Detailed syntax of ES6

First, define the variable let (similar to var)

There has always been a bug in js which is var:

1. Variables declared by var will have variable promotion

console.log(name);  //jhonvar name = 'jhon';

2. Var does not have block-level scope

var name2 = 'jjjon';
{    var name2 = 'tom';
}
console.log(name2);     //tom

3. var can be used to define a variable multiple times, and the subsequent variable replaces the previous variable

var name3 = 'jond';var age = 18;var name3 = 19;var name3 = 'rose';
console.log(name3);     //rose

Newly defined variables let:

1. Variables declared by let will not be promoted. They can only be used later if they are defined in the front.

console.log(name4);     //报错let name4 = '1112';

2. let has block-level scope

let name5 = '222';
{
    let name5 = ' ttt';
}
console.log(name5);　　//222

3. let cannot redefine a variable multiple times

let name6 = 'aa';
let name6 = 'bb';　　  //报错console.log(name6);　　//aa

Second, const declares a constant

Constant: refers to data that will not change

1. The value cannot be changed

const pi = 3.01415;//pi = 3;     //报错
    {
        const arr = [5,6,8,9,];
        arr.push(7);
        console.log(arr);　　//(5) [5, 6, 8, 9, 7]
        arr = 10;     //值不能改变，否则报错
    }

2. Constants have block-level scope

{
    const ban = "vin";
}
console.log(ban);     //报错

3. There is no variable promotion, declare first and then use

console.log(ba);     //报错const ba = 'liu';

4. Constants with the same name cannot be declared

5. An initial value must be assigned, otherwise an error will be reported

const bb;     //报错

6. If an object is declared, the address of the object cannot be changed, but its internal attributes can be changed

const obj = {
    na:"jjjj",
    age:20};
console.log(obj.na);　　//jjjjobj.na = "ccs";
console.log(obj.na);　　//ccs

For example: application scenarios, fixed addresses can use constants

// var path = 1122// var path = '1243';const path = 'path';
console.log(path);　　//path

3. String expansion

1. Determine whether the string "hello word" exists "word"

var str = 'hello word';var result = str.indexOf('word');
console.log(result);　　//6

2. ES6 provides includes(): returns a Boolean value, used to determine whether the string contains certain characters

var bool = str.includes('word');
console.log(bool);    //true

3, startsWith(str[,num]): Returns a Boolean value, used to determine whether the string starts with a certain character

 bool2 = str.startsWith('hello'

//Pass in 2 parameters to this method
var bool3 = str.startsWith('word',6);
console.log(bool3); //true

4, endsWith(str[,num]): Returns a Boolean value, used to determine whether the string ends with certain characters

var bool4 = str.endsWith('d');
console.log(bool4);　　//true//给这个方法传入两个参数var bool5 = str.endsWith('o',7);
console.log(bool5);　　//false

5, repeat(num): Pass in a number, this method will repeat the string the number of times corresponding to the number

var str322= '我爱我家，\n';
console.log(str322.repeat(3));        //3行 我爱我家，

Four, 5.0 template syntax:`Template string`

var obj33 = {
    name:'zhuzhu',
    age:18,
    sex:'男',
    hobby:'女',
    veight:120,
    height:180};// 字符串拼接方法var str4 = '大家好，我叫:'+obj33.name+'，今年'+obj33.age+'，性别'+obj33.sex+'，爱好'+obj33.hobby+'。';
console.log(str4);        //大家好，我叫:zhuzhu，今年18，性别男，爱好女。// 但是字符串的拼接太麻烦了，有没有简单的方法来解决这个问题呢，有的，模板字符串就可以了var temp111 = `大家好，我叫${obj33.name}，今年${obj33.age}，性别${obj33.sex}，爱好${obj33.hobby}`;
console.log(temp111);    //大家好，我叫zhuzhu，今年18，性别男，爱好女//1，可以是变量let name8 = "美女";
let temp22 = `我叫${name8}`;
console.log(temp22);    //我叫美女// 2，可以是方法function getName(){    return "宝宝";
}
let temp3 = `我叫${getName()}`;
console.log(temp3);        //我叫宝宝// 3,可以是表达式let aa = 1 ;
let bb = 2;
let temp4 = `a + b=${aa+bb}`;
console.log(temp4);        //a + b=3

Five, 6.0 Arrow function: ( )=>{}

// 最原始函数var arr = [2,3,5,7];
 $(arr).each(function(index,item){
   console.log(item);
 }); // 箭头函数改造// 改造一：匿名函数中的funtion关键字我们可以省略，参数与方法体之间中=>$(arr).each((index,item)=>{console.log(item);})
 // 改造二：如果方法体中的代码只有一句我们可以去掉{},并且代码结尾的分号要去掉$(arr).each((index,item) =>console.log(item));

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