Since Python can't run in the browser, you'll need to set up a server to run the Python function, which you can request in JavaScript.

Here is a basic example using the Flask framework:

from flask import Flask

app = Flask(__name__)

@app.route("/doubletwo")
def doubletwo():
  return 2 * 2

app.run("0.0.0.0")

Run that on a separate repl, then copy its' URL. Now, request it using JavaScript's fetch API:

async function main() {
    let r = await fetch("https://your-url.repl.co/doubletwo") // replace this with your url
    if (r.ok) {
        let text = await r.text()
        alert("Return value from function: " + text)
    } else {
        alert("There was an error")
    }
}

main()