generator based async control flow

Usage no npm install needed!

<script type="module">
  import gentrify from '';



Browser demo

Small, light, fast generator-based async control flow for ES6. gentrified code can invoke async functions as if they were synchronous. No more callback hell.

gentrify is only 40 lines, and fits nicely in your brain's L-1 cache. It is about 10x lower overhead than similar solutions with promises. Async call stack depth can exceed a million without hitting stack overflow. Explicit tail calls are optimized.

Async functions are written as generators and use the yield operator to toss async requests at the gentrify trampoline. Request types include other generators, promises and callback-based APIs, which are resolved and bounced back via the return value of yield. Errors in async functions are thrown as exceptions in the caller.

This project is an experiment; caveat user.


var run = require('gentrify').run,
    topics = ["generator", "es6"];

function* githubSearch(topics) {
    for (var t of topics) {
        // invoke a promise-based API
        var data = yield $.getJSON(`${t}&sort=stars`);
        console.log(data.items.slice(0, 5).map(x => `${x.full_name},${x.stargazers_count}`).join('\n'));
        console.log("sleeping 3 seconds...");
        // invoke another async function
        yield sleep(3000);
    return "done!";

function* sleep(ms) {
    // invoke a callback-based API
    return yield cb => setTimeout(cb, ms);
    .then(x => console.log(x))
    .catch(x => console.log("Error", x));

How does gentrify compare to X

Some functions are born async, some achieve asyncness, but most have asyncness thrust upon them. A function way down the stack needs to be async and pretty soon, every intermediate function sports a callback monkey on its back.

The gentrify trampoline adds very little overhead for the common case of intermediate functions. Generator-generator calls are accumulated in an array. Since it is on the heap, you can have async call depths in the millions without overflowing the stack. Moreover, the generator can return an explicitly flagged tail call, which the trampoline can optimize to minimal stack and heap overhead.

CSP channels are a powerful concept for processing streams and events. But making every intermediate single-output function into a goroutine feels like overkill.

Co is very small and easy to understand. It deprecates thunks, and wraps everything in promises, which limit stack depth to about 1,000 - which should be good enough in most cases. Co is the most popular and battle-tested of the lot, and should be preferred for serious usage.

Consider the following function which recursively computes the sum of the first n integers. The idea is to measure the "sync convenience tax" we pay for using these libraries.

function* sumn(x) {
    return x <= 0 ? 0 : (yield sumn(x - 1));

See a browser demo or run npm test.

sumn(1000) takes 13-15ms on co and js-csp. gentrify takes 1-3ms.

A gentrified sumn can recurse millions of times without breaking a sweat or blowing up the stack - a feat which can't be equalled by either of the above, which run out of stack around n = 1,000. Even a plain synchronous recursive function runs out of stack around n = 100,000.

(Tests done on node v6.1.0 on a 2016 Macbook Pro)


function* asyncfunc(...args)

A gentrifiable async function is specified as follows:

  • It must be a generator function.
  • It can yield the following async operations:
    • Generator object. This in turn must have been created from a gentrifiable generator function.
    • Promise or thenable.
    • Thunk. This is a function taking a single callback argument. Used to invoke NodeJS-style callback APIs. e.g. function(cb) { return fs.readFile("name", "utf-8", cb); }
  • It uses return to pass its result to its caller.
  • It can signal a tail call by returning a generator object wrapped by e.g.
  • Errors from yieldables are thrown and can be caught by the caller.
  • It signals errors to its caller by throwing an exception or returning an Error instance. In both cases, the caller receives an exception.

Arrays and objects are not supported as yieldables. Use Promise.all() for parallel operations. val => )

Returns a promise that resolves a generator (not a generator function).

var run = require('gentrify').run;
function* githubSearch(list) {...}

run(githubSearch(["ajax", "cors"]))
    .then(res => console.log(res), err => console.log("error", err.message));, function cb(err, res){})

Runs the generator and invokes the callback cb with the error or result.

var run = require('gentrify').run;
function* githubSearch(list) {...}

run(githubSearch(["ajax", "cors"], (err, res) => {
    if (err) {
        return console.log("error", err.message);

Wraps the generator object in a structure recognized as a tail call by the trampoline. The wrapped object can be supplied as the argument to the return statement of a generator function to trigger tail call optimization.

var run = require('gentrify').run,
    tc = require('gentrify').tc;

function* sumn(n, acc=0) {
    return n <= 0 ? acc : tc(sumn(n - 1, n + acc));

    .then(res => console.log(res), err => console.log("error", err.message));

Platform Compatibility

gentrify requires a browser which implements ES6 (as of late 2016, most
desktop browsers including Chrome 53.0+, Firefox 48.0+ and Safari 10.0+ are supported).

Node v6.1.0 and above work out of the box.

Workarounds for older browsers and node exist, but I haven't tested any of them.