Kit Menke's Blog

Working with JavaScript can definitely be painful, but I always love stumbling across interesting and strange features of the language. I just found wtfjs.com which led me to these two answers on stackoverflow.

The first answer made me realize that undefined is not a keyword in JavaScript; it actually is a type that also happens to have a global variable with the same name. This means that you can change the value of undefined (the global variable) to be something else.

And, like shown above, the correct way to check if a variable is undefined is by using typeof.

The second feature, is something that I often used but had no idea why it actually worked: void(0).

According to Breton’s excellent explanation, void is actually a prefix operator. When you prefix any expression with void, the result evaluates to undefined. So, using our correct method for checking undefined above you get this in the console:

Awesome!