What is greenlet?

Greenlet is a tiny JavaScript library that takes an async function and moves it into a separate background thread in the browser. Normally, JavaScript in a web page runs on a single thread, which means that heavy or slow computations can make the page feel sluggish or unresponsive while they run. Browsers offer a feature called Web Workers that lets you run code in a separate thread, but setting one up directly involves some boilerplate. Greenlet removes that setup overhead. You pass your function to greenlet once, and it returns a new version of that function that runs in its own worker thread whenever you call it. The function must be self-contained, meaning it cannot reference variables or imports from the surrounding file, because it runs in a completely separate context. The README notes that greenlet works best when the data being passed in and out is small, since larger data has to be copied between threads. The README shows an example where a function fetches data from a network API and extracts just one field from the response. The whole function is wrapped with greenlet so the fetch and JSON parsing happen in the background without blocking anything else on the page. The library is very small, around one kilobyte compressed. It supports all major browsers including Internet Explorer 10 and later. If your site uses a Content Security Policy for security, you need to add specific worker permissions to your policy for greenlet to work. The library is MIT licensed and was created by Jason Miller, who also made a related library called workerize for moving entire modules into workers.