ES 6语言基础

if(true) {
    var a = 'Tom'
}
console.log('a',a) // Tom

if(true) {
    let a = 'Tom'
}
console.log('a',a) // Uncaught ReferenceError: a is not defined

const a = 1
a = 2 // Uncaught TypeError: Assignment to constant variable
const b // Uncaught SyntaxError: Missing initializer in const declaration

let f = v => v
// 等同于
var f = function (v) {
    return v
}

let f = () => 5
// 等同于
var f = function () { return 5 }
let sum = (num1, num2) => num1 + num2;
// 等同于
var sum = function(num1, num2) {
    return num1 + num2
}

let sum = (num1, num2) => {
    let num = 0
    return num1 + num2 + num;
}

mounted () {
    this.foo = 1
    setTimeout(function(){
        console.log(this.foo)  // 打印出1
    }.bind(this),200)
}
//相当于
mounted () {
    this.foo = 1
    setTimeout(() => {
        console.log(this.foo)  // 同样打印出1
    },200)
}

const foo = 'bar'
const baz = {foo}
// 等同于
const baz = {foo: foo}
console.log(baz) // {foo: "bar"}

{
    name: 'item',
    data () {
        return {
            name:'bar'
        }
    }
    mounted () {
    },
    methods: {
        clearSearch () {
        }
    }
}
// 相当于
{
    name: 'item',
    data :function() {
        return {
            name:'bar'
        }
    }
    mounted :function() {
    },
    methods: {
        clearSearch :function() {
        }
    }
}

// 数组
const input = [1, 2];
const [first, second] = input;
console.log(first,second) // 1 , 2
// 对象
const o = {
    a: "foo",
    b: 30,
    c: "Johnson"
};
const {a, b, c} = o;
console.log(a,b,c) // foo , 30 , Johnson

export var name = "小明";
export let age = 20;
// 上面的写法等价于下面的写法
var name = "小明";
let age = 20;
export {
    name:name,
    age:age
}
// export对象简写的方式
export {name,age}

export function sum(num1,num2){
    return num1 + num2;
}
// 等价于
let sum = function (num1,num2){
    return num1 + num2;
}
export sum

//other.js
var name = "小明"
let age = 20
// export对象简写的方式
export {name,age}
//import.js
import {name,age} from "other.js"
console.log(name) // 小明
console.log(age) // 20

//other.js
export default function sum(num1,num2) {
    return num1 + num2;
}
//import.js
import sum from "other.js"
console.log(sum(1,1)) // 2

module.exports = xxx
// 相当于
export xxx

var promise = new Promise(function(resolve, reject) {
    ...
    if (/* 异步操作成功 */){
        resolve(value);
    } else {
        reject(error);
    }
});

promise.then(function(value) {
    // success逻辑
}, function(error) {
    // failure逻辑
}).catch(function(){
    // error逻辑
});

getJSON("/1.json").then(function(post) {
    return getJSON(post.nextUrl);
}).then(function (data) {
    console.log("resolved: ", data);
}, function (err){
    console.log("rejected: ", err);
});

var getJSON = function(url) {
    // 返回一个Promise对象
    var promise = new Promise(function(resolve, reject){
        var client = new XMLHttpRequest(); //创建XMLHttpRequest对象
        client.open("GET", url);
        client.onreadystatechange = onreadystatechange;
        client.responseType = "json";              //设置返回格式为json
        client.setRequestHeader("Accept", "application/json");//设置发送格式为json
        client.send();//发送
        function onreadystatechange() {
            if (this.readyState !== 4) {
                return;
            }
            if (this.status === 200) {
                resolve(this.response);
            } else {
                reject(new Error(this.statusText));
            }
        };
    });
    return promise;
};
getJSON("/data.json").then(function(data) {
    console.log(data);
}, function(error) {
    console.error(error);
});

async function myFunc() {
    //异步操作
}

// myFunc()返回一个Promise对象
async function myFunc() {
    return 'hello';
}
// 使用then方法就可以接收到返回值
myFunc().then(value => {
    console.log(value); // hello
})

async function myFunc() {
    return 'hello';
}
let foo = await myFunc(); // hello

try {
    let foo = await myFunc(); // hello
} catch (e) {
    // 错误逻辑
    console.log(e)
}

async mounted () {
    // 代码编写自上而下，一行一行，以便于阅读
    let resp = await ajax.get('weibo/list')
    let top = resp[0]
    console.log(top)
}