Question

In Javascript i have read that we can define our own value for undefined. Initially undefined == null is true.

1. After we change the value of undefined will undefined == null still be true?
2. By assigning our own value to undefined does it mean that we can assign real numbers to it?
3. Why are there two absence of value thingies like null and undefined? Couldn’t we just do with one? Is there any difference between them?

Answer 1

undefined is a variable on the global object which is window in browser environments. Its initial value is the primitive undefined.

Being a property on the global object, historically you could have changed its value as,

window.undefined = "42"; // or just undefined = "42"

The meaning of life is now clearly defined. But since EcmaScript-5 is out, this has been disallowed, and even though it is still a property of the global object, it has been made non-writable now.

The primitives null and undefined are not the same thing if no tampering has occurred.

Null is a data type that has the sole value null. Undefined is another data type whose sole value is the primitive undefined. You can verify whether they represent the same object or not easily.

null === undefined // false

However,

null == undefined // true

is true, because they are both casted to the boolean value false before a comparison is made. The rules for converting both these values to boolean are clearly defined in section 9.2 of the spec.

9.2 ToBoolean

Argument Type | Result
-------------------------------------------------------------------------
Undefined     | false
Null          | false
Boolean       | The result equals the input argument (no conversion).
Number        | ..
String        | ..
Object        | ..