Section 3.8. undefined

3.8. undefined

Another special value used occasionally by JavaScript is the undefined value. undefined is returned when you use either a variable that has been declared but never had a value assigned to it or an object property that does not exist. Note that this special undefined value is not the same as null.

Although null and the undefined value are distinct, the == equality operator considers them equal to one another. Consider the following:

 my.prop == null 

This comparison is TRue if either the my.prop property does not exist or it does exist but contains the value null. Since both null and the undefined value indicate an absence of value, this equality is often what we want. However, if you truly must distinguish between a null value and an undefined value, use the === identity operator or the typeof operator (see Chapter 5 for details).

Unlike null, undefined is not a reserved word in JavaScript. The ECMAScript v3 standard specifies that there is always a global variable named undefined whose initial value is the undefined value. Thus, in a conforming implementation, you can treat undefined as a keyword, as long as you don't assign a value to the variable.

If you are not sure that your implementation has the undefined variable, you can simply declare your own:

 var undefined; 

By declaring but not initializing the variable, you assure your implementation has the undefined value. The void operator (see Chapter 5) provides another way to obtain the undefined value.

When the undefined value is used in a Boolean context, it converts to false. When used in a numeric context, it converts to NaN. And when used in a string context, it converts to "undefined".