JavaScript's void Operator

Originally published in the A Drip of JavaScript newsletter.

If you've been doing web development for any length of time, you've probably seen links that look something like this:

<a href="javascript:void 0;" onclick="doSomething()">Do something</a>

While this is a rather terrible line of HTML, there is something interesting going on in it: namely, the void 0 that is executed when a user clicks the link. What exactly does void 0 do? Lets ask our trusty console.

// Outputs: undefined
console.log(void 0);

That's interesting. If void 0 is equivalent to undefined, what about using void with other values?

// Outputs: undefined
console.log(void "test");

// Outputs: undefined
console.log(void {});

It turns out that void's one and only purpose is to make an expression evaluate to undefined. While this isn't useful in most situations, it can come in handy occasionally.

As we've talked about before, undefined isn't a JavaScript keyword, but a property of the global object. That means that undefined can be "shadowed" within a function's scope. Here is an example:

function shadowLog () {
    var undefined = "shadowed";
    console.log(undefined);
}

// Outputs: "shadowed"
shadowLog();

Under ordinary circumstances one can expect the undefined variable to refer to the undefined primitive. However, if you are dealing with variable shadowing or redefining undefined, then it can be helpful to use void as a guarantee that you are working with the undefined primitive, like so:

(function shadowFunc () {
    var undefined = "shadowed";
    var undefinedVar;

    // Outputs: false
    console.log(undefined === void 0);

    // Outputs: false
    console.log(undefinedVar === undefined);

    // Outputs: true
    console.log(undefinedVar === void 0);
})();

When using void, it is conventional to use 0 as the operand. Using void 0 here allows us to easily check whether a variable is undefined, even though the name undefined was shadowed in the function's scope.

That's it for this week. Thanks for reading!

Josh Clanton