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: ' + 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?
1 Answer
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 = ` 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}`)) 
