Async-itertools

This module implements a number of asynchronous iterator building blocks inspired by constructs from Python, APL, Haskell, and SML. Each has been recast in a form suitable for JavaScript.

The module standardizes a core set of fast, memory efficient tools that are useful by themselves or in combination. Together, they form an “iterator algebra” making it possible to construct specialized tools succinctly and efficiently in pure JavaScript.

Installation

npm install async-itertools

Production

Generally, you'll use this library to transform existing iterators; but we provide a number of differerent methods to create iterators as well.

emptySync & emptyAsync

Create empty iterators

import { emptySync, emptyAsync } from "async-iterator";
for (const a of emptySync()) {
  // dream the impossible
}
for (const a of emptyAsync()) {
  // do the impossible!
}

countSync & countBigSync; countAsync & countBigAsync

Create a sequence of integers

import {
  countSync,
  countBigSync,
  countAsync,
  countBigAsync,
} from "async-iterator";
for (const a of countSync(0, 9)) {
  console.log(a);
} // logs 1, 2, 3, 4, 5, 6, 7, 8, 9
for (const a of countBigSync(9n, 0n)) {
  console.log(a);
} // logs 9n, 8n, 7n, 6n, 5n, 4n, 3n, 2n, 1n, 0n
for await (const a of countAsync(0, 9)) {
  console.log(a);
} // logs 1, 2, 3, 4, 5, 6, 7, 8, 9
for await (const a of countBigAsync(9n, 0n)) {
  console.log(a);
} // logs 9n, 8n, 7n, 6n, 5n, 4n, 3n, 2n, 1n, 0n

syncFrom & asyncFrom

Create iterators from given items

import { syncFrom, asyncFrom } from "async-iterator";
for (const a of syncFrom(1, 2, 3)) {
  console.log(a);
} // logs 1, 2, 3
for await (const a of asyncFrom(4, 5, 6)) {
  console.log(a);
} // logs 4, 5, 6

Transformation: transducers

This library employs transducers, (see this), to transform iterators.

This library provides two methods -- transduceSync and transduceAsync -- to apply transducers to synchronous and asynchronous iterators.

import { transduceSync } from "async-iterator";
// import { transduceSync } from "async-iterator/transduce";
for (const item of transduceSync(/*list of transducers*/)(/*some iterator*/)) {
  // do something with transduced item
}

import { transduceAsync } from "async-iterator";
// import { transduceAsync } from "async-iterator/transduce";
for await (const item of transduceAsync(/*list of transducers*/)(/*some asynchronous iterator*/)) {
  // do something with transduced item
}

In addition, the library provides a number of built-in transducers that can be applied.

map

Similiar to Array.prototype.map, maps items with a given transformation function.

// import { transducers } from "async-iterator";
// const { map } = transducers;
import { map } from "async-iterator/transducers";
const addOne = map((x) => x + 1);
const abs = map(Math.abs);
for (const x of transduceSync(addOne, abs)([-3, -2, -1, 0, 1, 2, 3])) {
  console.log(x);
}
// logs: 2, 1, 0, 1, 2, 3, 4

filter

Similiar to Array.prototype.filter, filters items that do not match a given predicate

import { filter } from "async-iterator/transducers";
const removeStrings = map((x) => typeof x !== "string");
const keepPositive = map((x) => x > 0);
for (const x of transduceSync(
  removeStrings,
  keepPositive
)(["alice", -3, -2, "bob", -1, 0, 1, "claire", 2, 3])) {
  console.log(x);
}
// logs: 0, 1, 2, 3

accumulate

Apply function successively to items in iterator. Similar to Array.prototype.reduce.

import { accumulate } from "async-iterator/transducers";
const sum = accumulate((a, b) => a + b, 0);
for (const x of transduceSync(sum)([1, 2, 3, 4])) {
  console.log(x);
}
// logs: 1, 3, 6, 10

group

Place items into groups of size N.

import { group } from "async-iterator/transducers";
const triplet = group(3);
for (const x of transduceSync(triplet)([1, 2, 3, 4, 5, 6, 7, 8, 9])) {
  console.log(x);
}
// logs: [1, 2, 3], [4, 5, 6], [7, 8, 9]

take

Take only the first N items and drop the rest (see 'reject').

import { take } from "async-iterator/transducers";
const take5 = take(5);
for (const x of transduceSync(take5)([1, 2, 3, 4, 5, 6, 7, 8, 9])) {
  console.log(x);
}
// logs: 1, 2, 3, 4, 5

reject

Reject first N items and take the rest (see 'take').

import { reject } from "async-iterator/transducers";
const rejectDozen = reject(12);
for (const x of transduceSync(take5)([
  1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14,
])) {
  console.log(x);
}
// logs 13, 14

Multiple different types of transducers can be applied.

import { map, filter, take } from "async-iterator/transducers";
const transformation = transduceSync(
  map((x) => x + 2),
  filter((x) => x % 2),
  take(4)
);
for (const x of transformation([
  1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14,
])) {
  console.log(x);
}
// logs 3, 5, 7, 9

Utiilities

This library provides a number of iterator related utilities.

isIterator & isAsyncIterator & exahusable

Test of object is an iterator or asyncIterator, or either.

import { isIterator, isAsyncIterator, exhaustable } from "async-iterator";
const iterator = (function* () {})();
const asyncIterator = (async function* () {})();
const block = {};
console.log(isIterator(iterator)); // true
console.log(isAsyncIterator(iterator)); // false
console.log(exhaustable(iterator)); // true
console.log(isIterator(asyncIterator)); // false
console.log(isAsyncIterator(asyncIterator)); // true
console.log(exhaustable(asyncIterator)); // true
console.log(isIterator(block)); // false
console.log(isAsyncIterator(block)); // false
console.log(exhaustable(block)); // false

exhaust & exhaustSync & exhaustAsync

Exhaust all items from iterator. Warning: Initial object may have items removed

import { exhaust, exhaustSync, exhaustAsync } from "async-iterator";
const iterator = [1, 2, 3];
const aIterator = (async function* () {
  yield 4;
  yield 5;
  yield 6;
})();
const oneTwoThree = exhaust(iterator); //[1,2,3]
const fourFiveSix = await exhaust(iterator); //[4,5,6]
const oneTwoThreeB = exhaustSync(iterator); //[1,2,3]
const fourFiveSixB = await exhaustAsync(iterator); //[]

teeSync & teeAsync

Tee iterator onto n other iterators Warning: Initial object may be emptied

import { teeSync, teeAsync } from "async-iterator";
const iterator = (async function* () {
  yield 1;
  yield 2;
  yield 3;
})();

const [i0, i1, i2] = teeAsync(3)(iterator);

for (const x of i0) {
  console.log(x);
} // logs 1, 2, 3

for (const x of transduce(map((x) => x - 1))(i1)) {
  console.log(x);
} // logs 0, 1, 2

for (const x of transduce(filter((x) => x > 2))(i2)) {
  console.log(x);
} // logs  3

AsyncChannel

AsyncChannel is an experimental primative object.

// file://declare.mjs
import { AsyncChannel } from "async-iterator";
export const c = new AsyncChannel();
setTimeout(async () => {
  for await (const i of c) {
    console.log(i);
  }
});

Put items directly on an async channel.

// file://use.mjs
import { c } from "./declare.mjs";
c.put("hello"); //logs "hello"
c.put("world"); //logs "world"

Automatically place items onto channels via decorators

// file://use-websocket.mjs
import { c } from "./declare.mjs";
import { withWebSocket } from "async-iterator/channel-decorators";
const socket = new WebSocket(/*ws url*/);
withWebSocket(c, socket);

// file://use-event-emitter.mjs
import { c } from "./declare.mjs";
import { withEmitter } from "async-iterator/channel-decorators";
const source = new EventSource(/*sse url*/);
withEmitter(c, source);