FormData parsing for Fastly Compute JavaScript

by Katsuyuki Omuro

Parses body streams of type multitype/form-data and provides them as a polyfill for Request.prototype.formData() and Response.prototype.formData(). This is meant to be used with Fastly Compute, but can probably be used with any platform that happens to provide Request/Response but not .prototype.formData().

Eventually, we expect to see native implementations to be coming to the Fastly Compute JavaScript runtime, and they will be more performant, but this polyfill can be used in the meantime.

Prerequisites

This polyfill does depend on the data structures File and FormData, which at the current time will themselves also need polyfills. blob-polyfill and formdata-polyfill seem to work.

You may face a gotcha when doing this; see the Caveats section below.

Usage

In your Compute project:

npm install blob-polyfill formdata-polyfill @h7/compute-js-formdata

Then, at the beginning of your program:

Object.assign(globalThis, require('blob-polyfill'));
require('formdata-polyfill');
require('@h7/compute-js-formdata');

Now, you can use formData():

const req = event.request;
const formData = await req.formData();
formData.get('foo'); // Value of form field 'foo'

Caveats

blob-polyfill

blob-polyfill does not automatically make its exports available globally, so you'll have to be sure to copy its exported members such as Blob and File to globalThis so that they are available to the other polyfills and to your program.

You can do this like this:

// require and merge with globalThis
Object.assign(globalThis, require('blob-polyfill'));

// require other polyfills
require('formdata-polyfill');
require('@h7/compute-js-formdata');

If you're using ES modules (i.e., import rather than require()) you'll have to be extra careful, because import statements get hoisted.

You'll want to place your blob-polyfill import in a separate file, e.g.:

// load-blob-polyfill.js
import * as blobPolyfill from 'blob-polyfill';
Object.assign(globalThis, blobPolyfill);

And then import that before the FormData polyfill:

// load blob polyfill and make global
import './load-blob-polyfill.js'

// load other polyfills
require('formdata-polyfill');
require('@h7/compute-js-formdata');

License

MIT.