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与forEach一样，不会等待在其中调用的异步函数完成。 （除非您使用await或.then明确告诉解释器这样做，否则不会等待任何异步调用）

让t1等待t2每次调用：

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而不是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

以下是将.reduce()与 async/await 结合使用时按顺序运行 Promise 的样子：

 async function main() { const t2 = (v) => Promise.resolve(v*2) const arr1 = [1,2,3,4,5]; const arr1_mapped = await arr1.reduce(async (allAsync, val) => { const all = await allAsync all.push(await t2(val) /* <-- your async transformation */) return all }, []) console.log(arr1_mapped) } main()