Strict equality operators in JavaScript: === and !==

The first time you see this, you think it’s a typo. But yes, === and !== are valid operators in JavaScript.

These operators do a strict comparison of equality by comparing both the type and the value. It’s easier to explain if I show you an example.

// This line will return true because both things mean 2
'2' == 2 

// This line will return false because on one side you have a string and
// on the other side you have a number.
'2' === 2 

Here’s another example:

var x = null;

if (x == undefined) // true
    // do something

if (x === undefined) // false!
    // do something

The first if statement evaluates to true because JavaScript says that null means the same as undefined, but the second if statement evaluates to false because null is not exactly the same as undefined.

Why would you care about this? Well, for the simple JavaScript that we write to manipulate stuff on a page of HTML, it probably doesn’t matter. But if you’re writing a complicated JavaScript library, this may be something that you might use. If anything, it’s good to know what it means so that you don’t freak out when you see it in someone else’s code.