Implement Last-Event-ID support (closes #50)

This change implements support for the `lastEventId` attribute and
setting the `Last-Event-ID` header on reconnection requests, per the
EventSource specification.

Author: mpetazzoni

Diff for: README.md

@@ -94,6 +94,8 @@ is the UNIX timestamp of the _reception_ of the event.
 Additionally, the events will have the following fields:
 
 - `id`: the event ID, if present; `null` otherwise
+- `lastEventId`: the last seen event ID, or the empty string if no event
+  with an ID was received
 - `data`: the event data, unparsed
 
 `SSE`, like `EventSource`, will emit the following events:
@@ -155,10 +157,18 @@ request that the outgoing HTTP request be made with a CORS credentials
 mode of `include`, as per the [HTML Living
 Standard](https://fetch.spec.whatwg.org/#concept-request-credentials-mode).
 
+## Reconnecting after failure
+
+SSE.js does not (yet) automatically reconnect on failure; you can listen
+for the `abort` event and decide whether to reconnect and restart the
+event stream by calling `stream()`.
+
+SSE.js _will_ set the `Last-Event-ID` header on reconnection to the last
+seen event ID value (if any), as per the EventSource specification.
+
 ## TODOs and caveats
 
 - Internet Explorer 11 does not support arbitrary values in
   `CustomEvent`s.  A dependency on `custom-event-polyfill` is necessary
   for IE11 compatibility.
 - Improve `XmlHttpRequest` error handling and connection states
-- Automatically reconnect with `Last-Event-ID`

Diff for: lib/sse.js

@@ -48,6 +48,8 @@ var SSE = function (url, options) {
   this.progress = 0;
   /** @type {string} */
   this.chunk = '';
+  /** @type {string} */
+  this.lastEventId = '';
 
   /**
    * @type AddEventListener
@@ -222,9 +224,14 @@ var SSE = function (url, options) {
       }
     }.bind(this));
 
-    var event = new CustomEvent(e.event || 'message');
-    event.data = e.data || '';
+    if (e.id !== null) {
+      this.lastEventId = e.id;
+    }
+
+    const event = new CustomEvent(e.event || 'message');
     event.id = e.id;
+    event.data = e.data || '';
+    event.lastEventId = this.lastEventId;
     return event;
   };
 
@@ -261,6 +268,9 @@ var SSE = function (url, options) {
     for (let header in this.headers) {
       this.xhr.setRequestHeader(header, this.headers[header]);
     }
+    if (this.lastEventId.length > 0) {
+      this.xhr.setRequestHeader("Last-Event-ID", this.lastEventId);
+    }
     this.xhr.withCredentials = this.withCredentials;
     this.xhr.send(this.payload);
   };

Diff for: lib/sse.test.js

@@ -115,6 +115,21 @@ describe('SSE Lifecycle', () => {
     sse.xhr.trigger('abort', {});
     expect(sse.readyState).toBe(sse.CLOSED);
   });
+
+  it('should sent Last-Event-ID on reconnection', () => {
+    sse.stream();
+    expect(sse.xhr.setRequestHeader).toHaveBeenCalledTimes(0);
+    sse.xhr.responseText = 'id: event-1\ndata: Test message\n\n';
+    sse.xhr.trigger('progress', {});
+    expect(sse.lastEventId).toBe('event-1');
+
+    sse.xhr.trigger('abort', {});
+    expect(sse.readyState).toBe(sse.CLOSED);
+    expect(sse.lastEventId).toBe('event-1');
+
+    sse.stream();
+    expect(sse.xhr.setRequestHeader).toHaveBeenCalledWith('Last-Event-ID', 'event-1');
+  });
 });
 
 describe('SSE Event handling and Listeners', () => {
@@ -147,22 +162,71 @@ describe('SSE Event handling and Listeners', () => {
     expect(listener).toHaveBeenCalledTimes(1);
     expect(listener.mock.calls[0][0].data).toBe('Test message');
     expect(listener.mock.calls[0][0].event).toBe(undefined);
-    expect(listener.mock.calls[0][0].id).toBe("1");
+    expect(listener.mock.calls[0][0].id).toBe('1');
+    expect(listener.mock.calls[0][0].lastEventId).toBe('1');
   })  
 
   it('should handle multiple data events', () => {
-    sse.xhr.responseText = 'data: First message\n\n';
+    sse.xhr.responseText = 'id: id1\ndata: First message\n\n';
+    sse.xhr.trigger('progress', {});
+    sse.xhr.responseText += 'id: id2\ndata: Second message\n\n';
+    sse.xhr.trigger('progress', {});
+
+    expect(listener).toHaveBeenCalledTimes(2);
+    expect(listener.mock.calls[0][0].data).toBe('First message');
+    expect(listener.mock.calls[0][0].event).toBe(undefined);
+    expect(listener.mock.calls[0][0].id).toBe('id1');
+    expect(listener.mock.calls[0][0].lastEventId).toBe('id1');
+    expect(listener.mock.calls[1][0].data).toBe('Second message');
+    expect(listener.mock.calls[1][0].event).toBe(undefined);
+    expect(listener.mock.calls[1][0].id).toBe('id2');
+    expect(listener.mock.calls[1][0].lastEventId).toBe('id2');
+  });
+
+  it('should set lastEventId only when id field is in the event', () => {
+    sse.xhr.responseText = 'id: id1\ndata: First message\n\n';
     sse.xhr.trigger('progress', {});
     sse.xhr.responseText += 'data: Second message\n\n';
     sse.xhr.trigger('progress', {});
 
     expect(listener).toHaveBeenCalledTimes(2);
     expect(listener.mock.calls[0][0].data).toBe('First message');
     expect(listener.mock.calls[0][0].event).toBe(undefined);
-    expect(listener.mock.calls[0][0].id).toBe(null);
+    expect(listener.mock.calls[0][0].id).toBe('id1');
+    expect(listener.mock.calls[0][0].lastEventId).toBe('id1');
     expect(listener.mock.calls[1][0].data).toBe('Second message');
     expect(listener.mock.calls[1][0].event).toBe(undefined);
     expect(listener.mock.calls[1][0].id).toBe(null);
+    expect(listener.mock.calls[1][0].lastEventId).toBe('id1');
+  });
+
+  it('should reset lastEventId when id is empty', () => {
+    sse.xhr.responseText = 'data: First message\n\n';
+    sse.xhr.trigger('progress', {});
+    sse.xhr.responseText += 'data: Second message\nid: id2\n\n';
+    sse.xhr.trigger('progress', {});
+    sse.xhr.responseText += 'data: Third message\n\n';
+    sse.xhr.trigger('progress', {});
+    sse.xhr.responseText += 'data: Fourth message\nid\n\n';
+    sse.xhr.trigger('progress', {});
+
+    expect(listener).toHaveBeenCalledTimes(4);
+    expect(listener.mock.calls[0][0].data).toBe('First message');
+    expect(listener.mock.calls[0][0].event).toBe(undefined);
+    expect(listener.mock.calls[0][0].id).toBe(null);
+    expect(listener.mock.calls[0][0].lastEventId).toBe('');
+    expect(listener.mock.calls[1][0].data).toBe('Second message');
+    expect(listener.mock.calls[1][0].event).toBe(undefined);
+    expect(listener.mock.calls[1][0].id).toBe('id2');
+    expect(listener.mock.calls[1][0].lastEventId).toBe('id2');
+    expect(listener.mock.calls[2][0].data).toBe('Third message');
+    expect(listener.mock.calls[2][0].event).toBe(undefined);
+    expect(listener.mock.calls[2][0].id).toBe(null);
+    expect(listener.mock.calls[2][0].lastEventId).toBe('id2');
+    expect(listener.mock.calls[3][0].data).toBe('Fourth message');
+    expect(listener.mock.calls[3][0].event).toBe(undefined);
+    expect(listener.mock.calls[3][0].id).toBe('');
+    expect(listener.mock.calls[3][0].lastEventId).toBe('');
   });
 
   it('should handle repeat data elements', () => {