no-void

Disallow void operators

The void operator takes an operand and returns undefined: void expression will evaluate expression and return undefined. It can be used to ignore any side effects expression may produce:

The common case of using void operator is to get a “pure” undefined value as prior to ES5 the undefined variable was mutable:

// will always return undefined
(function(){
    return void 0;
})();

// will return 1 in ES3 and undefined in ES5+
(function(){
    undefined = 1;
    return undefined;
})();

// will throw TypeError in ES5+
(function(){
    'use strict';
    undefined = 1;
})();
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Another common case is to minify code as void 0 is shorter than undefined:

foo = void 0;
foo = undefined;
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When used with IIFE (immediately-invoked function expression), void can be used to force the function keyword to be treated as an expression instead of a declaration:

var foo = 1;
void function(){ foo = 1; }() // will assign foo a value of 1
+function(){ foo = 1; }() // same as above
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function(){ foo = 1; }() // will throw SyntaxError
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Some code styles prohibit void operator, marking it as non-obvious and hard to read.

Rule Details

This rule aims to eliminate use of void operator.

Examples of incorrect code for this rule:

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

void foo
void someFunction();

var foo = void bar();
function baz() {
    return void 0;
}
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Options

This rule has an object option:

allowAsStatement set to true allows the void operator to be used as a statement (Default false).

allowAsStatement

When allowAsStatement is set to true, the rule will not error on cases that the void operator is used as a statement, i.e. when it’s not used in an expression position, like in a variable assignment or a function return.

Examples of incorrect code for { "allowAsStatement": true }:

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/*eslint no-void: ["error", { "allowAsStatement": true }]*/

var foo = void bar();
function baz() {
    return void 0;
}
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Examples of correct code for { "allowAsStatement": true }:

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/*eslint no-void: ["error", { "allowAsStatement": true }]*/

void foo;
void someFunction();
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When Not To Use It

If you intentionally use the void operator then you can disable this rule.

Version

This rule was introduced in ESLint v0.8.0.

Resources

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no-void

Rule Details

Options

allowAsStatement

When Not To Use It

Related Rules

Version

Further Reading

Resources