JavaScript summed up #3: Type checking

Since JavaScript currently is a dynamically typed language, the topic of type checking is quite interesting. In certain situations it’s rather important to know of which type the object at hand really is.

Here, we have some dummy objects that we want to type-check:

var num = 12;
var string = "Hello World";
var array = [1,2,3];
var object = { firstProp: 1, secondProp: 2 };
function Car(){};
var car = new Car();

Let’s look at two different ways to check the type of an object.

typeof

With the help of the typeof keyword, you have one possibility to check an object’s type. An example usage could look like this:

if ( typeof num == "number" ) ... // true
if ( typeof array == "array") ... // false, because "object" is returned
if ( typeof car == "Car" ) ... // false, because "object" is returned

The typeof way works nicely with most object types. But unfortunately, it leaves a little uncertainty when dealing with types like Array or your own types (like “Car” in our dummy examples). That’s because typeof only returns the not so helpful “object” in those cases. So let’s look at another way…

constructor

Every JavaScript object has a property called “constructor” – which proves to be helpful in our case:

if ( car.constructor == Car ) ... // true
if ( array.constructor == Array ) ... // true

The constructor-variant has the advantage of returning a reference to the function that was used when creating the object in question – not just “object”. Since a reference is returned, you have to remember not to use quotation marks (in contrast to the typeof variant, where a string is returned).