# 如何实现 promise.map，限制 promise 并发数

更多描述

实现一个 promise.map，进行并发数控制，有以下测试用例

pMap([1, 2, 3, 4, 5], (x) => Promise.resolve(x + 1));

pMap([Promise.resolve(1), Promise.resolve(2)], (x) => x + 1);

// 注意输出时间控制
pMap([1, 1, 1, 1, 1, 1, 1, 1], (x) => sleep(1000), { concurrency: 2 });

Issue

欢迎在 Gtihub Issue 中回答此问题: Issue 89 (opens new window)

Author

回答者: dmwin72015 (opens new window)

没人回答

Author

回答者: shfshanyue (opens new window)

以下代码见如何实现 promise.map - codepen (opens new window)

function pMap(list, mapper, concurrency = Infinity) {
  // list 为 Iterator，先转化为 Array
  list = Array.from(list);
  return new Promise((resolve, reject) => {
    let currentIndex = 0;
    let result = [];
    let resolveCount = 0;
    let len = list.length;
    function next() {
      const index = currentIndex;
      currentIndex++;
      Promise.resolve(list[index])
        .then((o) => mapper(o, index))
        .then((o) => {
          result[index] = o;
          resolveCount++;
          if (resolveCount === len) {
            resolve(result);
          }
          if (currentIndex < len) {
            next();
          }
        });
    }
    for (let i = 0; i < concurrency && i < len; i++) {
      next();
    }
  });
}

Author

回答者: heretic-G (opens new window)

Promise.map = function (queue = [], opt = {}) {
  let limit = opt.limit || 5;
  let queueIndex = 0;
  let completeCount = 0;
  let _resolve;
  let result = Array(queue.length);

  for (let i = 0; i < limit; i++) {
    next(queueIndex++);
  }

  function next(index) {
    if (queue.length === 0) return;
    let curr = queue.shift();
    if (typeof curr === "function") {
      curr = curr();
    }
    Promise.resolve(curr)
      .then(
        (res) => {
          result[index] = res;
        },
        (res) => {
          result[index] = res;
        }
      )
      .finally(() => {
        completeCount += 1;
        if (completeCount === result.length) {
          return _resolve(result);
        }
        next(queueIndex++);
      });
  }
  return new Promise((resolve) => {
    _resolve = resolve;
  });
};

function add(a, b) {
  return Promise.resolve(a + b);
}

function sum(arr) {
  if (arr.length <= 2) {
    return add(arr[0] || 0, arr[1] || 0);
  }
  let mid = (arr.length / 2) | 0;
  let promiseArr = [];
  for (let i = 0; i < mid; i++) {
    promiseArr.push(add(arr[i], arr[mid + i]));
  }
  return Promise.map(promiseArr).then((res) => {
    if (arr.length % 2 !== 0) {
      res.push(arr.pop());
    }
    return sum(res);
  });
}

Author

回答者: yazhouio (opens new window)

function pMap(list, mapper, cur) {
  cur = cur || list.length;
  let step = Promise.resolve();
  do {
    let temp = list.splice(0, cur);
    step = step.then(() =>
      Promise.all(
        temp.map((i, index) => Promise.resolve(i).then((e) => mapper(e, index)))
      )
    );
  } while (list.length);

Author

回答者: Vi-jay (opens new window)

function pMap([...arr], fn, { concurrency = Infinity } = {}) {
  let queue = [],
    results = [];
  return new Promise((resolve) =>
    (function closure() {
      const times = concurrency - queue.length;
      for (let i = 0; i < times; i++) {
        if (!arr.length) return;
        const promise = Promise.resolve(arr.shift())
          .then(fn)
          .then((data) => results.push(data))
          .finally(() => {
            queue = queue.filter((item) => promise !== item);
            if (!queue.length && !arr.length) return resolve(results);
            return closure();
          });
        queue.push(promise);
      }
    })()
  );
}

Author

回答者: yxw007 (opens new window)

function parallelMap(arr, fn, concurrency = Number.MAX_SAFE_INTEGER) {
  return new Promise((resolve) => {
    let ret = [];
    let index = -1;
    function next() {
      ++index;
      Promise.resolve(arr[index])
        .then((val) => fn(val, index))
        .then((res) => {
          ret.push(res);
          if (ret.length === arr.length) {
            resolve(ret);
          } else if (index < arr.length) {
            next();
          }
        });
    }

    for (let i = 0; i < arr.length && i < concurrency; i++) {
      next();
    }
  });
}

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