This assumes you are already familiar with Express. If not, check out my article FreeKB - Node.js - Getting Started with Client Server Express on Docker. 

req.query can be used to return URL parameters. Let's say you have the following Express app and you submit a request to the / endpoint with URL parameter foo=bar (e.g. http://www.example.com/?foo=bar)

import express from "express"
const app = express()
const timestamp = () => Date()

app.get('/', (req, res) => {
  console.log(`${timestamp()} Hello Console`)
  console.log(req.query)
  res.send(`${timestamp()} Hello World`)
})

app.listen(12345)
console.log(`Node.js app is up and running!`);

Running this app should return something like this in the console log.

{"foo": "bar"}

And here is now you can return a specific request URL parameter, "foo" in this example.

import express from "express"
const app = express()
const timestamp = () => Date()

app.get('/', (req, res) => {
  console.log(`${timestamp()} Hello Console`)
  console.log(req.query.foo)
  res.send(`${timestamp()} Hello World`)
})

app.listen(12345)
console.log(`Node.js app is up and running!`);

And you probably should have an if / else statement to determine if the URL parameter exists.

import express from "express"
const app = express()
const timestamp = () => Date()

app.get('/', (req, res) => {
  console.log(`${timestamp()} Hello Console`)
  if (req.query.foo == undefined) {
    console.log("req.query.foo is undefined")
  } else {
    console.log(req.query.foo)
  }
  res.send(`${timestamp()} Hello World`)
})

app.listen(12345)
console.log(`Node.js app is up and running!`);

Be aware that if you are using a Node.js client/server model, you can use the above in server, but in client, you'd go with Javascript, something like this.

const url = window.location.href

Or, you can include parameters in a Javascript script tag.

<script src="example.js" foo="bar"></script>

And then access the value like this in Javascript or Node.js client.

const foo = document.currentScript.getAttribute('foo');