eventsource

What is eventsource?

The eventsource npm package is a Node.js implementation of the EventSource client, which is used for receiving server-sent events (SSE). It allows clients to subscribe to a stream of updates from a server over an HTTP connection. The package is designed to be compliant with the W3C EventSource specification and is typically used for real-time data feeds, such as live notifications, stock prices updates, or any other continuous data stream.

What are eventsource's main functionalities?

Connecting to an SSE server

This code demonstrates how to connect to an SSE server and listen for messages. The 'onmessage' handler is called whenever a new message is received, and the 'onerror' handler is called if there's an error with the connection.

const EventSource = require('eventsource');
const es = new EventSource('http://example.com/events');
es.onmessage = function(event) {
  console.log(event.data);
};
es.onerror = function(err) {
  console.error('EventSource failed:', err);
};

Listening for specific event types

This code snippet shows how to listen for specific event types using the 'addEventListener' method. In this example, the client listens for 'userupdate' events and processes the received data.

const EventSource = require('eventsource');
const es = new EventSource('http://example.com/events');
es.addEventListener('userupdate', function(event) {
  const userData = JSON.parse(event.data);
  console.log('User update:', userData);
});

Reconnecting after connection loss

This example illustrates the automatic reconnection feature of the EventSource API. The 'onopen' handler is called when the connection is successfully established, and the 'onerror' handler indicates that the connection has been lost and a reconnection attempt will be made.

const EventSource = require('eventsource');
const es = new EventSource('http://example.com/events');
es.onopen = function() {
  console.log('Connection to server opened.');
};
es.onerror = function() {
  console.log('Connection lost, the browser will automatically attempt to reconnect.');
};

Other packages similar to eventsource

sse-client

The sse-client package is another implementation of the EventSource API for Node.js. It provides similar functionality for connecting to SSE servers and handling server-sent events. Compared to eventsource, it may have different API nuances or additional features, but the core functionality remains the same.

oceanwind

Oceanwind is not a direct alternative to eventsource, but it is an example of a package that uses server-sent events to provide real-time updates to clients. It is a library for managing real-time communication between servers and clients, and it may include an SSE implementation as part of its feature set.

README

EventSource

This library is a pure JavaScript implementation of the EventSource client. The API aims to be W3C compatible.

You can use it with Node.js or as a browser polyfill for browsers that don't have native EventSource support.

Install

npm install eventsource

Example

npm install
node ./example/sse-server.js
node ./example/sse-client.js    # Node.js client
open http://localhost:8080      # Browser client - both native and polyfill
curl http://localhost:8080/sse  # Enjoy the simplicity of SSE

Browser Polyfill

Just add example/eventsource-polyfill.js file to your web page:

<script src=/eventsource-polyfill.js></script>

Now you will have two global constructors:

window.EventSourcePolyfill
window.EventSource // Unchanged if browser has defined it. Otherwise, same as window.EventSourcePolyfill

If you're using webpack or browserify you can of course build your own. (The example/eventsource-polyfill.js is built with webpack).

Extensions to the W3C API

Setting HTTP request headers

You can define custom HTTP headers for the initial HTTP request. This can be useful for e.g. sending cookies or to specify an initial Last-Event-ID value.

HTTP headers are defined by assigning a headers attribute to the optional eventSourceInitDict argument:

var eventSourceInitDict = {headers: {'Cookie': 'test=test'}};
var es = new EventSource(url, eventSourceInitDict);

Allow unauthorized HTTPS requests

By default, https requests that cannot be authorized will cause the connection to fail and an exception to be emitted. You can override this behaviour, along with other https options:

var eventSourceInitDict = {https: {rejectUnauthorized: false}};
var es = new EventSource(url, eventSourceInitDict);

Note that for Node.js < v0.10.x this option has no effect - unauthorized HTTPS requests are always allowed.

HTTP status code on error events

Unauthorized and redirect error status codes (for example 401, 403, 301, 307) are available in the status property in the error event.

es.onerror = function (err) {
  if (err) {
    if (err.status === 401 || err.status === 403) {
      console.log('not authorized');
    }
  }
};

HTTP/HTTPS proxy

You can define a proxy option for the HTTP request to be used. This is typically useful if you are behind a corporate firewall.

var es = new EventSource(url, {proxy: 'http://your.proxy.com'});

License

MIT-licensed. See LICENSE