How can I use Async Generators in JavaScript?

Question

I have an api thats going to return a cursor for fetching more data. I've mocked it out like this:

function fetch(n) {
  return Promise.resolve({
    results: [n],
    next: next < 10 && n + 1,
  })
}

What I'm trying to do is figure out how I can use async/await along with generators in order to interact with this api.

Here's basically what I've prototyped:

async function* api(url) {
  let result = await fetch(url)
  yield result
  while (result.next) {
    result = await fetch(result.next)
    yield result
  }
}

The idea is that I should be able to create an async generator and yield from that generator in order to iterate through the cursor:

async function main() {
  const gen = api(0)
  const zero = await gen.next()
  console.log(zero.result)
  const one = await gen.next()
  console.log(one.result)
  const rest = await Promise.all([...gen])
  console.log(rest.map(r => r.result))
}

All things considered, I think this is a pretty sweet way of handling paginated data and being able to pull out all of the data with [...gen] is pretty damn cool.

Only problem is, it doesn't work! Apprently you can't use async with function*:

❯❯❯ node --version
v7.0.0
❯❯❯ node --harmony --harmony-async-await async-generator.js
/Users/chetcorcos/code/async-generator.js:11
async function* api(url) {
              ^
SyntaxError: Unexpected token *
    at Object.exports.runInThisContext (vm.js:76:16)
    at Module._compile (module.js:545:28)
    at Object.Module._extensions..js (module.js:582:10)
    at Module.load (module.js:490:32)
    at tryModuleLoad (module.js:449:12)
    at Function.Module._load (module.js:441:3)
    at Module.runMain (module.js:607:10)
    at run (bootstrap_node.js:382:7)
    at startup (bootstrap_node.js:137:9)
    at bootstrap_node.js:497:3

But I really feel like this should be possible. There's a popular library called co that I've been poking around with but I don't think that's what I want.

Any ideas how to get this concept of "async generators" to work?

Answer 1

You can do this using the Babel plugin transform-async-generator-functions.

The usage is like this:

const g = async i => [ 1, 2, 3 ]
  .map(x => x * 10 ** i);

const f = async function * () {
  for (let i = 0; i < 10; i++) {
    const xs = await g(i);
    for (const x of xs) {
      yield x;
    }
  }
};

const main = async () => {
  for await (const x of f()) {
    console.log(x);
  }
};

main().catch(e => console.error(e));

Here is an example repo showing how to setup your project.

The important part is the .babelrc file:

{
  "presets": [ "env" ], 
  "plugins": [ "transform-async-generator-functions" ]
 }