Why Do Top JavaScript Libraries Use void 0 Instead of undefined?

In JavaScript, undefined is a primitive value that represents “not defined”, appearing when a variable is declared without a value or a function returns nothing.

let a;
console.log(a); // undefined

function doNothing() {}
console.log(doNothing()); // undefined

Many high‑quality libraries such as React, Vue, or Lodash use the terse expression void 0, which raises the question of how it differs from undefined and why it is chosen.

The undefined value was originally not a keyword but a normal property on the global object ( window), meaning it could be overwritten, which could break code that relies on it.

In early JavaScript (non‑strict mode) or old browsers you could even assign a new value to undefined, causing all checks against it to fail—a potential disaster for robust libraries.

Since ES5, the global undefined property is read‑only ( writable: false), preventing reassignment. However, you can still shadow it in a local scope by declaring a variable named undefined.

The Dark History of undefined : It’s Not a Keyword

Keywords like true, false, and null cannot be used as variable names, but undefined originally could, making it mutable.

true = 1; // Uncaught SyntaxError: Invalid left-hand side in assignment

Because void is a unary operator, it evaluates its operand and always returns undefined, providing absolute safety.

console.log(void 1);        // undefined
console.log(void 'hello'); // undefined
console.log(void (1 + 1)); // undefined
console.log(void alert('hi')); // shows alert then undefined

The void operator cannot be overwritten, unlike a variable, so its result is reliably undefined in any environment.

The expression void 0 is simply the shortest form of using void (compared to void(0) or void 'any'), with 0 being one of the shortest possible operands.

Which Is Better?

From a safety perspective: void 0 guarantees a true, untampered undefined, which is why many high‑quality libraries prefer it.

From a minification perspective: void 0 is shorter than the literal undefined, saving a few bytes in large codebases.

From everyday development and readability: In modern ES5+ environments with linters, the risk of undefined being overwritten is negligible, so using the literal undefined is clear and intuitive.