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;
})();
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:
Open in Playground
/*eslint no-void: "error"*/
void foo
void someFunction();
var foo = void bar();
function baz() {
return void 0;
}
Options
This rule has an object option:
allowAsStatementset totrueallows the void operator to be used as a statement (Defaultfalse).
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 }:
Open in Playground
/*eslint no-void: ["error", { "allowAsStatement": true }]*/
var foo = void bar();
function baz() {
return void 0;
}
Examples of correct code for { "allowAsStatement": true }:
Open in Playground
/*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.
Related Rules
Version
This rule was introduced in ESLint v0.8.0.
Further Reading