In Javascript, to check whether a value is not a number.

isNaN.js

/*
 * In ES6, You can use Number.isNaN to check whether a number is not a number. Or in ES5, you should use a polyfill 
 * condition to check, `n !== n`
 */
Number.isNaN = Number.isNaN || function(value) {     
    return value !== value;
}