Make EventEmitter more Node-like (#4865)

Add behaviors to make `EventEmitter` a bit more like its NodeJS counterpart. This should make using it a bit less surprising.

lib/EventEmitter.js

@@ -11,14 +11,22 @@ export default class EventEmitter {
     }
 
     this.listeners[event].add(cb)
+    return this
   }
 
   emit (event, ...data) {
-    if (!this.listeners[event]) return
-    this.listeners[event].forEach(cb => cb(...data)) // eslint-disable-line standard/no-callback-literal
+    const listeners = this.listeners[event]
+    const hasListeners = listeners && listeners.size
+    if (!hasListeners) {
+      return false
+    }
+
+    listeners.forEach(cb => cb(...data)) // eslint-disable-line standard/no-callback-literal
+    return true
   }
 
   off (event, cb) {
     this.listeners[event].delete(cb)
+    return this
   }
 }

test/unit/EventEmitter.test.js

@@ -71,6 +71,30 @@ describe('EventEmitter', () => {
 
       expect(run).toThrow(/The listener already exising in event: sample/)
     })
+
+    it('should support chaining like the nodejs EventEmitter', () => {
+      const emitter = new EventEmitter()
+      let calledA = false
+      let calledB = false
+
+      emitter
+        .on('a', () => { calledA = true })
+        .on('b', () => { calledB = true })
+
+      emitter.emit('a')
+      emitter.emit('b')
+
+      expect(calledA).toEqual(true)
+      expect(calledB).toEqual(true)
+    })
+
+    it('should return an indication on emit if there were listeners', () => {
+      const emitter = new EventEmitter()
+      emitter.on('a', () => { })
+
+      expect(emitter.emit('a')).toEqual(true)
+      expect(emitter.emit('b')).toEqual(false)
+    })
   })
 
   describe('Without a listener', () => {