server.js

import { Meteor } from 'meteor/meteor';

WebApp.connectHandlers.use('/file', (req, res) => {
    // We only care about PUT methods
    res.setHeader("Access-Control-Allow-Methods", "PUT");
    // I am running meteor as a backend, see https://iamlawrence.me/agnostic-meteor
    // Therefore we need to enable CORS
    res.setHeader("Access-Control-Allow-Origin", "*");
    res.setHeader("Access-Control-Allow-Headers", "Content-Type");

    // When sending a CORS request, a 'preflight' request is sent.
    // This will determine what methods are allowed on this endpoint.
    // Just respond happily with a 200 response
    // See more here: https://developer.mozilla.org/en-US/docs/Web/HTTP/Access_control_CORS#Preflighted_requests
    if (req.method === 'OPTIONS') {
        res.writeHead(200);
        res.end();
        return;
    } else if (req.method === 'PUT') {
        // In this example, I only care about images...
        if (!req.headers['content-type'].startsWith('image')) {
            res.writeHead(400);
            res.end();
        }
        
        // Because the 'readable' and 'end' events are async
        //   we wrap them with wrapAsync, because we want to
        //   receive the entire file before sending our response.
        // Read up on this here http://docs.meteor.com/api/core.html#Meteor-wrapAsync
        let getFile = Meteor.wrapAsync(done => {
            let chunks = [];
            req.on('readable', () => {
                chunks.push(req.read());
            });
            req.on('end', () => {
                done(undefined, Buffer.concat(chunks));
            });
        });

        // We grab the file using the wrapped async method 
        // that we just created above.
        let buffer = getFile();

        // Now you can do whatever you want with this file.
        // Pass it onto S3, write it to the File System,
        //   exec the file directly... 
        //   Actually, please don't do that. Please.

        // We can finally reply happily!
        res.writeHead(200);
        res.end();
    }
});