Null and undefined in JavaScript

1. Undefined and null are automatically converted to false in the Boolean unary count.

2. Null = = Undefined;//true

Null = = Undefined;//false

2. typeof Null;//object

typeof undefined;//undefined

3. Number (null);//0

Number (undefined);//nan

4. Current Usage:
Null means "no object", meaning there should be no value here. Typical usage:
(1) As a function parameter, the parameter of the function is not an object.
(2) As the end point of the prototype chain of the object.
Object.getprototypeof (object.prototype);//null

Undefined means "missing value". Typical usage:
(1) The variable is declared and has not been assigned, and the variable is equal to undefined.
var data;
data;//undefined
(2) When the function is called, the argument that should be supplied is not provided, and the parameter is equal to undefined.
function f (data) {Console.log (data);}
f ();//undefined
(3) The object does not have an assigned property, and the value of this property is undefined.
var obj = new Object ();
obj.p;//undefined
(4) When the function does not return a value, undefined is returned by default.
var res = (function () {}) ();//undefined

Null and undefined in JavaScript