", { ignoreJSX: "single-line" }] */
const Component = (
    <div>
        <p />
    </div>
)
const Component = (
    <div
        prop={true}
    />
)

Examples of correct code for this rule with the all and { "ignoreJSX": "single-line" } options:

/* eslint no-extra-parens: ["error", "all", { ignoreJSX: "single-line" }] */
const Component = (<div />)
const Component = (<div><p /></div>)

enforceForArrowConditionals

Examples of correct code for this rule with the "all" and { "enforceForArrowConditionals": false } options:

/* eslint no-extra-parens: ["error", "all", { "enforceForArrowConditionals": false }] */

const b = a => 1 ? 2 : 3;
const d = c => (1 ? 2 : 3);

enforceForSequenceExpressions

Examples of correct code for this rule with the "all" and { "enforceForSequenceExpressions": false } options:

/* eslint no-extra-parens: ["error", "all", { "enforceForSequenceExpressions": false }] */

(a, b);

if ((val = foo(), val < 10)) {}

while ((val = foo(), val < 10));

enforceForNewInMemberExpressions

Examples of correct code for this rule with the "all" and { "enforceForNewInMemberExpressions": false } options:

/* eslint no-extra-parens: ["error", "all", { "enforceForNewInMemberExpressions": false }] */

const foo = (new Bar()).baz;

const quux = (new Bar())[baz];

(new Bar()).doSomething();

enforceForFunctionPrototypeMethods

Examples of correct code for this rule with the "all" and { "enforceForFunctionPrototypeMethods": false } options:

/* eslint no-extra-parens: ["error", "all", { "enforceForFunctionPrototypeMethods": false }] */

const foo = (function () {}).call();

const bar = (function () {}).apply();

const baz = (function () {}.call());

const quux = (function () {}.apply());

functions

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

/* eslint no-extra-parens: ["error", "functions"] */

((function foo() {}))();

var y = (function () {return 1;});

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

/* eslint no-extra-parens: ["error", "functions"] */

(0).toString();

(Object.prototype.toString.call());

({}.toString.call());

(function(){} ? a() : b());

(/^a$/).test(x);

a = (b * c);

(a * b) + c;

typeof (a);

Further Reading

• MDN: Operator Precedence

Related Rules

• arrow-parens
• no-cond-assign
• no-return-assign

Version

This rule was introduced in ESLint 0.1.4.

Resources

• Rule source
• Documentation source