null (JavaScript: The Definitive Guide, 4th Edition)

3.7. null

The JavaScript keyword null is a special value that indicates no value. null is usually considered to be a special value of object type -- a value that represents no object. null is a unique value, distinct from all other values. When a variable holds the value null, you know that it does not contain a valid object, array, number, string, or boolean value.[9]

[9]C and C++ programmers should note that null in JavaScript is not the same as 0, as it is in those languages. In certain circumstances null is converted to 0, but the two are not equivalent.