The void operator evaluates the given
expression and then returns undefined.
Syntax
void expression
Description
This operator allows evaluating expressions that produce a value into places where an expression that evaluates to undefined is desired.
The void operator is often used merely to obtain the
undefined primitive value, usually using void(0) (which is
equivalent to void 0). In these cases, the global variable
undefined can be used.
It should be noted that the precedence
of the void operator should be taken into account and that
parentheses can help clarify the resolution of the expression following the
void operator:
void 2 === "2"; // (void 2) === '2', returns false
void (2 === "2"); // void (2 === '2'), returns undefined
Examples
Immediately Invoked Function Expressions
When using an immediately-invoked function expression, the function keyword cannot be at the immediate start of the statement, because that would be parsed as a function declaration, and would generate a syntax error when the parentheses representing invocation is reached — if the function is unnamed, it would immediately be a syntax error if the function is parsed as a declaration.
function iife() {
console.log("Executed!");
}(); // SyntaxError: Unexpected token ')'
function () {
console.log("Executed!");
}(); // SyntaxError: Function statements require a function name
In order for the function to be parsed as an expression, the function keyword has to appear at a position that only accepts expressions, not statements. This can be achieved be prefixing the keyword with a unary operator, which only accepts expressions as operands. Function invocation has higher precedence than unary operators, so it will be executed first. Its return value (which is almost always undefined) will be passed to the unary operator and then immediately discarded.
Of all the unary operators, void offers the best semantic, because it clearly signals that the return value of the function invocation should be discarded.
void function () {
console.log("Executed!");
}();
// Logs "Executed!"
This is a bit longer than wrapping the function expression in parentheses, which has the same effect of forcing the function keyword to be parsed as the start of an expression instead of a statement.
(function () {
console.log("Executed!");
})();
JavaScript URIs
When a browser follows a javascript: URI, it evaluates the code in the URI
and then replaces the contents of the page with the returned value, unless the returned
value is undefined. The void operator can be used to return
undefined. For example:
<a href="javascript:void(0);">Click here to do nothing</a>
<a href="javascript:void(document.body.style.backgroundColor='green');">
Click here for green background
</a>
Note:
javascript:pseudo protocol is discouraged over other alternatives, such as unobtrusive event handlers.
Non-leaking Arrow Functions
Arrow functions introduce a short-hand braceless syntax that returns an expression. This can cause unintended side effects by returning the result of a function call that previously returned nothing. To be safe, when the return value of a function is not intended to be used, it can be passed to the void operator to ensure that (for example) changing APIs do not cause arrow functions' behaviors to change.
button.onclick = () => void doSomething();
This ensures the return value of doSomething changing from
undefined to true will not change the behavior of this code.