The EventSource's message object not have a retry property bug

Start with 5-network/12-server-sent-events/article.md, especially the server response format section, and compare its EventSource event properties with the linked MDN guidance. Check whether the example or wording needs correction around retry, then update the article so the documented behavior and example agree.

The EventSource's message object not have a retry property bug

server.js

import fs from 'node:fs';
import express from 'express';

const app = express();
app.use(express.static('public'));

const convertImageToBase64URL = (filename, imageType = 'png') => {
  try {
    const buffer = fs.readFileSync(filename);
    // const base64String = Buffer.from(buffer, 'base64');
    const base64String = Buffer.from(buffer).toString('base64');
    // console.log(`base64String`, base64String.slice(0, 100));
    return `data:image/${imageType};base64,${base64String}`;
  } catch (error) {
    throw new Error(`file ${filename} no exist ❌`)
  }
}

app.get('/sse', (req, res) => {
  res.writeHead(200, {
    // 'Content-Type': 'text/event-stream',
    'Content-Type': 'text/event-stream; charset=utf-8',
    'Cache-Control': 'no-cache',
    'Connection': 'keep-alive',
    'Access-Control-Allow-Origin': "*",
  });
  let i = 0;
  let uid = setInterval(() => {
    console.log(`i`, i);
    if(i < 10) {
      i++;
      const data = convertImageToBase64URL("./public/test.png");
      console.log(`SSE ${i}`, data.slice(0, 100));
      // event id `lastEventId` ✅
      const id = 2023;
      // ms
      const retryTimes = 1000 * 60;
      res.write(`id: ${id}\n`);
      res.write(`retry: ${retryTimes}\n`);
      // custom event type `custom_message`\n ✅
      res.write(`event: custom_event_message\n`);
      res.write(`data: ${data}\n\n`);
    } else {
      clearInterval(uid);
    }
  }, 3000);
});
// http://localhost:3000/sse


const port = 3000;
app.listen(port, () => {
  console.log(`SSE server is running on: http://localhost:${port}/`);
});

client.js

window.addEventListener(`load`, (e) => {
  console.log(`page loaded ✅`);
  if (!!window.EventSource) {
    const img = document.querySelector(`#sse`);
    const source = new EventSource('http://localhost:3000/sse');
    source.onopen = (event) => {
      console.log(`✅ Connection to server opened.`, event);
    };
    source.addEventListener(`custom_event_message`, (event) => {
      console.log(`\nevent`, event);
      const timestamp = event.timeStamp;
      console.log(`event.timeStamp`, timestamp);
      const retry = event.retry;
      console.log(`event.retry`, retry);
      // undefined ❌
      const id = event.lastEventId;
      console.log(`event.lastEventId`, id);
      const type = event.type;
      console.log(`event.type`, type);
      const data = event.data;
      // console.log(`🚀 event.data =`, data);
      img.src = `${data}`;
    });
    source.onerror = (err) => {
      console.log(`❌ EventSource failed.`, err);
      // setTimeout(() => {
      //   console.log(`⚠️ After 3 seconds, auto close connection!`);
      //   source.close();
      // }, 3000);
    };
  } else {
    console.log(`Your browser doesn't support SSE ❌`);
  }
});

refs

https://javascript.info/server-sent-events#server-response-format

https://developer.mozilla.org/en-US/docs/Web/API/Server-sent_events/Using_server-sent_events#retry