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第165天学习打卡(项目谷粒商城 7 ES6 ECMAScript6.0 )

ES6(ECMAScript6.0)

是JavaScript语言的下一代标准.

ECMAScript是浏览器脚本语言的规范,而熟知的各种js语言,如javascript则是规范的具体实现。

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快捷键shift +!+ 回车 就会生成一个html文档

1、let声明变量

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<!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>
</head>
<body>
    <script>
        //var声明的变量往往是越域
        //let声明的变量有严格局部作用域
        {
            var a = 1;
            let b = 2;
        }
        console.log(a); // 1
        console.log(b);//ReferenceError: b is not defined
        
    </script>
    
</body>
</html>

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浏览器中显示的内容:

image-20210622184633122

let只能声明一次变量

在这个下面修改的数据不用再次打卡Open with Live Server只需要把里面的数据进行保存,然后在原来已经打开的Open with Live Server里面就能查看到

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2. const声明常量(只读变量)

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<!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>
</head>
<body>
    <script>
        //var声明的变量往往是越域
        //let声明的变量有严格局部作用域
        // {
        //     var a = 1;
        //     let b = 2;
        // }
        // console.log(a); // 1
        // console.log(b);//ReferenceError: b is not defined

        //var 可以声明多次
        //let只能声明一次
        // var m = 1
        // var m = 2
        // let n = 3
        // //let n = 4
        // console.log(m) //2
        // console.log(n) //Identifier 'n' has already been declared


        //var 会变量提升
        //let 不存在变量提升
        // console.log(x); //undefined
        // var x = 10;
        // console.log(y); //ReferenceErroe: y is not defined
        // let y = 20

        //const
        //1.声明之后不允许改变
        //2.一旦声明必须初识化,否则会报错
        const a = 1;
        a = 3; //Uncaught TypeError: Assignment to constant variable
        


    </script>
    
</body>
</html>

3、解构表达式

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1) 数组解构

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2) 对象解构

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4、字符串扩展

1)几个新的API

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2)字符串模板

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<!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>
</head>

<body>

    <script>
        let arr = [1, 2, 3];

        // let a = arr[0];
        // let b = arr[1];
        // let c = arr[2];

        //数组解构
        //以前我们想要获取其中的值,只能通过角标。 ES6可以这样
        // let [a, b, c] = arr; //x,y,z 将与arr中的每个位置对应来取值

        // console.log(a, b, c)

        const person = {
            name: "jack",
            age: 21,
            language: ['java', 'js', 'css']
        }

        // const name = person.name;
        // const age = person.age;
        // const language = person.language;


        //对象解构
        //解构表达式获取值,将person里面每一个属性和左边对应赋值
        //扩展:如果想要将name的值赋给其他变量,可以如下, abc是新的变量名
        const { name: abc, age, language } = person;

        console.log(abc, age, language)


        //4.字符串扩展
        let str = "hello.vue"
        console.log(str.startsWith("hello"));//true
        console.log(str.endsWith(".vue")); //true
        console.log(str.includes("e")); //true
        console.log(str.includes("hello"));//true

        //字符串模板
        let ss = `<div>
            <span>hello world</span>
            </div>`;
        console.log(ss);


        // 字符串插入变量和表达式。变量名写在${}中, ${}中可以放入JavaScript表达式

        function fun(){
            return "这是一个函数"
        }

        let info = `我是${abc}, 今年${age + 9}了, 我想说: ${fun()}`;
        console.log(info);

    </script>
</body>

</html>

5、函数参数默认值

1)函数参数默认值

image-20210622195440744

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2)不定参数

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3)箭头参数

一个参数时的写法

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多个参数时的写法:

image-20210622202025856

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4)实战:箭头函数结合解构表达式

image-20210622202221408

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<!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>
</head>

<body>

    <script>
        //在ES6以前,我们无法给一个函数参数设置默认值,只能采用变通写法
        function add(a, b) {
            //判断b是否为空,为空就给默认值1
            b = b || 1;
            return a + b;
        }
        //传一个参数
        console.log(add(10));

        //现在可以这么写,直接给参数写上默认值,没传就会使用默认值
        function add2(a, b = 1){
            return a + b;
        }
        console.log(add2(20));//20是传给a的, 没有给b传值, b就是默认为1


        // 不定参数
        function fun(...values){
            console.log(values.length)
        }
        fun(1, 2) //2
        fun(1, 2, 3, 4) //4


        //箭头函数
        //以前声明一个方法
        // var print = function(obj){
        //     console.log(obj);
        // }

        var print = obj => console.log(obj);
        print("hello");
        
        //以前的写法 传递多个参数
        var sum = function(a, b){
            return a + b;
        }

        //现在的写法传递多个参数
        var sum2 = (a, b) => a + b;
        console.log(sum2(11, 12));

        var sum = function(a, b){
           c = a + b;
           return a + c;
        }

        var sum3 = (a, b) => {
            c = a + b;
           return a + c;

        }
        console.log(sum3(10, 20));


        const person = {
            name: "jack",
            age: 21,
            language: ['java', 'js', 'css']
        }
        function hello(person){
            console.log("hello," + person.name)  //这里是不会打印值的
        }


        // var hello2 = (param) => console.log("hello," + param.name);//这里传的参数是person里面的
        // hello2(person); //调用person

        var hello2 = ({name}) => console.log("hello," + name);//这里传的参数是person里面的
        hello2(person); //调用person

    </script>


</body>

</html>

6、对象优化

1)新增的API

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2)声明对象简写

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3)对象的函数属性简写

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4)对象的拓展运算符

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

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<!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>
</head>

<body>

    <script>
        const person = {
            name: "jack",
            age: 21,
            language: ['java', 'js', 'css']
        }
        console.log(Object.keys(person)); //["name", "age", "language"]
        console.log(Object.values(person));//["jack", 21, Array(3)]
        console.log(Object.entries(person));//[Array(2), Array(2), Array(2)]  这个打印的是key value



        const target = {a : 1};
        const source1 = {b : 2};
        const source2 = {c : 3};

        //{a:1, b:2, c:3}
        Object.assign(target, source1, source2);

        console.log(target);

        //声明对象简写
        const age = 23
        const name = "张三"
        const person1 = {age : age, name : name}

        const person2 = {age, name}
        console.log(person2);

        //对象的函数属性简写
        let person3 = {
            name: "jack",
            //以前
            eat: function(food){
                console.log(this.name + "在吃" + food);
            },

            //箭头函数this不能使用,不然this.name打印出来的结果是这样的 在吃苹果  , 必须是  对象.属性 person3.name
            eat2: food => console.log(person3.name + "在吃" + food),

            eat3(food){
                console.log(this.name + "在吃" + food);
            }
        }
        person3.eat("香蕉")
        person3.eat2("苹果")
        person3.eat3("橘子")


        //4)对象拓展运算符
        //1 拷贝对象(深拷贝)
        let p1 = {name: "Amy", age: 15}
        let someone = {...p1}//这里的三个点是对p1里面的内容的拷贝 到当前的someone
        console.log(someone) //{name: "Amy", age: 15}


        //2.合并对象
        let age1 = {age: 15}
        let name1 = {name: "Amy"}
        let p2 = {name: "zhangsan"}
         p2 = {...age1, ...name1}
        console.log(p2)

    </script>
</body>

</html>

7、map 和reduce

数组中新增了map和reduce方法

  1. map

image-20210622214021143

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  1. reduce

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8、Promise

image-20210622215746893

快捷键操作: alt + shift + f整理代码

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