ES6 new variable modifiers let and const, new basic data type symbol

ES6 Brief introduction

ES6, Full name ECMAScript 6.0 , yes JavaScript The next version of the standard ,2015.06 release .

ES6 Mainly to solve ES5 The innate deficiency of , such as JavaScript There's no concept of class in it , But the current browser's JavaScript yes ES5 edition , Most advanced browsers also support ES6, But only achieved ES6 Part of the features and functions of .

New variable modifier let and const

let yes javascript Key words of  

Its characteristics ：

1、 Within a code block , Variable application let After keyword declaration , This code block has a local scope （ Called block level scopes ）, This variable cannot be accessed outside the code block .

2、 You can declare that you do not assign values , But the variable declaration must be written before the access statement , Otherwise, an error will be reported .

3、 In the same block of code , Use of the let Declared variable name , Not enough to declare a variable name with the same name

Code display ：

<script>
        console.log(a)
			let a=0;
        // Application error 
</script>

{
    let a=0;
}
console.log(a)

{
let a
a=0
let a=9
}

contrast var keyword ：

1、var The declared variable has no scope

2、 You can declare that you do not assign values , You can also write the access before the declaration , Because there is precompiled , In the browser. js This variable is implicitly declared in advance when the engine scan script is executed .

3、 You can declare two variable names with the same name at the same time

const yes JS A constant modifier of

var and let The value of the declared variable can be modified , and const Once the declared variable is assigned, it cannot be changed , When it is used to declare variables at the same time , It must be initialized, otherwise an error will be reported , Often used for modular development .

It is associated with let Same keyword , The code block of the variable declared with it will form a block level scope , At the same time in the same code block , You cannot declare two identical variable names at the same time

const Declared object , The value of the object can be modified and increased , Because it is a reference object , It is a storage space , The storage space does not change when the value is modified .

symbol

ES5 The basic data types of are number、boolean、string、undefined、null

ES6 New basic data type , Used to ensure the uniqueness of a data , Avoid repetition in the program .

Illustrate with examples ：

<script>
    var re=symbol("aaa")
    var re2=symbol("aaa")
    console.log(re==re2)//false
    // Come to the conclusion ： Even if the values of two numbers are the same , But it is symbol The data is not equal .
</script>