Local Node http proxy with CORS headers

cors-proxy.js

// The use case is that you are developing a web app locally that makes Ajax requests to some external
// API that doesn't have sufficiently permissive CORS headers so your requests fail. The solution is to point the app
// to this local proxy which will allow CORS.
const http = require('http')
const axios = require('axios')

const HOSTNAME = '127.0.0.1'
const PROXY_BASE_URL = process.env.PROXY_BASE_URL;
const PORT = process.env.PORT || 9999

function addCorsHeaders(res) {
  const headers = {
    'Access-Control-Allow-Credentials': true,
    'Access-Control-Allow-Headers': '*',
    'Access-Control-Allow-Methods': 'GET,POST,PUT,PATCH,DELETE,OPTION',
    'Access-Control-Allow-Origin': '*',
  }
  Object.keys(headers).forEach((header) => {
    res.setHeader(header, headers[header])
  })
}

function proxyRequestHeaders(headers) {
  const OMIT_HEADERS = ['host']
  return Object.entries(headers).reduce((acc, [key, value]) => {
    if (!OMIT_HEADERS.includes(key)) acc[key] = value
    return acc
  }, {})
}

function addProxyResponseHeaders(headers, res) {
  for (const [key, value] of Object.entries(headers)) {
    res.setHeader(key, value)
  }
  addCorsHeaders(res)
}

const server = http.createServer(async (req, res) => {
  if (req.method === 'OPTIONS') {
    addCorsHeaders(res)
    res.end()
  } else if (req.method === 'GET') {
    console.log(`cors-proxy req.url=${req.url}`)
    const url = `${PROXY_BASE_URL}${req.url}`
    const headers = proxyRequestHeaders(req.headers)
    const validateStatus = () => true // make axios not throw on non-2xx status
    const transformResponse = (data) => data; // make axios not parse JSON body
    try {
      const response = await axios.request({ url, headers, validateStatus, transformResponse })
      res.statusCode = response.status
      addProxyResponseHeaders(response.headers, res)
      res.end(response.data)
    } catch (error) {
      console.log('cors-proxy error', error)
      res.statusCode = 500
      res.end(toString(error))
    }
  } else {
    res.statusCode = 400
    const message = `cors-proxy: request method ${req.method} unsupported (url=${req.url})`
    console.log(message)
    res.end(message)
  }
})

server.listen(PORT, HOSTNAME, () => {
  console.log(`cors-proxy running at http://${HOSTNAME}:${PORT}/ with PROXY_BASE_URL=${process.env.PROXY_BASE_URL}`)
});