no-extra-parens

Disallow unnecessary parentheses

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

Problems reported by this rule can be fixed automatically, except when removing the parentheses would create a new directive, because that could change the semantics of the code. For example, the following script prints object to the console, but if the parentheses around "use strict" were removed, it would print undefined instead.

<!--
// this is a script
// -->

("use strict");

function test() {
    console.log(typeof this);
}

test();

In this case, the rule will not try to remove the parentheses around "use strict" but will still report them as a problem.

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.
• "ternaryOperandBinaryExpressions": false allows extra parentheses around binary expressions that are operands of ternary ?:.
• "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.
• "allowParensAfterCommentPattern": "any-string-pattern" allows extra parentheses preceded by a comment that matches a regular expression.

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);

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

class A {
    [(x)] = 1;
}

class B {
    x = (y + z);
}

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

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

(0).toString();

({}.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);

class A {
    [x] = 1;
}

class B {
    x = y + z;
}

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 a1(b) {
  return (b = 1);
}

function a2(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;

ternaryOperandBinaryExpressions

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

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

(a && b) ? foo : bar;

(a - b > a) ? foo : bar;

foo ? (bar || baz) : qux;

foo ? bar : (baz || qux);

ignoreJSX

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

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

allowParensAfterCommentPattern

To make this rule allow extra parentheses preceded by specific comments, set this option to a string pattern that will be passed to the RegExp constructor.

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

/* eslint no-extra-parens: ["error", "all", { "allowParensAfterCommentPattern": "@type" }] */

const span = /**@type {HTMLSpanElement}*/(event.currentTarget);

if (/** @type {Foo | Bar} */(options).baz) console.log('Lint free');

foo(/** @type {Bar} */ (bar), options, {
    name: "name",
    path: "path",
});

if (foo) {
    /** @type {Bar} */
    (bar).prop = false;
}

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);

Related Rules

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

Version

This rule was introduced in ESLint v0.1.4.

Further Reading

Resources

• Rule source
• Tests source