What Does Void 0 Do In JavaScript? - Mastering JS

Tutorials / Fundamentals / What Does void 0 Do in JavaScript? Jul 23, 2019

The void operator in JavaScript evaluates an expression and returns undefined. At first glance, this operator doesn't seem useful, but there are 3 cases where you may see the void operator in practice. Here are the 3 major use cases:

No Overwriting undefined

Surprisingly enough, undefined is a valid variable name in JavaScript. The below script will print out "test" twice.

const undefined = 'test'; function foo() { return undefined; } function bar(undefined) { return undefined; } console.log(foo()); console.log(bar('test'));

However, void is an operator. That means, if you try to name a variable void, JavaScript will throw the below error.

SyntaxError: Unexpected token void

Some JavaScript projects prefer void 0 instead of undefined to avoid accidentally creating variables named undefined. There is even an ESLint rule to disallow undefined for this reason.

No-Op for <a> Tags

Another use case for the void operator is to make an <a> tag with a javascript: URI do nothing. An <a> tag is not valid HTML unless it has a name or href property, so javascript: void(0) is a common trick to make an <a> tag render correctly without adding an actual link.

<a href="javascript:void(0)">Click Here</a>

Below is how the above HTML looks:

Click Here

Immediately Invoked Function Expressions (IIFEs)

To declare a function and execute it in the same statement, you normally would use parentheses to force JavaScript to handle the function declaration as an expression.

(function() { console.log('Hello, World'); })(); // Prints "Hello, World"

You may occasionally see the void operator as an alternative to parentheses. Because void is a unary operator, it tells JavaScript to treat the function declaration as an expression, so the below code prints "Hello, World."

void function() { console.log('Hello, World'); }();

The difference is that a normal IIFE can still return a value, whereas using void for IIFE will always evaluate to undefined.

(function() { return 42; })(); // 42 void function() { return 42; }(); // undefined Did you find this tutorial useful? Say thanks by starring our repo on GitHub!

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