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.
| API | |||||
|---|---|---|---|---|---|
| SSE | 6.0 | 79.0 | 6.0 | 5.0 | 11.5 |
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>Live preview
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:
| Events | Description |
|---|---|
| onopen | When a connection to the server is opened |
| onmessage | When a message is received |
| onerror | When an error occurs |