An incrementally improved approach to a common JavaScript function/utility widely adopted all over. Returns the lowercase type or [[Class]] of a generic variable.

typeOf.js

var typeOf = (function(Object, RegExp){
  // WTFPL License - http://en.wikipedia.org/wiki/WTFPL
  // thanks to @jdalton and @ljharb
  var toString = Object.prototype.toString,
      cache = (Object.create || Object)(null);
  return function typeOf(Unknown) {
    var asString = typeof Unknown;
    return asString == 'object' ? (
      Unknown === null ? 'null' : (
        cache[asString = toString.call(Unknown)] || (
          cache[asString] = asString
            .slice(asString.indexOf(' ') + 1, -1)
            .toLowerCase()
        )
      )
    ) : asString;
  };
}(Object, RegExp));

jsPerf comparison here plus a mandatory thanks to @jdalton for ints and tips

the maximum amount of retained data in terms of strings and respective bytes is:

(Object.getOwnPropertyNames(this).filter(
  function(s){
    return  typeof this[s] == 'function' &&
            s[0] == s[0].toUpperCase() &&
            !/_/.test(s);
  },
  this
).map(
  function (s) {
    // toLowerCase() not needed
    return '[object ' + s + ']' + s.toLowerCase();
  }
).join('').length * 2 / 1024).toFixed(2) + 'KB';

Right now the amount in nodejs is 1.42KB and that's the worst case scenario assuming every single available instance of each type is passed to this function.

This looks wrong: RegExp.lastMatch.toLowerCase(). Not part of ES5.

nothing is wrong ;-) typeOf([]) is the string 'array' indeed, as example. RegExp.lastMatch is universally available since ever and it will be there for backward compatibility.

FWIW, RegExp.lastMatch has been in http://javascript.spec.whatwg.org/#regexp.lastmatch as a TODO item since its inception. Still needs to be specced, though.

What's Special About This Code

• it does not require pre memory allocation as a statically pre-filled map/object would do
• it is incrementally optimized and compatible with every new native type that will be passed to the function (int32array, htmlcanvaselement, any other available type)
• it is compatible with every JS environment being string representation agnostic and filled on demand using those representations ([array Array] or [JavaPackage nu.validator] cases )
• it has ~zero runtime impact avoiding dynamic pre-filling of any native type