undefined in javascript

undefined-js-notes.txt

The definition of undefined in ECMAScript 1st edition is

Undefined is a primitive value used when a variable has not been assigned a value.

It is also used:

as the returned value when trying to retrieve an object property that does not exist (after looking for it over the prototype chain).
1. If O doesn’t have a property with name P, go to step 4.
2. Get the value of the property.
3. Return Result(2).
4. If the [[Prototype]] of O is null, return undefined.
5. Call the [[Get]] method of [[Prototype]] with property name P.
6. Return Result(5).

as the default value for parameters that are not specified in a function call
If the caller supplies fewer parameter values than there are formal parameters, the extra formal parameters have value undefined.

as the void operator returned value
The production UnaryExpression : void UnaryExpression is evaluated as follows:
1. Evaluate UnaryExpression.
2. Call GetValue(Result(1)).
3. Return undefined.

as the default value returned by return in function calls
The RETURN statement
…
If Expression is omitted, the return value is the undefined value.

as the default function call result if it includes no return statement
… a function with an “empty body”; if it is invoked, it merely returns undefined.