Leaking and Non-Leaking Arrow Functions in JavaScript

When writing arrow functions that shouldn't return any value you may be tempted to return an || operation such as:

/* Leaking Arrow Function */
const arrowThatShouldntReturn = () => someOtherFunction() || undefined;

This may work if someOtherFunction returns a falsy value, but won't work for truthy values.

The Void Operator

The void unary operator evaluates the given expression and then returns undefined. You'll see this operator used all over the React codebase to create non-leaking arrow functions!

To use it we can define our functions as:

/* Non-leaking Arrow Function */
const arrowThatShouldntReturn = () => void someOtherFunction(); // returns undefined

Or when we don't wish to do anything yet:

/* Non-leaking Arrow Function */
const arrowThatShouldntReturn = () => void; // returns undefined

The first method will evaluate someOtherFunction() and return undefined regardless of the returned value. The second method is equivalent to void undefined which evaluates undefined and returns undefined.

Are there other use cases? Yes.

Before ES6

In old times you used to define variables using var. This immediately adds the variable you're declaring to the global object. It also pretty much didn't have restrictions, so you could just var undefined = true and it'd work. This is one of the reasons void 0 used to be the preferred method. There's even an ESLint rule for it! It's not necessary in strict environments though.

HTML

You probably also met the void operator in anchor tags! Ever met this bad boy?

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

Yup, that's it. We used it to pretty much create links that preventDefault.

IIFE

You can use it with IIFEs too!

(function() { return true })(); // returns true

void function() { return true }(); // returns undefined

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