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.

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