Grammatical composition, keywords and reserved words, variables

1. Syntax

1.1 If the introduction of external JS, then in the script tag can not write the JS code

<script src= "" ></script>

1.2 Case-sensitive

1.3 Identifier composition begins with: A (letter), _, $

1.4 Cannot put keyword, reserved word, true false null as identifier

2. Variables

var a;//create (DECLARE) variable var a=1;//create variable, and variable initialization; a=1+a;//variable operation

3. Data type

var a;//create variable var a=1;//create variable, and variable initialization; Action of a=1+a;//variable a = 1;//value Numbera = 0.01;//integer and real number share a data type Numbera = ' Hello world ';//String St Ringa = "JavaScript";//double-quoted string Stringa = true;//Boolean Booleana = false;//to True the opposite Boolean value Booleana =null; The null type is a privileged value, meaning ' empty ' Nulla =undefined;//undefined and null are very similar undefined
A = {}; Objects Object

The difference between 3.1undefined and null

Undefined

A. There is no need to assign a value of undefined to a variable, or it can be ' empty ', and JS will automatically assign undefined to undefined variables.

B. Declaring variables must be initialized.

var a;//create variable alert (a); Undefined undefined variables will pop up

Null

A. Objects that represent "empty"

var B = {};alert (typeof B); B is a null type, the returned value [Object Object], the string of the return type is Objectalert (b);//Popup [Object Object]

Example:

alert (undefined = = null); Returns Truealert (undefined = = = null);//returns Falsealert (typeof undefined = = typeof null); Returns false

4.TYPEPOF (String that returns the type of data)

Example 1:

var a=1;//create variable alert (typeof a); Will pop up number A is the # type, the value is 1, and the string that returns the type is #

Example 2:

var B;alert (typeof B)//b is the undefined type, the return value is undefined, and the string of the return type is undefined

Example 3

var B = Null;alert (typeof b)//b is a null type, the returned value [Object Object], the string of the return type is an object

5. Initialization of different data types

var box = '; String initialization is generally an empty string var box = 0;//number initialization, generally with 0var box = False;//boolean initialization, generally false or True;var box = {}; Obeject, generally with empty objects

Implicit conversion of 6.Boolean

Grammatical composition, keywords and reserved words, variables