Harish Mallipeddi

Javascript gotcha - undefined vs null

I always get confused over when to use what. So for my own reference (and possibly others), here’s an explanation:

• undefined is a special value whose type is also undefined
• null is a special object and hence its type is object
• When a variable has no value assigned to it yet, its type is undefined
• When an property is not defined on an object, that property is undefined
• A variable can be explicitly set to null and then the type of that variable will be object

More examples

5 == 5; // true
undefined == undefined; // true
typeof 5 == "number"; // true
typeof undefined == "undefined"; // true
typeof nada == "undefined"; // true
typeof window.doesnotexist == "undefined"; // true 
window.doesnotexist == null; // true
window.doesnotexist === undefined; // true
window.doesnotexist === null; // false
null == null; // true
typeof null == "object"; // true
null == undefined; // true
null === undefined; // false
!null && !undefined; // true

What to test for in if expressions?

If you just want to know if “x” is true-ish, use if (x) { }

If you want to know if it’s false-y, use if (!x) { }

(x == null) only tests if x is falsey, since (false == 0) and (” == false) and (undefined == null).

If you’re going to bother writing out (x == true), then it must matter that x is boolean “true” and not 1 or “asdf” or [1,2,3], so you should use ===. If it doesn’t matter, then skip the “== true” to make it clearer.

undefined and null are oddballs, since they’re false-y but != false. (null == undefined) but (null !== undefined).

Original Source

Do you’ve any other rule-of-thumbs that you follow related to this?