var test_string = 'Hello',
      test_array = ['one', 'two', 'three'],
      test_number = 42;
      test_object = { key: 'value' };
      test_function = function() {};

  test_string.constructor == String;// true
  test_array.constructor == Array;// true
  test_number.constructor == Number;// true
  test_object.constructor == Object;// true
  test_function.constructor == Function;// true

  test_string.constructor == Object;// false
  test_array.constructor == Object;// false
  test_number.constructor == Object;// false
  test_object.constructor == Function;// false
  test_function.constructor == Object;// false

The only drawback is that it’s a bit cumbersome, and a little confusing. I like to wrap this in a bit of sugar.

  function is_typeof(type, suspect) {
    if (type === undefined) {
      throw new Error('is_typeof(type, suspect): type is undefined.');
    }

    if (suspect === undefined) {
      throw new Error('is_typeof(type, suspect): suspect is undefined.');
    }

    return suspect.constructor == type;
  }

Note: I use snake case because I like it, feel free to judge me and change it to something you’d prefer.

I go with is_typeof because it’s a little more definitive and it bears resemblance to the well-known typeof keyword so it indicates what it does. I’m open to better names for this one but I think it’s short sweet and to the point.

This may look a bit weird, but it makes for a pretty clear syntax. Let’s take a look at this in action:

  function Unicorn(arg) {

    if (is_typeof(String, arg)) {
      this.name = arg;
    }

    if (is_typeof(Object, arg)) {
      var obj = arg;
      for (var prop in obj) {
        if(obj.hasOwnProperty(prop)) {
          this[prop] = obj[prop];
        }
      }
    }

    return Unicorn;
  };

Here’s a simple example of a basic custom object, with the ability to accept either a string or an object in the constructor. This gives us a sort-of dual API to create Unicorns. How sweet is that. Seriously, you can make some pretty useful functions with this pattern. Although I should point out that you shouldn’t do this just because you can, this technique is an investment in complexity and too much of it is unhealthy. If you’re testing, which you should be, this adds a significant amount of surface area to test. It works great for constructors and cases where you want the same basic action to be run with more or less data.

This is only one simple technique using the is_typeof method. Another use is hardening up your code by making strict type requirements, among others. It’s a good trick to have in your bag and it’s syntax makes it pretty easy to figure out at a glance. It really helps you write better code.

If you want to take this to the next level you can curry this function to make shorthand methods for any type you like:

  function is_string(suspect) {
    return is_typeof(String, suspect);
  }

This makes code like the following pretty short and sweet:

  var greeting = 'Hello World!';

  if (is_string(greeting)) {
    alert(greeting);
  }