JavaScript async/await for Promises inside Array.m

问题:

This question already has answers here:

Using async/await with a forEach loop (20 answers)

Closed 3 years ago.

Given the following code

class SomeClass {
  async someFunc() {
    const urlParameters = [0, 1, 2];
    const batchAjaxResult = await urlParameters.map((parameter) => { 
        const result = await $.get(`someUrl/${parameter}`);
        return {
          parameter,
          result
        }
     });

  console.log(batchAjaxResult);

  }
}

JavaScript will return an Array of resolved Promises instead of the actual Promises result.

This is probably due to Array.map() not being implemented as a Promise.

Is there a Promise-based version of Array.map?

This is question differs from How to return the response from an asynchronous call, because it's about How to return batched responses wrapped inside Array.map.

回答1:

You could use this simple function which chains the promises to achieve sequential execution:

function asyncMap(arr, mapper) {
  var q = Promise.resolve();
  return Promise.all(arr.map(v => q = q.then(() => mapper(v))));
}


// Usage

asyncMap([1, 2, 3], v => {
  return new Promise(resolve => {
    console.log("Start " + v);
    setTimeout(() => {
      console.log("End " + v);
      resolve("foo" + v);
    }, 500);
  });
}).then(a => console.log(a));

回答2:

By writing this question I managed to get down to the root of the problem.

It seems like Bluebird has such an feature called Promise.map

Similarily there is a native implementation called Promise.all that creates a "bulk Promise"

I changed the code as follows and it's working:

class SomeClass {
  async someFunc() {
      const urlParameters = [0, 1, 2];
      const batchAjaxResult = await Promise.all(
          urlParameters.map(async (parameter) => {
            const result = await $.get(`someUrl/${parameter}`);
            return {
              parameter,
              result
            }
         })
      );
      console.log(batchAjaxResult);

  }
}