Filtering an array with a function that returns a

问题:

Given

let arr = [1,2,3];

function filter(num) {
  return new Promise((res, rej) => {
    setTimeout(() => {
      if( num === 3 ) {
        res(num);
      } else {
        rej();
      }
    }, 1);
  });
 }

 function filterNums() {
   return Promise.all(arr.filter(filter));
 }

 filterNums().then(results => {
   let l = results.length;
   // length should be 1, but is 3
 });

The length is 3 because Promises are returned, not values. Is there a way to filter the array with a function that returns a Promise?

Note: For this example, fs.stat has been replaced with setTimeout, see https://github.com/silenceisgolden/learn-esnext/blob/array-filter-async-function/tutorials/array-filter-with-async-function.js for the specific code.

回答1:

As mentioned in the comments, Array.prototype.filter is synchronous and therefore does not support Promises.

Since you can now (theoretically) subclass built-in types with ES6, you should be able to add your own asynchronous method which wraps the existing filter function:

Note: I've commented out the subclassing, because it's not supported by Babel just yet for Arrays

class AsyncArray /*extends Array*/ {
  constructor(arr) {
    this.data = arr; // In place of Array subclassing
  }

  filterAsync(predicate) {
     // Take a copy of the array, it might mutate by the time we've finished
    const data = Array.from(this.data);
    // Transform all the elements into an array of promises using the predicate
    // as the promise
    return Promise.all(data.map((element, index) => predicate(element, index, data)))
    // Use the result of the promises to call the underlying sync filter function
      .then(result => {
        return data.filter((element, index) => {
          return result[index];
        });
      });
  }
}
// Create an instance of your subclass instead
let arr = new AsyncArray([1,2,3,4,5]);
// Pass in your own predicate
arr.filterAsync(async (element) => {
  return new Promise(res => {
    setTimeout(() => {
      res(element > 3);
    }, 1);
  });
}).then(result => {
  console.log(result)
});

Babel REPL Demo

回答2:

Here's a way:

var wait = ms => new Promise(resolve => setTimeout(resolve, ms));
var filter = num => wait(1).then(() => num == 3);

var filterAsync = (array, filter) =>
  Promise.all(array.map(entry => filter(entry)))
  .then(bits => array.filter(entry => bits.shift()));

filterAsync([1,2,3], filter)
.then(results => console.log(results.length))
.catch(e => console.error(e));

The filterAsync function takes an array and a function that must either return true or false or return a promise that resolves to true or false, what you asked for (almost, I didn't overload promise rejection because I think that's a bad idea). Let me know if you have any questions about it.

var wait = ms => new Promise(resolve => setTimeout(resolve, ms));
var filter = num => wait(1).then(() => num == 3);

var filterAsync = (array, filter) =>
  Promise.all(array.map(entry => filter(entry)))
  .then(bits => array.filter(entry => bits.shift()));

filterAsync([1,2,3], filter)
.then(results => console.log(results.length))
.catch(e => console.error(e));

var console = { log: msg => div.innerHTML += msg + "<br>",
                error: e => console.log(e +", "+ (e.lineNumber-25)) };

<div id="div"></div>

回答3:

Here is a 2017 elegant solution using async/await :

Very straightforward usage:

const results = await filter(myArray, async num => {
  await doAsyncStuff()
  return num > 2
})

The helper function (copy this into your web page):

async function filter(arr, callback) {
  const fail = Symbol()
  return (await Promise.all(arr.map(async item => (await callback(item)) ? item : fail))).filter(i=>i!==fail)
}

Demo:

// Async IIFE
(async function() {
  const myArray = [1, 2, 3, 4, 5]

  // This is exactly what you'd expect to write 
  const results = await filter(myArray, async num => {
    await doAsyncStuff()
    return num > 2
  })

  console.log(results)
})()


// Arbitrary asynchronous function
function doAsyncStuff() {
  return Promise.resolve()
}


// The helper function
async function filter(arr, callback) {
  const fail = Symbol()
  return (await Promise.all(arr.map(async item => (await callback(item)) ? item : fail))).filter(i=>i!==fail)
}

I'll even throw in a CodePen.

回答4:

Promise Reducer to the rescue!

[1, 2, 3, 4].reduce((op, n) => {
    return op.then(filteredNs => {
        return new Promise(resolve => {
            setTimeout(() => {
                if (n >= 3) {
                    console.log("Keeping", n);
                    resolve(filteredNs.concat(n))
                } else {
                    console.log("Dropping", n);
                    resolve(filteredNs);
                }
            }, 1000);
        });
    });
}, Promise.resolve([]))
.then(filteredNs => console.log(filteredNs));

Reducers are awesome. "Reduce my problem to my goal" seems to be a pretty good strategy for anything more complex than what the simple tools will solve for you, i.e. filtering an array of things that aren't all available immediately.

回答5:

Late to the game but since no one else mentioned it, Bluebird supports Promise.map which is my go-to for filters requiring aysnc processing for the condition,

function filterAsync(arr) {
    return Promise.map(arr, num => {
        if (num === 3) return num;
    })
        .filter(num => num !== undefined)
}

回答6:

A valid way to do this (but it seems too messy):

let arr = [1,2,3];

function filter(num) {
  return new Promise((res, rej) => {
    setTimeout(() => {
      if( num === 3 ) {
        res(num);
      } else {
        rej();
      }
    }, 1);
  });
}

async function check(num) {
  try {
    await filter(num);
    return true;
  } catch(err) {
    return false;
  }
}

(async function() {
  for( let num of arr ) {
    let res = await check(num);
    if(!res) {
      let index = arr.indexOf(num);
      arr.splice(index, 1);
    }
  }
})();

Again, seems way too messy.

回答7:

A variant of @DanRoss's:

async function filterNums(arr) {
  return await arr.reduce(async (res, val) => {
    res = await res
    if (await filter(val)) {
      res.push(val)
    }
    return res
  }, Promise.resolve([]))
}

Note that if (as in current case) you don't have to worry about filter() having side effects that need to be serialized, you can also do:

async function filterNums(arr) {
  return await arr.reduce(async (res, val) => {
    if (await filter(val)) {
      (await res).push(val)
    }
    return res
  }, Promise.resolve([]))
}

回答8:

For typescript folk (or es6 just remove type syntax)

function mapAsync<T, U>(array: T[], callbackfn: (value: T, index: number, array: T[]) => Promise<U>): Promise<U[]> {
  return Promise.all(array.map(callbackfn));
}

async function filterAsync<T>(array: T[], callbackfn: (value: T, index: number, array: T[]) => Promise<boolean>): Promise<T[]> {
  const filterMap = await mapAsync(array, callbackfn);
  return array.filter((value, index) => filterMap[index]);
}

es6

function mapAsync(array, callbackfn) {
  return Promise.all(array.map(callbackfn));
}

async function filterAsync(array, callbackfn) {
  const filterMap = await mapAsync(array, callbackfn);
  return array.filter((value, index) => filterMap[index]);
}