Javascript的promise,async和await的区别详解

// 以下三个请求依次执行
req1 = () => { return fetch("http://example.com/api/v1/get")}
req2 = () => { return fetch("http://example.com/api/v1/post")}
req3 = () => { return fetch("http://example.com/api/v1/delete")}
//写法1
req1().then(res=>{
    console.log("1: ",res)
    req2().then(res =>{
        console.log("2: ",res)
        req3().then(res =>{
            console.log("3: ",res)
        })
    })
})

// 写法2
req1().then(res =>{
    console.log("1: ", res)
    return req2()
})
.then(res =>{
    console.log("2: ", res)
    return req3()
})
.then(res =>{
    console.log("3: ", res)
})
// 写法3
function f1(){
    req1()
    req2()
    req3()
}
// 写法4
async  function f2(){
    await req1
    await req2
    await req3
}
// 写法5
async  function f3(){
    req1().then(res => {
        console.log("1:", res)
    })
    await f3_1()
}
async function f3_1(){
    req1().then(res => {
        console.log("2:", res)
    })
    await f3_2()
}
async function f3_2(){
    req2().then(res=>{
         console.log("3: ",res)
    })
}
// 写法6
ff()
async function ff(){
    await req1_good()
}
async function req1_good(){
    fetch("http://example.com/api/v1/get").then(res =>{
        console.log("1: ",res)
    })
    await req2_good()
}
async  function req2_good() {
    fetch("http://example.com/api/v1/post").then(res =>{
        console.log("2: ",res)
    })
    await req3_good()
}
async function req3_good() {
     fetch("http://example.com/api/v1/delete").then(res => {
         console.log("3: ",res)
     })
}