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W3C compliant EventSource client for Node.js and browser (polyfill)


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.

npm install eventsource
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)

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.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).

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);

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

var eventSourceInitDict = {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.

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');

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: '' });