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🦎 Move an async function into its own thread.
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Greenlet npm travis gzip size install size

Move an async function into its own thread.

A simplified single-function version of workerize, offering the same performance as direct Worker usage.

The name is somewhat of a poor choice, but it was available on npm.

Greenlet only supports browser environments, since it uses Web Workers. For use in a NodeJS environment, Web Workers must be polyfilled using a library like node-webworker.

Installation & Usage

npm i -S greenlet

Accepts an async function with, produces a copy of it that runs within a Web Worker.

⚠️ Caveat: the function you pass cannot rely on its surrounding scope, since it is executed in an isolated context.

greenlet(Function) -> Function


Greenlet is most effective when the work being done has relatively small inputs/outputs.

One such example would be fetching a network resource when only a subset of the resulting information is needed:

import greenlet from 'greenlet'

let getName = greenlet( async username => {
    let url = `${username}`
    let res = await fetch(url)
    let profile = await res.json()

console.log(await getName('developit'))

πŸ”„ Run this example on JSFiddle

Transferable ready

Greenlet will even accept and optimize transferables as arguments to and from a greenlet worker function.

Browser support

Thankfully, Web Workers have been around for a while and are broadly supported by Chrome, Firefox, Safari, Edge, and Internet Explorer 10+.

If you still need to support older browsers, you can just check for the presence of window.Worker:

if (window.Worker) {
} else {


If your app has a Content-Security-Policy, Greenlet require worker-src data: and script-src data: in your config.

License & Credits

In addition to the contributors, credit goes to @sgb-io for his annotated exploration of Greenlet's source. This prompted a refactor that clarified the code and allowed for further size optimizations.

MIT License

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