Server-Sent Events allow a web page to get updates from a server.

Server-Sent Events – One Way Messaging

A server-sent event is when a web page automatically gets updates from a server.

This was also possible before, but the web page would have to ask if any updates were available. With server-sent events, the updates come automatically.

Examples: Facebook/Twitter updates, stock price updates, news feeds, sport results, etc.

Receive Server-Sent Event Notifications

The EventSource object is used to receive server-sent event notifications:


var source = new EventSource("demo_sse.php");
source.onmessage = function(event) {
    document.getElementById("result").innerHTML += + "

Example explained:

  • Create a new EventSource object, and specify the URL of the page sending the updates (in this example “demo_sse.php”)
  • Each time an update is received, the onmessage event occurs
  • When an onmessage event occurs, put the received data into the element with id=”result”

Check Server-Sent Events Support

In the tryit example above there were some extra lines of code to check browser support for server-sent events:

if(typeof(EventSource) !== "undefined") {
    // Yes! Server-sent events support!
    // Some code.....
} else {
    // Sorry! No server-sent events support..

Server-Side Code Example

For the example above to work, you need a server capable of sending data updates (like PHP or ASP).

The server-side event stream syntax is simple. Set the “Content-Type” header to “text/event-stream”. Now you can start sending event streams.

Code in PHP (demo_sse.php):

Code in ASP (VB) (demo_sse.asp):

<% Response.ContentType = "text/event-stream" Response.Expires = -1 Response.Write("data: The server time is: " & now()) Response.Flush() %>

Code explained:

  • Set the “Content-Type” header to “text/event-stream”
  • Specify that the page should not cache
  • Output the data to send (Always start with “data: “)
  • Flush the output data back to the web page

The EventSource Object

In the examples above we used the onmessage event to get messages. But other events are also available:

Events Description
onopen When a connection to the server is opened
onmessage When a message is received
onerror When an error occurs