JavaScript: The Definitive Guide, 6th Edition

null and undefined

null is a language keyword that evaluates to a special value that is usually used to indicate the absence of a value. Using the typeof operator on null returns the string “object”, indicating that null can be thought of as a special object value that indicates “no object”. In practice, however, null is typically regarded as the sole member of its own type, and it can be used to indicate “no value” for numbers and strings as well as objects. Most programming languages have an equivalent to JavaScript’s null: you may be familiar with it as null or nil.

JavaScript also has a second value that indicates absence of value. The undefined value represents a deeper kind of absence. It is the value of variables that have not been initialized and the value you get when you query the value of an object property or array element that does not exist. The undefined value is also returned by functions that have no return value, and the value of function parameters for which no argument is supplied. undefined is a predefined global variable (not a language keyword like null) that is initialized to the undefined value. In ECMAScript 3, undefined is a read/write variable, and it can be set to any value. This error is corrected in ECMAScript 5 and undefined is read-only in that version of the language. If you apply the typeof operator to the undefined value, it returns “undefined”, indicating that this value is the sole member of a special type.

Despite these differences, null and ...