How does “+var === +var” work internally to verify if var is numeric?

Question

Seeing this question: Is there a (built-in) way in JavaScript to check if a string is a valid number? and this: jsperf, one of the presented approaches is this (mutatis mutandis

Answer 1

+ converts the value to a number.

a gets converted to 123 and 123 === 123.

b gets converted to NaN but NaN !== NaN (because NaN is never equal to another NaN according step 4a of the equality rules).