Server-Sent Event

Introduction

With server-sent events, it's possible for a server to send new data to a web page at any time, by pushing messages to the web page. These incoming messages can be treated as Events + data inside the web page.
It is similar with web socket, but 1-way direction, from server to client
You'll need a bit of code on the server to stream events to the front-end, but the client side code works almost identically to websockets in part of handling incoming events.

Implementation

Backend

app.get('/', (req, res) => {
    // Content Type must defined as text/event-stream
    res.setHeader('Content-Type', 'text/event-stream');
    let counter = 0;
    const intervalId = setInterval(() => {
        counter++;
        // Write the event stream format
        // the text must follow the below, including numbers of \n
        res.write(`event:message\n data: Message ${counter}\n\n`);
    }, 2000);

    // When client closes connection, stop sending events
    req.on('close', () => {
        clearInterval(intervalId);
        res.end();
    });
});

Frontend

// declare the server source
const evtSource = new EventSource({apiUrl}, {
  withCredentials: true,
});
// listen to the server message
evtSource.onmessage = (event) => {
  const newElement = document.createElement("li");
  const eventList = document.getElementById("list");

  newElement.textContent = `message: ${event.data}`;
  eventList.appendChild(newElement);
};