test_app/frontend.js

function updateLights(state) {
    document.getElementById("red").classList.toggle("red", state === "red");
    document.getElementById("green").classList.toggle("green", state === "green");
}

function fetchStatus() {
    fetch("/status")
        .then(response => response.json())
        .then(data => updateLights(data.state))
        .catch(console.error);
}

function toggleLight() {
    fetch("/toggle", { method: "POST" })
        .then(response => response.json())
        .then(data => updateLights(data.state))
        .catch(console.error);
}

setInterval(fetchStatus, 1000);