ES6-简介、语法

ES6

ES6简介

ECMAScript 6（简称ES6）是于2015年6月正式发布的JavaScript语言的标准，正式名为ECMAScript 2015（ES2015）。它的目标是使得JavaScript语言可以用来编写复杂的大型应用程序，成为企业级开发语言 [1] 。

1.优点

提升JS编写大型的复杂应用程序的能力（这次升级加入了模块化的概念、细化和优化语法、封装了一些方法）。

2.不足

对浏览器存在兼容性问题，可以使用Bable：ES6->ES5（转换器）

ES6语法

1.let声明变量

let申明的变量是有局部作用域
let声明的变量不能跟已有变量重名

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<head><meta charset="UTF-8"><meta http-equiv="X-UA-Compatible" content="IE=edge"><meta name="viewport" content="width=device-width, initial-scale=1.0"><title>Document</title><script>{//let申明的变量是有局部作用域let a = 1;//console.log(a);}//let声明的变量不能跟现有变量重名let b = true;//let b = false;console.log(b);</script>
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2.const声明常量

常量必须初始化

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<head><meta charset="UTF-8"><meta http-equiv="X-UA-Compatible" content="IE=edge"><meta name="viewport" content="width=device-width, initial-scale=1.0"><title>Document</title><script>const PI = 3.14;//PI = 3.1415;    //Assignment to constant variable//常量必须初始化const A = 1;</script>
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3.解构赋值

解构赋值是对赋值运算符的扩展。他是一种针对数组或者对象进行模式匹配，然后对其中的变量进行赋值。在代码书写上简洁且易读，语义更加清晰明了；也方便了复杂对象中数据字段获取。

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<head><meta charset="UTF-8"><meta http-equiv="X-UA-Compatible" content="IE=edge"><meta name="viewport" content="width=device-width, initial-scale=1.0"><title>Document</title><script>//数组的解构let a = 1,b = true,c = "abc";let [x,y,z] = [2,false,"bcd"];let res = a + x;console.log(x,y,z);//对象的解构let sid = 1,sname = "mary";let student1 = {"sid":sid,"sname":sname};console.log(student1);let student2 = {sid,sname};console.log(student2);let teacher = {"tid":1,"tname":"张雪峰"};// let id = teacher.tid;// let name = teacher.tname;//变量名必须与属性名一致let {tname,tid} = teacher;console.log(tname,tid);</script>
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4.模板字符串

模板字符串相当于加强版的字符串，用反引号 `,除了作为普通字符串，还可以用来定义多行字符串，还可以在字符串中加入变量和表达式。

定义多行字符串
引用变量
调用函数

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<head><meta charset="UTF-8"><meta http-equiv="X-UA-Compatible" content="IE=edge"><meta name="viewport" content="width=device-width, initial-scale=1.0"><title>Document</title><script>//使用模板字符串定义多行字符串let str1 = `hello world`;console.log(str1);//模板字符串引用变量let name = "mary";let str2 = `hello ${name}`;console.log(str2);//模板字符串调用函数function show() {return "tom";}let str3 = `hello ${show()}`;console.log(str3);</script>
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5.定义方法简写

<!DOCTYPE html>
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<head><meta charset="UTF-8"><meta http-equiv="X-UA-Compatible" content="IE=edge"><meta name="viewport" content="width=device-width, initial-scale=1.0"><title>Document</title><script>let student1 = {sid:1,sname:"mary",sleep:function(){console.log("睡觉");},eat:function(){console.log("吃饭");}};student1.sleep();let student2 = {sid:1,sname:"tom",sleep(){console.log("睡觉");},eat(){console.log("吃饭");}};student2.eat();</script>
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6.对象拓展运算符

拓展运算符：（…），用于取出参数对象所有可遍历属性然后拷贝到当前对象

<!DOCTYPE html>
<html lang="en">
<head><meta charset="UTF-8"><meta http-equiv="X-UA-Compatible" content="IE=edge"><meta name="viewport" content="width=device-width, initial-scale=1.0"><title>Document</title><script>let teacher1 = {"tid":1,"tname":"张雪峰"};let teacher2 = {...teacher1};console.log(teacher2);let stu1 = {"sid":1},stu2 = {"sname":"mary"},stu3 = {"sage":18};let student = {...stu1,...stu2,...stu3};console.log(student);</script>
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7.函数的默认参数

<!DOCTYPE html>
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<head><meta charset="UTF-8"><meta http-equiv="X-UA-Compatible" content="IE=edge"><meta name="viewport" content="width=device-width, initial-scale=1.0"><title>Document</title><script>function show(a,b=10){let res = a + b;console.log(res);}show(1,2);show(1);</script>
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8.不定参数

不定参数用来表示不确定参数个数，形如，…变量名，由…加上一个具名参数标识符组成。具名参数只能放在参数列表的最后，并且有且只有一个不定参数。

<!DOCTYPE html>
<html lang="en">
<head><meta charset="UTF-8"><meta http-equiv="X-UA-Compatible" content="IE=edge"><meta name="viewport" content="width=device-width, initial-scale=1.0"><title>Document</title><script>function show(a,...b){console.log(b.length);}show(1);show(2,true);show(3,4.2,"abc",true);</script>
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9.箭头函数

箭头函数提供了一种更加简洁的函数书写方式。基本语法是：参数 => 函数体

无参的箭头函数必须在参数的地方写()
一个参数的箭头，例：res = >
有多个参数的箭头函数必须在参数的地方写()
函数体有多行语句时需要将{}写出来

<!DOCTYPE html>
<html lang="en">
<head><meta charset="UTF-8"><meta http-equiv="X-UA-Compatible" content="IE=edge"><meta name="viewport" content="width=device-width, initial-scale=1.0"><title>Document</title><script>function show(a) {return a;}//console.log(show(1));//箭头函数let showAgain = b => b;//console.log(showAgain(2));function f1() {console.log("Hello World");}f1();//无参的箭头函数必须在参数的地方写()let f2 = () => console.log("Hello World");f2();function f3(a,b) {var res = a + b;return res;}console.log(f3(3,4));//有多个参数的箭头函数必须在参数的地方写()//函数体有多行语句时需要将{}写出来let f4 = (x,y) => {var res = x + y;return res;}console.log(f4(3,4));</script>
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