js基础语法

var 声明的变量往往会越域
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);   //ReferenceError: y is not defined
let y = 20;  

数组解构

let arr = [1,2,3];
let a = arr[0];
let b = arr[1];
let c = arr[2];
let [a,b,c] = arr;

对象解构

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

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 {} 中可以放入JavaScript 表达式

let name = 'yj'
let age = 20

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

let info = `我是${name}，今年${age + 10}了, 我想说： ${fun()}`;
console.log(info);

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;
}

console.log(add(10));

现在可以这么写

function add(a, b = 1) {
    return a + b;
}

console.log(add(20));

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);
}
//现在可以这么写
var print = obj => console.log(obj);
print("hello");

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

var sum = (a, b) => a + b;
console.log(sum(11, 12));

function hello(name) {
    console.log("hello," + name)
}

var hello = ({name}) => console.log("hello," + name);
hello('yj');

对象

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)]
//将key,value返回出来
console.log(Object.entries(person));//[Array(2), Array(2), Array(2)]

Object.assign

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};

对象的函数属性简写

let person = {
    name: "jack",
    // 以前：
    eat: function (food) {
        console.log(this.name + "在吃" + food);
    },
    //箭头函数this不能使用，对象.属性
    eat2: food => console.log(person.name + "在吃" + food),
    eat3(food) {
        console.log(this.name + "在吃" + food);
    }
};

person.eat("香蕉");
person.eat2("苹果");
person.eat3("橘子");

拷贝对象（深拷贝）

let p1 = {name: "Amy", age: 15};
let someone = {...p1};
console.log(someone);  //{name: "Amy", age: 15}

合并对象

let age1 = {age: 15};
let name1 = {name: "Amy"};
let p2 = {...age1, ...name1};
console.log(p2)

数组中新增了map和reduce方法
map()：接收一个函数，将原数组中的所有元素用这个函数处理后放入新数组返回

let arr = ['1', '20', '-5', '3'];

arr = arr.map((item)=>{
    return item * 2
});

arr = arr.map(item => item * 2);

reduce() 为数组中的每一个元素依次执行回调函数，不包括数组中被删除或从未被赋值的元素

let arr = ['1', '20', '-5', '3'];

let result = arr.reduce((a, b) => {
    console.log("上一次处理后的值，或者是初始值：" + a);
    console.log("数组中当前被处理的元素：" + b);
    return a + b;
}, 100);

console.log(result)

Promise
比如有三个请求
1、查出当前用户信息
2、按照当前用户的id查出他的课程
3、按照当前课程id查出分数

菜鸟写法

    $.ajax({
        url: "mock/user.json",
        success(data) {
            console.log("查询用户：", data);
            $.ajax({
                url: `mock/user_corse_${data.id}.json`,
                success(data) {
                    console.log("查询到课程：", data);
                    $.ajax({
                        url: `mock/corse_score_${data.id}.json`,
                        success(data) {
                            console.log("查询到分数：", data);
                        },
                        error(error) {
                            console.log("出现异常了：" + error);
                        }
                    });
                },
                error(error) {
                    console.log("出现异常了：" + error);
                }
            });
        },
        error(error) {
            console.log("出现异常了：" + error);
        }
    });

采用Promise优化

function get(url, data) {
    return new Promise((resolve, reject) => {
        $.ajax({
            url: url,
            data: data,
            success: function (data) {
                resolve(data);
            },
            error: function (err) {
                reject(err)
            }
        })
    });
}

get("mock/user.json")
        .then((data) => {
            console.log("用户查询成功~~~:", data);
            return get(`mock/user_corse_${data.id}.json`);
        })
        .then((data) => {
            console.log("课程查询成功~~~:", data);
            return get(`mock/corse_score_${data.id}.json`);
        })
        .then((data) => {
            console.log("课程成绩查询成功~~~:", data)
        })
        .catch((err) => {
            console.log("出现异常", err)
        });