Have you ever tried to validate NaN using == or ===? could you correctly guess the output of below statements? 1. x=parseInt("string") 2. console.log(x) 3. x==NaN 4. x===NaN 5. NaN===NaN 6. isNaN(x) well, if you correctly answerd all the statements then congtars, but if don't; don't worry, i will try to give a walk through. Let's first the output first - x=parseInt("string") NaN console.log(x) NaN x==NaN false x===NaN false NaN===NaN false isNaN(x) true It is not possible to rely on the equality operators (== and ===) to determine whether a value is NaN or not, because both NaN == NaN and NaN === NaN evaluate to false. Hence, the necessity of an isNaN function. source - http://droidscript.org