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;
→ 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 ...

Get Eloquent JavaScript now with O’Reilly online learning.

O’Reilly members experience live online training, plus books, videos, and digital content from 200+ publishers.