eightyfive eightyfive - 8 months ago 538
Javascript Question

How to parse multipart/form-data body with Koa?

Because I spent some (too much) time figuring out this simple requirement. I am documenting here the way to achieve

body parsing with Koa.

In my case, the reason of the confusion was the number of alternatives available out there:

And I wanted to find the most minimalist/close to
way/philosophy of doing things.

So here it is. Below. In accepted answer. Hope this helps.


You have to use koa-multer as stated in the official Koa wiki.

So a simple setup would look like:

const koa = require('koa');
const multer = require('koa-multer');

const app = koa();


app.use(function *() {
  this.body = this.req.body;

A couple of notes:

  • Multer will only parse bodies of requests of type multipart/form-data
  • Notice the use of this.req.body instead of Koa's supercharged this.request (not sure if this is intentional but this is confusing for sure... I would expect the parsed body to be available on this.request...)

And sending this HTML form as FormData:

  <input type="hidden" name="topsecret" value="1">
  <input type="text" name="area51[lat]" value="37.235065">
  <input type="text" name="area51[lng]" value="-115.811117">

Would give you access to nested properties as expected:

// -> console.log(this.req.body)
  "topsecret": 1,
  "area51": {
    "lat": "37.235065",
    "lng": "-115.811117",