A Better is_a Function for JS

In 2007 I have posted about get_class and is_a functions in JavaScript in order to simulate original PHP functions.

Well ... that was crap, since a much simpler and meaningful version of the is_a function can be easily summarized like this:

var is_a = function () {
  function verify(what) {
    // implicit objet representation
    // the way to test primitives too
    return this instanceof what;
  }
  return function is_a(who, what) {
    // only undefined and null
    // return always false
    return who == null ?
      false :
      verify.call(who, what)
    ;
  };
}();

... or even smaller with explicit cast ...

function is_a(who, what) {
  // only undefined and null
  // return always false
  return who == null ?
    false :
    Object(who) instanceof what
  ;
}

An "even smaller" alternative via @kentaromiura

function is_a(who, what) {
  return who != null && Object(who) instanceof what;
}

Here a usage example:

alert([
  is_a(false, Boolean),   // true
  is_a("", String),       // true
  is_a(123, Number),      // true
  is_a(/r/, RegExp),      // true
  is_a([], Array),        // true
  is_a(null, Object),     // false
  is_a(undefined, Object) // false
]);

As twitted few minutes ago, an alternative would be to pollute the Object.prototype:

Object.defineProperty(Object.prototype, "is_a", {
  value: function (constructor) {
    return this instanceof constructor;
  }
});

// (123).is_a(Number); // true

However, this way would not scale with null and undefined so that per each test we need to check them and this is boring.
Finally, I would not worry about cross frame variables since via postMessage everything has to be serialized and unserialized.