null == undefined , null === undefined, which one will return true in JavaScript ?

Answer 1

The first one (null == undefined).

If you want to know why Strict Equality Comparison (===) works different from Abstract Equality Comparison (==), you should review this article:

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Equality_comparisons_and_sameness

Answer 2

null == undefined.