What is the difference between (NaN != NaN) and (NaN !== NaN)?

Question

First of all I want to mention that I know how isNaN() and Number.isNaN() work. I am reading The Definite Guide by David Flanagan and he g

Answer 1

For purposes of NaN, != and !== do the same thing.

However, many programmers avoid == or != in JavaScript. For example, Douglas Crockford considers them among the "bad parts" of the JavaScript language because they behave in unexpected and confusing ways:

JavaScript has two sets of equality operators: === and !==, and their evil twins == and !=. The good ones work the way you would expect.

...My advice is to never use the evil twins. Instead, always use === and !==.