Disallow use of the void operator. (no-void)

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;
})();

Another common case is to minify code as void 0 is shorter than undefined:

foo = void 0;
foo = undefined;

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
function(){ foo = 1; }() // will throw SyntaxError

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:

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

void foo
void someFunction();

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

Options

This rule has an object option:

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 }:

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

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

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

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

void foo;
void someFunction();

When Not To Use It

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

Further Reading

Related Rules

Version

This rule was introduced in ESLint 0.8.0.

Resources