How can I convert an async iterator to an array?

Question

Given I have an async generator:

async function* generateItems() {
    // ...
}

What's the simplest way to iterate all the results into an array? I've tried the following:

// This does not work
const allItems = Array.from(generateItems());

// This works but is verbose
const allItems = [];
for await (const item of generateItems()) {
    allItems.push(item);
}

(I know this is potentially bad practice in a Production app, but it's handy for prototyping.)

Answer 1

You can use the new Array.fromAsync() static method.

This will return a promise whose fulfillment value is a new Array instance.

Example from the ES proposal:

async function* f() {
  for (let i = 0; i < 4; i++) yield i;
}

// Resolves to [0, 1, 2, 3].
await Array.fromAsync(f());

Availability

Although it is still in stage 3, all major browsers and Node support it in their latest versions.

Answer 2

Here is a simple function to convert async iterator to an array without having to include a whole package.

async function toArray(asyncIterator){ 
    const arr=[]; 
    for await(const i of asyncIterator) arr.push(i); 
    return arr;
}

Answer 3

Your solution is the only one I know of. Here it is as a TypeScript function:

async function gen2array<T>(gen: AsyncIterable<T>): Promise<T[]> {
    const out: T[] = []
    for await(const x of gen) {
        out.push(x)
    }
    return out
}

TypeScript Playground

Answer 4

Looks like the async-iterator-to-array npm library does this:

Install

npm install --save async-iterator-to-array

Usage

const toArray = require('async-iterator-to-array')

async function * iterator (values) {
  for (let i = 0; i < values.length; i++) {
    yield values[i]
  }
}

const arr = await toArray(iterator([0, 1, 2, 3, 4]))

console.info(arr) // 0, 1, 2, 3, 4