async-to-gen

async-to-gen

Turn your JavaScript with async functions into ES6 generators so they can be used in modern browsers or in node.js (v0.12 or newer).

Async functions are an exciting new proposed addition to JavaScript. The v8 team is hard at work getting it right, which means it could appear in future versions of node.js. However if you're impatient like me, then you probably just can't wait to get rid of your promise triangles and callback hell.

You can use Babel to accomplish this, but async-to-gen is a faster, simpler, zero-configuration alternative with minimal dependencies for super-fast npm install and transform time.

Also, async-to-gen provides support for async generators which return Async Iterators, a great syntax and primitive for producing and operating on streams of data.

Get Started!

Use the command line:

npm install --global async-to-gen

async-to-gen --help
async-to-gen input.js > output.js

Or the JavaScript API:

npm install async-to-gen

var asyncToGen = require('async-to-gen');
var fs = require('fs');

var input = fs.readFileSync('input.js', 'utf8');
var output = asyncToGen(input).toString();
fs.writeFileSync('output.js', output);

// source maps!
var map = asyncToGen(input, { sourceMaps: true }).generateMap();
fs.writeFileSync('output.js.map', output);

Use async-node

Wherever you use node you can substitute async-node and have a super fast async functions aware evaluator or REPL.

$ async-node
> async function answer() {
... return await 42
... }
undefined
> promise = answer()
Promise { <pending> }
> promise.then(console.log)
Promise { <pending> }
42

Use the require hook

Using the require hook allows you to automatically compile files on the fly when requiring in node, useful during development:

require('async-to-gen/register')
require('./some-module-with-async-functions')

As always, don't forget to use async-to-gen to compile files before distributing your code on npm, as using the require hook affects the whole runtime and not just your module.

Use in Build Systems:

Rollup: rollup-plugin-async

Common Usage

Async I/O

Async functions are great for writing asynchronous code that looks synchronous, and that's perfect for writing code that needs to perform async I/O operations.

One of the original motivations for Node.js was non-blocking I/O, perfect! However its core libraries do not yet support Promises, nor do many popular libraries written for Node 😭.

Do not fret, we can fix this with promisify-node!

const promisify = require('promisify-node');
const fs = promisify('fs');

async function emojify(filePath) {
  const contents = await fs.readFile(filePath, 'utf8')
  const edited = contents.replace(/:\)/g, '😉')
  await fs.writeFile(filePath, edited, 'utf8')
}

Mocha

Writing tests in mocha? Async functions are super handy for testing any code using promises and already work out of the box! To enable async functions in mocha include the require hook when you run your tests:

mocha --require async-to-gen/register test.js

Then in your tests, use async functions in your it clauses:

describe('My Promising Module', () => {

  it('promises to give a value', async () => {
    expect(await myFunction('input')).to.equal('output')
  })

})

Testing your express app? Try supertest-as-promised and async functions:

const express = require('express')
const request = require('supertest-as-promised')

const app = express()
app.get('/foo', (req, res) => res.send('bar'))

describe('My express app', () => {

  it('loads foo', async () => {
    const response = await request(app).get('/foo')
    expect(response).to.equal('bar')
  })

})

Jest

Writing tests in Jest? Use the scriptPreprocessor entry in your jest configuration in package.json:

{
  "name": "my-project",
  "jest": {
    "scriptPreprocessor": "async-to-gen"
  },
  "devDependencies": {
    "async-to-gen": "*"
  }
}

Scripts

Have interactive scripts that require lots of input from the user? Async functions make writing those much easier! Check out interactive-script.

npm install interactive-script

Then write your script:

const interactive = require('interactive-script')
interactive(async (say, ask) => {
  say('What is your favorite number?')
  let num;
  do {
    num = Math.ceil(Math.random() * 100)
  } while (!await ask(`Is it ${num}? `, 'yN'))
  say(`Great, I think ${num} is a fine number.`)
})

And run it with async-node:

async-node myScript.js

Streams

Streaming data can be a challenging thing to get right. While Observables have provided a great library for streamed data, Async Generators provides language-level support for this concept!

Consider subscribing to a web socket within an program using async functions:

async function* stockTickerInEuro(symbol) {
  var socket = await openWebSocket('ws://stocks.com/' + symbol)
  try {
    for await (var usd of socket) {
      var euro = usd * await loadExchangeRateUSD2EUR()
      yield euro
    }
  } finally {
    closeWebSocket(socket)
  }
}

Then calling this function produces an Async Iterator (an Iterator of Promises) of stock ticker values.

var ticker = stockTickerInEuro('AAPL')
ticker.next().then(step => console.log(step.value))

Or use for-await loops within another async function:

async function bloombergTerminal() {
  for await (var price of stockTickerInEuro('AAPL')) {
    console.log(price)
  }
}

NOTE: The behavior of for-await loops using this tool is not identical to the proposed spec addition. Where the proposed spec's for-await expects a Symbol.asyncIterator method for the iterated source, this tool expects Symbol.iterator instead since it transforms it to a for-of loop.

Dead-Simple Transforms

When async-to-gen transforms async functions, it makes as few edits as possible, and does not affect the location of lines in a file, leading to easier to understand stack traces when debugging.

It also includes a very small conversion function at the bottom of the file. How small? 204 chars for async functions and 533 chars for async generators.

Before:

async function foo() {
  return await x
}

After:

function foo() {return __async(function*(){
  return yield x
}())}

function __async(g){/* small helper function */}

Using with Babel

Don't bother using both! If you're already using Babel (maybe you need JSX, other proposed features, or are supporting older versions of Node) then you might be excited to hear that using babel-preset-es2017 in your .babelrc will provide support for async functions!

Babel is an amazing tool that you should consider using, however async-to-gen intentionally makes some different choices to provide a different set of trade-offs. Babel is general-purpose and supports a wider set of features but requires some configuration and more dependencies and those features may cost build performance or output code quality. async-to-gen can only do one thing, but that simplicity lends itself towards faster builds and cleaner output.

Ultimately, if you only need to add async function support you should try async-to-gen but if you need more features then you should use Babel.