HTML SSE

The Server-Sent Events (SSE) API enables pushing messages/updates from a server to the web page via HTTP connection.

Server-Sent Events - One Way Messaging

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

Normally, a web page has to request data from the server, but with server-sent events, the updates are pushed automatically.

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

Browser Support

The numbers in the table specify the first browser version that fully support the Server-Sent Events API.

Receive Server-Sent Event Notifications

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

Example

  <script>
const x = document.getElementById("result");

  // Check browser support for SSE
if(typeof(EventSource)
  !== "undefined") {
  var source = new EventSource("demo_sse.php");

  source.onmessage = function(event) {
    x.innerHTML +=
  event.data + "<br>";
  };
} else {
  x.innerHTML = "Sorry,
  no support for server-sent events.";
}
</script>

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 Browser 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):

 <?php
header('Content-Type: text/event-stream');

 header('Cache-Control: no-cache');

$time = date('r');
echo "data: The server time is: {$time}\n\n";
flush();
?>

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: