How to parse Multi-part form data in an Azure Function App with HTTP Trigger? (NodeJS)

To answer original question:

However, NodeJS multipart form data parsers are all in the form of httpserver or expressJS middleware.

Is there any available tools that can parse the multipart form data after it has all been received from the Function Application's wrapper?

Even 2 years later after you asked this question state of multipart form data parsers is not great, like you noticed majority of them assume req object which is a stream and tutorials/demos show how to parse multipart/form-data with express or httpServer.

However there is a parse-multipart npm package which can process req.body from azure function and return you array of objects with code similar to following:

const multipart = require("parse-multipart");
 

module.exports = function (context, request) {  
    context.log('JavaScript HTTP trigger function processed a request.'); 
    // encode body to base64 string
    const bodyBuffer = Buffer.from(request.body);
     
    const boundary = multipart.getBoundary(request.headers['content-type']);
    // parse the body
    const parts = multipart.Parse(bodyBuffer, boundary);
    context.res = { body : { name : parts[0].filename, type: parts[0].type, data: parts[0].data.length}}; 
    context.done();  
}; 

(original source: https://www.builtwithcloud.com/multipart-form-data-processing-via-httptrigger-using-nodejs-azure-functions/)

One area where I noticed parse-multipart can struggle is parsing forms with text fields. A slightly improved version which handles it better is called multipart-formdata:

require('multipart-formdata').parse(req.body, boundary)
//returns [{field, name, data, filename, type}, ...] where data is buffer you can use to save files

I have good experiences with the new azure-function-multipart package. Example code might look like this:

const {
  default: parseMultipartFormData,
} = require("@anzp/azure-function-multipart");

module.exports = async function (context, req) {
  const { fields, files } = await parseMultipartFormData(req);
  console.log("fields", fields)
  console.log("files", files)
};

See docs for more details.

You can try to use this adapter for functions and express, it may allow you to successfully use the multi-part middleware you want: https://github.com/yvele/azure-function-express

As a less desirable option, you can parse the body yourself, all the multi-part data will be available in req.body and will look something like this:

------WebKitFormBoundarymQMaH4AksAbC8HRW
Content-Disposition: form-data; name="key"

value
------WebKitFormBoundarymQMaH4AksAbC8HRW
Content-Disposition: form-data; name=""


------WebKitFormBoundarymQMaH4AksAbC8HRW--

I do think it's a good idea to support httpserver / express better in order to enable this extensibility.