According to MDN,

NaN is a property of the global object. In other words, it is a variable in global scope.

The initial value of NaN is Not-A-Number — the same as the value of Number.NaN. In modern browsers, NaN is a non-configurable, non-writable property. Even when this is not the case, avoid overriding it. It is rather rare to use NaN in a program.

Hear the value of NaN is Number.NaN. But, wait!

NaN === Number.NaN // false

Maybe because of the type you think🤔

NaN == Number.NaN // false

Somewhat reasonable explanation started here when I followed the links.

With a lot of confusion around NaN, let us see how we could arrive at a value of NaN. There are 5 different operations which could result in NaN.

• Numbers that can't be parsed

  parseInt('Integer') ⇒ NaN
  Number('%^@#') ⇒ NaN
  

• Math operation where the result is not a real number

  Math.sqrt(-1); ⇒ NaN
  

• The operand of an argument is NaN

  NaN + 20 ⇒ NaN
  60 * NaN ⇒ NaN
  

• Indeterminate form

  0 * Infinity ⇒ NaN
  

• Any operation that involves a string and is not an addition operation

  "Integer" * 5 ⇒ NaN
  

For geekier discussions, reach out to me on twitter at @radnerus93, 📥 DM always open.