disallow unnecessary parentheses (no-extra-parens)

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-invokes function expressions (also known as IIFEs) such as var x = (function () {})(); and ((function foo() {return 1;})()) to avoid conflicts with the wrap-iife 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

all

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

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

a = (b * c);

(a * b) + c;

typeof (a);

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

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);

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 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;

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();

({}.toString.call());

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

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

a = (b * c);

(a * b) + c;

typeof (a);

Further Reading

• MDN: Operator Precedence

Related Rules

• no-cond-assign
• no-return-assign

Version

This rule was introduced in ESLint 0.1.4.

Resources

• Rule source
• Documentation source