When is "undefined" equal to "null" in JavaScript?

Question

I've been watching some videos from Tut+ about Js.They said that sometimes "undefined" is equal to "null". So, when does this happen ?

Answer 1

undefined == null
// => true

undefined === null
// => false

== tests for equality, === tests for identity (or strict equality). If in doubt, use ===.

Answer 2

Just to add on , This question is somehow answered already, check here

You can just check if the variable has a truthy value or not. That means

 if( value ) {

 }

will evaluate to true if value is not:

null

undefined

NaN

empty string ("")

0

false