andreldm/app.js

## app.js
const express = require('express');
const proxy = require('express-http-proxy');
const app = express();

app.use(express.static('static'));
app.use('/proxy', proxy((req) => req.header('X-Proxy-Target')));
app.listen(3000, () => console.log("App running at port 3000"));

## index.html
<!DOCTYPE html>
<html lang="en">

<!-- place in a 'static' folder -->

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Express Proxy POC</title>
    <style>
        body>* {
            margin-top: 16px;
        }
    </style>
</head>

<body>
    <div>
        <label for="es">Elasticsearch URL</label>
        <input id="es" value="http://localhost:9200">
    </div>
    <div>
        <label for="query">Query</label>
        <input id="query" value="_cat/indices">
    </div>
    <div>
        <label for="method">Method</label>
        <input id="method" value="GET">
    </div>
    <div>
        <div>
            <label for="body">Body</label>
        </div>
        <textarea id="body" rows="10" cols="60"></textarea>
    </div>
    <div>
        <button onclick="test(false)">Test direct connection</button>
        <button onclick="test(true)">Test proxied connection</button>
    </div>

    <textarea id="result" rows="20" cols="80" readonly></textarea>
    <script>
        function getBody() { return getMethod() == 'GET' ? undefined : document.getElementById('body').value; }
        function getPath() { return `${document.getElementById('query').value}?format=json&pretty`; }
        function getMethod() { return document.getElementById('method').value; }
        function getURL() { return `${document.getElementById('es').value}/${getPath()}` }

        function test(proxy) {
            var url = getURL();
            var options = {
                body: getBody(),
                method: getMethod(),
                headers: { 'Content-Type': 'application/json' }
            }

            if (proxy) {
                url = 'http://localhost:3000/proxy/' + getPath();
                options.headers['X-Proxy-Target'] = getURL();
            }

            fetch(url, options)
                .then((response) => { return response.text(); })
                .then((data) => { document.getElementById('result').innerHTML = data; })
                .catch((error) => { document.getElementById('result').innerHTML = error; });
        }
    </script>
</body>

</html>

## package.json
{
  "name": "express-proxy-poc",
  "version": "0.0.1",
  "main": "app.js",
  "dependencies": {
    "express": "^4.17.1",
    "express-http-proxy": "^1.6.0"
  }
}
	const express = require('express');
	const proxy = require('express-http-proxy');
	const app = express();

	app.use(express.static('static'));
	app.use('/proxy', proxy((req) => req.header('X-Proxy-Target')));
	app.listen(3000, () => console.log("App running at port 3000"));
	<!DOCTYPE html>
	<html lang="en">

	<!-- place in a 'static' folder -->

	<head>
	<meta charset="UTF-8">
	<meta name="viewport" content="width=device-width, initial-scale=1.0">
	<title>Express Proxy POC</title>
	<style>
	body>* {
	margin-top: 16px;
	}
	</style>
	</head>

	<body>
	<div>
	<label for="es">Elasticsearch URL</label>
	<input id="es" value="http://localhost:9200">
	</div>
	<div>
	<label for="query">Query</label>
	<input id="query" value="_cat/indices">
	</div>
	<div>
	<label for="method">Method</label>
	<input id="method" value="GET">
	</div>
	<div>
	<div>
	<label for="body">Body</label>
	</div>
	<textarea id="body" rows="10" cols="60"></textarea>
	</div>
	<div>
	<button onclick="test(false)">Test direct connection</button>
	<button onclick="test(true)">Test proxied connection</button>
	</div>

	<textarea id="result" rows="20" cols="80" readonly></textarea>
	<script>
	function getBody() { return getMethod() == 'GET' ? undefined : document.getElementById('body').value; }
	function getPath() { return `${document.getElementById('query').value}?format=json&pretty`; }
	function getMethod() { return document.getElementById('method').value; }
	function getURL() { return `${document.getElementById('es').value}/${getPath()}` }

	function test(proxy) {
	var url = getURL();
	var options = {
	body: getBody(),
	method: getMethod(),
	headers: { 'Content-Type': 'application/json' }
	}

	if (proxy) {
	url = 'http://localhost:3000/proxy/' + getPath();
	options.headers['X-Proxy-Target'] = getURL();
	}

	fetch(url, options)
	.then((response) => { return response.text(); })
	.then((data) => { document.getElementById('result').innerHTML = data; })
	.catch((error) => { document.getElementById('result').innerHTML = error; });
	}
	</script>
	</body>

	</html>
	{
	"name": "express-proxy-poc",
	"version": "0.0.1",
	"main": "app.js",
	"dependencies": {
	"express": "^4.17.1",
	"express-http-proxy": "^1.6.0"
	}
	}