按顺序执行承诺映射

Question

我编写了一个在循环（映射）中调用的函数，并且该函数使用了 Promise。现在，我希望该函数同步运行并在调用其下一个实例之前退出。

function t1(){

  let arr1 = [1,2,3,4,5];

  return Promise.map(arr1, (val) =>{

    const params = {

      "param1" : val1

    };

    return t2(params);

  });

}

function t2(event){

  return Promise.resolve()

  .then({

  //do something

  //code doesn't reach here in sync manner. all five instance are invoked and then code reaches here for first instance and so on

  })

  .then({

  //promise chaining. do something more

  })

}

t2 被调用五次，但我希望每个实例只在实例返回值之前调用。目前它的行为不是那样，而是并行调用该函数五次。

由于项目限制，我无法使用 async/await。

Answer 1

您当前代码的问题在于Promise.prototype.map，像 一样forEach，不会等待在其中调用的异步函数完成。（除非您用await或明确告诉解释器这样做，否则不会等待异步调用.then）有t1AWAIT的每个呼叫t2：async function t1(){  let arr1 = [1,2,3,4,5];  const results = [];  for (const val of arr1) {    results.push(await t2(val));  }  return results;}或者，如果您想使用reduce代替async/ await：const delay = () => new Promise(res => setTimeout(res, 500));function t1(){  let arr1 = [1,2,3,4,5];  return arr1.reduce((lastProm, val) => lastProm.then(    (resultArrSoFar) => t2(val)      .then(result => [...resultArrSoFar, result])  ), Promise.resolve([]));}function t2(event){  return delay().then(() => {    console.log('iter');    return event;  });}t1()  .then(results => console.log('end t1', results));或者，如果您需要将顺序功能封装在 t2 中，那么让 t2 具有它生成的前一个 Promise 的半持久变量：const delay = () => new Promise(res => setTimeout(res, 500));const t1 = () => {  return Promise.all([1, 2, 3, 4].map(t2));};const t2 = (() => {  let lastProm = Promise.resolve();  return (event) => {    const nextProm = lastProm      .catch(() => null) // you may or may not want to catch here      .then(() => {        // do something with event        console.log('processing event');        return delay().then(() => event);      });    lastProm = nextProm;    return nextProm;  };})();t1().then(results => console.log('t1 done', results));

Answer 2

(function loop(index) {    const next = promiseArray[index];    if (!next) {        return;    }    next.then((response) => {        // do Something before next        loop(index + 1);    }).catch(e => {        console.error(e);        loop(index + 1);    });})(0 /* startIndex */)

Answer 3

以下是.reduce()与 async/await 结合使用时按顺序运行 Promise 的样子：async function main() {&nbsp; const t2 = (v) => Promise.resolve(v*2)&nbsp; const arr1 = [1,2,3,4,5];&nbsp;&nbsp;&nbsp; const arr1_mapped = await arr1.reduce(async (allAsync, val) => {&nbsp; &nbsp; const all = await allAsync&nbsp; &nbsp; all.push(await t2(val) /* <-- your async transformation */)&nbsp;&nbsp; &nbsp; return all&nbsp; }, [])&nbsp; console.log(arr1_mapped)}main()