5、 Constant, variable

1、 Literal （ Constant ）

1、 character string
Enclosed in single or double quotation marks
2、 Numbers
Don't distinguish between integers and decimals
3、 Boolean
Two values ：true、false
4、Object（ Array 、 object ）
5、 Custom constant
ES6 New in version , adopt const Custom constants , Constant names are usually capitalized , If it consists of more than one word , It can be separated by underscores

const  PI = 3.14
const  MY_SITE = 'http://www.baidu.com'

2、 Variable

1、 Variable = Variable name + A variable's value
2、 Declare variables ：var
Variables without assignment store undefined

var a;
console.log(a);  // undefined

3、 Defining variables
Declaration with assignment

var name;   // Declare a variable 
var address = "bj";  // Define a variable 

4、 Global and local variables
If a variable is declared outside the function , This variable is a global variable , You can access both inside and outside the function
If a variable is declared in the body of a function , This variable is a local variable , Can only be accessed within a function , Cannot be accessed outside a function

Don't use var You can also declare variables , This variable is called Global variables

str1 = "hello";  // Global variables 

5、 Define multiple variables at the same time

var c = d = 6;    // Equivalent to  var c = 6; d = 6; d Global variable 

var p = q = j = 888;  //q and j No, var, Global variable 
 //  Looking back and forth 
 // 1、888 Assign a value to j
 // 2、j = 888 The whole is a value , The left value , yes 888
 // 3、j = 888 The overall value 888 Assign a value to q
 // 4、q = 888 The overall value 888 Assign a value to p