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() {

  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()