What is the purpose of obj === void 0 or void(alert(…))?

I’ve seen code like this floating around the internet, always thinking the expression was a function all to void():

<a href="javascript:void(alert('Warning!!!'))">Click me!</a>


void is an operator, akin to typeof, instanceof, and new. The void operator specifies an expression to be evaluated without returning a value from the expression.

It always returns undefined.

Is this useful?

Since we know the undefined property can be manipulated in older browsers and we can use it as a custom variable name, let’s see if we can have some fun.

I’ve created an iife, manually overriding the property undefined to be true. The 2nd parameter is actually undefined since we did not pass it value when invoking the function.

(function(undefined, reallyUndefined) {
  console.log(undefined === true);              // true
  console.log(undefined === reallyUndefined);   // false
  console.log(reallyUndefined === void 0);      // true

So we can’t rely on undefined to be truly undefined. But we can rely on the return value of void 0 to be undefined.

And that is why you can find void 0 in unexpected places, such as all over the underscore.js source

 _.isUndefined = function(obj) {
  return obj === void 0;