Nullability

JavaScript has two different values used to represent "nothing": null and undefined.

Undefined

The value undefined roughly means "this value doesn't exist". Here are a few common places where you'll see it:

• A variable that isn't assigned a value
• The default return value of a function
• When accessing an unknown key of an object

Null

The value null is used for object non-existence. It roughly means, "there is no object here".

Conceptually, it's similar to the empty string for strings, or 0 for numbers.

In practice, undefined is often used for this same purpose.