This operator allows evaluating expressions that produce a value into places where anexpression that evaluates toundefined is desired.

Thevoid operator is often used merely to obtain theundefined primitive value, usually usingvoid(0) (which isequivalent tovoid 0). In these cases, the global variableundefined can be used.

It should be noted thatthe precedenceof thevoid operator should be taken into account and thatparentheses can help clarify the resolution of the expression following thevoid operator:

void 2 === "2"; // (void 2) === '2', returns falsevoid (2 === "2"); // void (2 === '2'), returns undefined

When using animmediately-invoked function expression, thefunction keyword cannot be at the immediate start of thestatement, because that would be parsed as afunction 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 anexpression, thefunction keyword has to appear at a position that only accepts expressions, not statements. This can be achieved by prefixing the keyword with aunary operator, which only accepts expressions as operands. Function invocation has higherprecedence than unary operators, so it will be executed first. Its return value (which is almost alwaysundefined) 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 thefunction keyword to be parsed as the start of an expression instead of a statement.

(function () {  console.log("Executed!");})();

Note that this trick only applies to IIFEs defined with thefunction keyword. Attempting to use thevoid operator to avoid parentheses for an arrow function results in a syntax error. Arrow function expressions always require parentheses around them when being called.

void () => { console.log("iife!"); }(); // SyntaxError: Malformed arrow function parameter list

When a browser follows a#"/en-US/docs/Web/JavaScript/Reference/Global_Objects/undefined">undefined. Thevoid operator can be used to returnundefined. For example:

<a href="#">Click here to do nothing</a><a href="#">  Click here for green background</a>

checkbox.onclick = () => doSomething();

checkbox.onclick = () => void doSomething();