Undefined vs. Null in JavaScript

undefined and null are two very special values in JavaScript. They are both of its own data type, and they both indicate the lack of something meaningful. In most cases, you can treat them as the same thing.

When you apply the typeof operator on undefined, it returns undefined.

javascript

1typeof undefined;

This proves that undefined is of its own data type. When a variable is initialized without a value, it will be assigned undefined automatically. It represents the absence of a meaningful value.

null is very similar. It defines an empty value. A value exists, but it is empty.

The fact that there are two values defining the same thing is just a mistake in JavaScript's design. In most cases, you can see them as interchangeable.

In fact, when you compare them, JavaScript will tell you they are the same.

javascript

1undefined == null;

But when you use === (equal value and equal type), JavaScript will return false.

javascript

1undefined === null;

That's because they are both unique and of different data types.

However, when you apply the same typeof operator on null. JavaScript will tell you it is an object.

javascript

1typeof null; // -> object

This is actually another mistake in JavaScript. null is of type null, not an object. The only reason that JavaScript hasn't fixed these mistakes is that it might break some older software.