promise的链式调用

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise/all

https://segmentfault.com/a/1190000007598894

1.promise.then   then的用法

function p1() {
    return new Promise((resolve) => {
        console.log(1);
        resolve();
    });
}

function p2() {
    return new Promise((resolve) => {
        console.log(2);
        resolve();
    });
}

function p3() {
    return new Promise((resolve) => {
        console.log(3);
        resolve();
    });
}

p1().then(()=>{
    return p2();
}).then(()=>{
    return p3()
}).then(()=>{
    console.log('all down');
})

2.promise.all()

var  p1=new Promise(function (resolve,reject) {
    resolve(1)
});
var  p2=new Promise(function (resolve,reject) {
    resolve(2)
});
var  p3=new Promise(function (resolve,reject) {
    resolve(3)
});

Promise.all([p1,p2,p3]).then(function (values) {
    console.log(values);
})