Why and how this simple node.js proxy works? - node.js

Question

I have a frontend-only web application on Netlify which has to consume an API on OpenSubtitles.org. Although OpenSubtitles.org enables CORS, sometimes I got preflight errors, so I decided to use a proxy.
I had problems using Netlify's proxy feature, so I decided I will create my own proxy on Heroku, and send my requests from the frontend to there, so these will be proxied to OpenSubtitles.org from a server.
I came up with the following based on the code I found here:
const express = require('express');
const request = require('request');
express()
.use('/', function(req, res) {
req.pipe(
request({
url: 'http://rest.opensubtitles.org/search' + req.url,
headers: {
'User-Agent': 'TemporaryUserAgent'
}
})
).pipe(res);
})
.listen(process.env.PORT || 8000);
I thought I deploy this, try it out, then I will enable CORS on it after that. However I've just realized it is working perfectly without doing anything else. How is it possible? Why can I call this from a frontend-only app on a different domain without explicitly enabling CORS?
Also, what if the server crashes, how to handle the errors there?

Answer 1

CORS is working because the url you're requesting responds with the header Access-Control-Allow-Origin set with a value of *. Since you're piping that response and its headers back to the original res object, it will enable CORS as if it was coming from your local proxy.
Below is a more straightforward example of how to proxy a request to another site and return its response intact using node streams.
const express = require('express')
const request = require('request')
const port = process.env.PORT || 1337
let server = express()
const proxyMiddleware = (req, res, next) => {
let url = `https://www.google.com/${req.url}`
let proxyRequest = request(url)
// Pass request to proxied request url
req.pipe(proxyRequest)
// Respond to the original request with the response from proxyRequest
proxyRequest.pipe(res)
}
server.use(proxyMiddleware)
server.listen(port, () => console.log(`Listening on ${port}`))