EventSource client for Node.js and Browser (polyfill)
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EventSource npm versionNPM Downloads


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

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.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');


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: ''});


MIT-licensed. See LICENSE

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