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What is Server Sent Event

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The Server Sent Events specification describes a built-in class EventSource, that keeps connection with the server and allows to receive events from it. similar to WebSocket it’s simpler. In many applications, the power of WebSocket is a little bit too much.

What difference:

  • One-directional: only server sends data

  • Only text

  • Regular HTTP

Getting messages

  1. To start receiving messages, we just need to create new EventSource(url).
  2. The browser will connect to url and keep the connection open, waiting for events.
  3. The server should respond with status 200 and the header Content-Type: text/event-stream
let eventSource = new EventSource("/events/channel");
eventSource.onmessage = function(event) {
  console.log("New message",;
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Upon creation, new EventSource connects to the server, and if the connection is broken – reconnects. that’s very convenient, as we don’t have to care about it.


let eventSource = new EventSource(...);
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Event types

By default EventSource object generates three events:

  • message – a message received, available as
  • open – the connection is open.
  • error – the connection could not be established, e.g. the server returned HTTP 500 status.

Hope it can help you :)

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