promise-map-limit

Map an array n items at a time with promises

API

mapLimit(iterable, limit, iteratee)

Parameters:

• iterable The collection to iterate over. The iterator interface will be used if it is implemented. If the collection does not implement the iterator interface then Object.keys() is used and values are iterated in the order of the keys.
• limit The concurrency limit. The maximum number of async operations at a time.
• iteratee The function that will be called with each value in iterable. It can return a Promise or a plain value if no async operation is needed. If the iteratee function throws then mapLimit aborts and rejects with the error.

Returns:

mapLimit returns a Promise<Array> that resolves with the transformed values from iterable. If iteratee throws or returns a rejected promise then mapLimit will abort and return a rejected promise with the same error.

Example

1const mapLimit = require('promise-map-limit');
2
3// This will work on max two items in the array at a time
4// When all items are processed it will resolve with the resulting array in the same order.
5mapLimit([1, 2, 3, 4], 2, n => Promise.resolve(n * 2)).then(array => {
6  // array == [2, 4, 6, 8]
7});
8
9// The iterator function can return the result synchronous
10mapLimit([1, 2, 3, 4], 2, n => n * 2).then(array => {
11  // array == [2, 4, 6, 8]
12});
13
14// Any value that implements the iterator interface can be used, such as Array, String, Map, Set
15mapLimit('abc', 2, str => str.toUpperCase()).then(array => {
16  // array == ['A', 'B', 'C']
17});
18
19// Any value that implements the iterator interface can be used, such as Array, String, Map, Set
20mapLimit('abc', 2, str => str.toUpperCase()).then(array => {
21  // array == ['A', 'B', 'C']
22});
23
24// Iterator object also implements the iterator interface
25const arr = ['a', 'b', 'c'];
26const iterator = a.entries();
27mapLimit(iterator, 2, entry => entry).then(array => {
28  // array == [
29  //   [0, 'a'],
30  //   [1, 'b'],
31  //   [2, 'c']
32  // ]
33});
34
35// If the iterator is omitted, each function in the array is executed
36// This will run max two functions at a time
37mapLimit([
38  () => Promise.resolve(1),
39  () => Promise.resolve(2),
40  () => 3, // can be sync
41  () => Promise.resolve(4),
42], 2).then(array => {
43  // array == [1, 2, 3, 4]
44});