异步进化

例如：
获取用户数据；
再根据用户数据，获得订单数据；
再根据订单数据，获得商品数据；

setTimeout

//回调地狱
setTimeout(()=>{
  let users = '用户数据'
  console.log(users);
  setTimeout(() => {
    let orders = '订单数据'
    console.log(orders);
    setTimeout(() => {
        let goods = '商品数据'
        console.log(goods);
    }, 1000);
  }, 1000);
},1000);

生成器函数

function getUsers(){
  setTimeout(() => {
    let data = '用户数据';
    console.log(data);
    //调用next方法，并且将数据传入
    iterator.next(data);
  }, 1000);
}
function getOrders(){
  setTimeout(() => {
    let data = '订单数据';
    console.log(data);
    iterator.next(data);
  }, 1000);
}
function getGoods(){
  setTimeout(() => {
    let data = '商品数据';
    console.log(data);
    iterator.next(data);
  }, 1000);
}
//声明生成器函数
function * gen(){
  let users = yield getUsers();
  let orders = yield getOrders();
  let goods = yield getGoods();
}
//调用生成器函数
let iterator = gen();
iterator.next();

Promise对象

const p = new Promise((resolve,reject)=>{
    setTimeout(() => {
        let data = '用户数据';
        console.log(data);
        resolve(data);
    }, 1000);
});
p.then(value=>{
    return new Promise((resolve,reject)=>{
        setTimeout(() => {
            let data = '订单数据';
            console.log(data);
            resolve([value,data]);
        }, 1000);
    });
}).then(value=>{
    return new Promise((resolve,reject)=>{
        setTimeout(() => {
            let data = '商品数据';
            console.log(data);
            resolve([value,data]);
        }, 1000);
    });
});

Async...await

function getUsers(){
    return new Promise((resolve,reject)=>{
        setTimeout(() => {
            let data = '用户数据';
            // console.log(data)
            resolve(data);
        }, 1000);
    })
}
function getOrders(){
    return new Promise((resolve,reject)=>{
        setTimeout(() => {
            let data = '订单数据';
            resolve(data);
        }, 1000);
    })
}
function getGoods(){
    return new Promise((resolve,reject)=>{
        setTimeout(() => {
            let data = '商品数据';
            resolve(data);
        }, 1000);
    })
}
async function main(){
    let users = await getUsers();
    console.log(users);
    let orders = await getOrders();
    console.log(orders);
    let goods = await getGoods();
    console.log(goods);
}
main();