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


    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.