stream-tools/twitch/public/labels.js

const labelEl = document.getElementById("label");

const label = location.pathname.split("/twitch/labels/")[1];
const labelLatestFollower   = "latest-follower"
const labelLatestSubscriber = "latest-subscriber"
const labelLatestCheer      = "latest-cheer"
const allowedLabels = [
    labelLatestFollower,
    labelLatestSubscriber,
    labelLatestCheer,
]

/**
 * @param {string} label
 */
async function sync() {
    if (label == null) {
        const exampleUrl = `${location.protocol}//${location.host}/twitch/labels/${labelLatestFollower}`;

        labelEl.innerHTML =
            "Needs label to listen to, " +
            `such as <a href="${exampleUrl}">${labelLatestFollower}</a>`;
        console.error("Needs label to listen to, such as " + exampleUrl);
        return;
    }

    let eventSource;
    try {
        eventSource = new EventSource("/twitch/sse/" + label);
    } catch (err) {
        console.error("Failed to connect to event source", err);
        return;
    }

    eventSource.addEventListener("error", () => {
        eventSource.close();
        console.error("Connection lost, or an error has occurred.");
        console.log("Attempting to reconnect...");

        setTimeout(() => {
            sync();
        }, 1000);
    });

    eventSource.addEventListener("update", event => {
        console.log(`[${label}] ${event.data}`);
        labelEl.innerText = event.data;
    });

    eventSource.addEventListener("open", () => {
        console.log("Connected.");
    });
}

sync();