Null, empty objects, and undefined

null: Is an object, but a null reference (not pointing to any object)

Empty Object : is an object, but its value is a reference to an object that does not have any properties

undefined: Undefined, so not an object, which itself is defined as a special type of "undefined"

1. When null participates in numeric operations, its value is automatically converted to 0, so the following expressions are evaluated to get the correct values:

Expression: 123 + null result value: 123

Expression: 123 * Null result value: 0

2. When undefined participates in any numerical calculation, the result must be Nan.

3. Alert (typeof undefined); //output "undefined"

Alert (typeof null); //output "Object"

Alert (null = = undefined); //output "True" ECMAScript that undefined is derived from null, so define them as equal

Alert (null = = = undefined); //output "false" = = = Absolute equals

Alert (typeof null = = typeof undefined); //output "false"

Null, empty objects, and undefined