Eloquent JavaScript

More on Types

The previous should enable you to write and understand simple JavaScript programs. However, before closing the chapter, a few more subtleties have to be cleared up.

Undefined Values

It is possible to define a variable using var something;, without giving it a value. What happens when you take the value of such a variable?

var mysteryVariable;
mysteryVariable;
→ undefined

In terms of tentacles, this variable ends in thin air—it has nothing to grasp. When you ask for the value of an empty place, you get a special value named undefined. Functions that do not return a specific value but are called for their side effects, such as print and alert, also return an undefined value.

There is also a similar value, null, whose meaning is “this value ...