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What is “void 0” in JavaScript?

Is it similar to undefined? Should we avoid it or not?

Photo by Andrea De Santis on Unsplash

Hi, I'm a software engineer, and welcome to my blog. Today, we are going to discuss void 0.

What we should know about void 0

In older versions of JavaScript, you wouldn't have found void 0 expressions. The keyword is **void **which **is **undefined.


void 0
void (0)
void "hello"
void (new Date())
//all will return undefined

undefined is mutable

Photo by Valentin Lacoste on Unsplash

So why do we use **void 0 **as an alias for undefined?

One of JavaScript's quirks is that it is not a reserved keyword. Instead, it is a property of the global object.

According to,

Undefined is nothing but a global variable name without a default value. Therefore, its primitive value is undefined. You can change the value of undefined:

var a = {};
a.b === undefined; // true because property b is not set
undefined = 42;
a.b === undefined; // false

Due to the mutability of undefined, it is generally a better idea to check for undefined-ness through typeof:

var a = {};
typeof a.b == 'undefined'; // always true

The undefined global variable has been disabled in ES5 as shown above.


Look at the variable disfunction when the variable will be printed. It is still undefined as a value.

The ESLint rule, no-undefined, disallows the use of undefined as a variable name and prevents shadowing issues.

How about void 0 as an undefined value? Should we avoid this?

Yes, you don't need void 0 in ES5 and newer. Keep your code readable and let the minifiers optimize the performance.


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