Sam Stites

A useful == in Javascript

17 May 2014 - San Francisco, CA

Today I was reviewing John Resig’s Secrets of a Javascript Ninja Quiz and noticed in section 21 that, when checking values in a cache, he uses a double-equals sign to see if a value exists on the object. Check it out:

function isPrime( num ) {
  if ( isPrime.cache[ num ] != null )
    return isPrime.cache[ num ];

  var prime = num != 1; // Everything but 1 can be prime
  for ( var i = 2; i < num; i++ ) {
    if ( num % i == 0 ) {
      prime = false;

  isPrime.cache[ num ] = prime

  return prime;

At first I thought this was a slip up, however after digging further, I noticed that this loose equality only applies to two objects: the undefined object and the null object - not all falsy values. While Dr. Axel Rauschmayer paints a clear picture as to why == is confusing and we should tend to avoid it, I must admit that Resig’s example might be the only place I have seen an example of where it could be considered legitmate: this provides a cleaner way to check for both undefined and null inputs in one line. While I still don’t think that you should ever use == in your code for other reasons, I think it’s very powerful to see specific cases where it does provide use. benvie provides a very thorough answer on the differences between how == and === work after digging directly into V8 which you can find on the SO question: JavaScript performance difference between double equals (==) and triple equals (===)