Promise 用法

1.单个请求时

let p = new Promise((resolve,reject)=>{
  setTimeout(function () {
    if (true) {
      resolve('xx')
    } else {
      reject('yy')
    }
  },100)
})
p.then()
  .catch()

2.多个请求时（各个请求没有相关）

let p1 = new Promise((resolve,reject)=>{
  setTimeout(()=>{resolve('p1')},1000)
});
let p2 = new Promise((resolve,reject)=>{
  setTimeout(()=>{reject('p2')},2000)
});

Promise.all([p1,p2]).then(param=>{
  console.log(param)
  console.log('所有请求完成')
}).catch(err=>{
  console.log(err)
})

3.有请求完成 race

let p1 = new Promise((resolve,reject)=>{
  setTimeout(()=>{resolve('p1')},1000)
});
let p2 = new Promise((resolve,reject)=>{
  setTimeout(()=>{reject('p2')},2000)
});

Promise.race([p1,p2]).then(param=>{
  console.log(param)
  console.log('有请求完成了')
}).catch(err=>{
  console.log(err)
})

4.一个请求依赖另一个求情的结果时

async function fn() {
  let a,a2;
  let p = new Promise((resolve,reject) => {
    a = '11';
    resolve('11')
  });
  await p;
  let p2 = new Promise((resolve,reject) => {
    a2 = a + '22';
    resolve('22')
  });
  await p2;
  console.log(a2)
}
fn()

写的比较简陋，朋友们多多指教