比较Promise, Generator, Async写法：

function takeLongTime(n) {    return new Promise(resolve => {        setTimeout(() => resolve(n + 200), n);    });}function step1(n) {    console.log(`step1 with ${n}`);    return takeLongTime(n);}function step2(n) {    console.log(`step2 with ${n}`);    return takeLongTime(n);}function step3(n) {    console.log(`step3 with ${n}`);    return takeLongTime(n);}

Promise

function doIt() {    console.time("doIt");    const time1 = 300;    step1(time1)        .then(time2 => step2(time2))        .then(time3 => step3(time3))        .then(result => {            console.log(`result is ${result}`);        });    console.timeEnd("doIt");}doIt();// step1 with 300// doIt: 13202.5498046875ms// step2 with 500// step3 with 700// result is 900

Generator

function* doii() {    console.time("do");    const time1 = 300;    const time2 = yield step1(time1);    const time3 = yield step2(time2);    const result = yield step3(time3);    console.log(`result is ${result}`);    console.timeEnd("do");}doii();let res = doii();res.next().value    .then( a => res.next(a).value )    .then( b => res.next(b).value )    .then( c => res.next(c).value )// step1 with 300// step2 with 500// step3 with 700// result is 900// do: 1506.312744140625ms

Async

async function doIt() {    console.time("doIt");    const time1 = 300;    const time2 = await step1(time1);    const time3 = await step2(time2);    const result = await step3(time3);    console.log(`result is ${result}`);    console.timeEnd("doIt");}doIt();// step1 with 300// step2 with 500// step3 with 700// result is 900// doIt: 1507.68505859375ms