Shiming Request.formData in Safari

I’m currently building a simple CRUD client-side only data logger PWA that contains no client-side JavaScript apart from what is inside the service worker. 

To do that I am following basic REST principles, and just using the Form element to submit data to the service-worker that then stores in it IndexedDB and renders the results back out to the client. I will explain more in another post, but in this post I want to quickly document how I fixed an issue in Safari.

When reading the request data that is posted by the user from a Form, you can use the event.request.formData() method to get a FormData, however it throws an exception in Safari (yet it works in Firefox and Chrome). It turns out the WebKit team haven’t implemented it properly yet.

I wrote a little shim that hack together a similar looking API to the FormData object so that you can process form data inside your service worker.

export default async (request) => {
  if (request.headers.get('content-type') !== 'application/x-www-form-urlencoded') {
    return null;

  const data = await request.text();
  const params = new URLSearchParams(data);

  return params;

The TL;DR is that for simple www-url-form-encoded forms, the data is passed in as a query string, which means that if you create a URL object and then return the data in the searchParams property, you get something that looks similar to the FormData object.

If your form is a multipart form (i.e, one that attaches a file), then you will need to use another solution that parses that data.

Hat-tip to Ingvar for mentioning that we don’t need to make a fake URL, we can just instantiate URLSearchParams. Also, Hat-tip again to him for making sure we should mention that this doesn’t work for multipart forms.

Picture of me smiling.

Paul Kinlan

I lead the Chrome Developer Relations team at Google.

We want people to have the best experience possible on the web without having to install a native app or produce content in a walled garden.

Our team tries to make it easier for developers to build on the web by supporting every Chrome release, creating great content to support developers on, contributing to MDN, helping to improve browser compatibility, and some of the best developer tools like Lighthouse, Workbox, Squoosh to name just a few.

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