fix: Provide correct return value from source.emit() (#17)

This rebases the work from #8 with minor modifications, and tests the return values directly (rather than adding a wide-bracket integration test).

Adopts arrow functions in the test file.

This was identified as the root cause of nock/nock#1485.

Thanks to @rbrtribeiro for the fix! 

Close #8 Close #9

index.js

@@ -15,14 +15,22 @@ function propagate(events, source, dest) {
     return explicitPropagate(events, source, dest)
   }
 
+  const shouldPropagate = eventName =>
+    events === undefined || events.includes(eventName)
+
   const oldEmit = source.emit
 
-  source.emit = function(eventType) {
-    oldEmit.apply(source, arguments)
+  // Returns true if the event had listeners, false otherwise.
+  // https://nodejs.org/api/events.html#events_emitter_emit_eventname_args
+  source.emit = (eventName, ...args) => {
+    const oldEmitHadListeners = oldEmit.call(source, eventName, ...args)
 
-    if (!events || ~events.indexOf(eventType)) {
-      dest.emit.apply(dest, arguments)
+    let destEmitHadListeners = false
+    if (shouldPropagate(eventName)) {
+      destEmitHadListeners = dest.emit(eventName, ...args)
     }
+
+    return oldEmitHadListeners || destEmitHadListeners
   }
 
   function end() {

tests/index.js

@@ -4,64 +4,64 @@ const { test } = require('tap')
 const { EventEmitter } = require('events')
 const propagate = require('..')
 
-test('propagates events', function(t) {
-  t.plan(12)
+test('propagates events', t => {
+  t.plan(16)
   const ee1 = new EventEmitter()
   const ee2 = new EventEmitter()
   propagate(ee1, ee2)
 
-  ee2.on('event-1', function(a, b, c) {
+  ee2.on('event-1', (a, b, c) => {
     t.equal(a, 'a')
     t.equal(b, 'b')
     t.equal(c, undefined)
   })
 
-  ee2.on('event-2', function(a, b, c) {
+  ee2.on('event-2', (a, b, c) => {
     t.equal(a, 'c')
     t.equal(b, 'd')
     t.equal(c, undefined)
   })
 
-  ee1.emit('event-1', 'a', 'b')
-  ee1.emit('event-1', 'a', 'b')
-  ee1.emit('event-2', 'c', 'd')
-  ee1.emit('event-2', 'c', 'd')
+  t.true(ee1.emit('event-1', 'a', 'b'))
+  t.true(ee1.emit('event-1', 'a', 'b'))
+  t.true(ee1.emit('event-2', 'c', 'd'))
+  t.true(ee1.emit('event-2', 'c', 'd'))
 })
 
-test('propagates can end', function(t) {
-  t.plan(1)
+test('propagates can end', t => {
+  t.plan(3)
 
   const ee1 = new EventEmitter()
   const ee2 = new EventEmitter()
   const prop = propagate(ee1, ee2)
 
-  ee2.on('event', function() {
+  ee2.on('event', () => {
     t.ok('true', 'propagated')
   })
 
-  ee1.emit('event')
+  t.true(ee1.emit('event'))
   prop.end()
-  ee1.emit('event')
+  t.false(ee1.emit('event'))
 })
 
-test('after propagation old one still emits', function(t) {
-  t.plan(2)
+test('after propagation old one still emits', t => {
+  t.plan(4)
 
   const ee1 = new EventEmitter()
   const ee2 = new EventEmitter()
   const prop = propagate(ee1, ee2)
 
-  ee1.on('event', function() {
+  ee1.on('event', () => {
     t.ok('true', 'propagated')
   })
 
-  ee1.emit('event')
+  t.true(ee1.emit('event'))
   prop.end()
-  ee1.emit('event')
+  t.true(ee1.emit('event'))
 })
 
-test('emit on source before destination', function(t) {
-  t.plan(1)
+test('emit on source before destination', t => {
+  t.plan(2)
 
   const source = new EventEmitter()
   const dest = new EventEmitter()
@@ -70,47 +70,47 @@ test('emit on source before destination', function(t) {
   // `count` should have been incremented by handler on source when handler on dest is invoked
   let count = 0
   propagate(source, dest)
-  source.on('event', function() {
+  source.on('event', () => {
     count++
   })
-  dest.on('event', function() {
+  dest.on('event', () => {
     t.equal(count, 1, 'emit on source first')
   })
 
   // Emit the events for assertion
-  source.emit('event')
+  t.true(source.emit('event'))
 })
 
-test('is able to propagate only certain events', function(t) {
-  t.plan(2)
+test('is able to propagate only certain events', t => {
+  t.plan(6)
   const ee1 = new EventEmitter()
   const ee2 = new EventEmitter()
   // propagate only event-1 and event-2, leaving out
   const p = propagate(['event-1', 'event-2'], ee1, ee2)
 
-  ee2.on('event-1', function() {
+  ee2.on('event-1', () => {
     t.ok(true, 'event 1 received')
   })
 
-  ee2.on('event-2', function(a, b, c) {
+  ee2.on('event-2', (a, b, c) => {
     t.ok(true, 'event 2 received')
   })
 
-  ee2.on('event-3', function(a, b, c) {
-    t.ok(false, 'event 3 should not have been received')
+  ee2.on('event-3', (a, b, c) => {
+    t.fail('event 3 should not have been received')
   })
 
-  ee1.emit('event-1')
-  ee1.emit('event-2')
-  ee1.emit('event-3')
+  t.true(ee1.emit('event-1'))
+  t.true(ee1.emit('event-2'))
+  t.false(ee1.emit('event-3'))
 
   p.end()
 
-  ee1.emit('event-1')
+  t.false(ee1.emit('event-1'))
 })
 
-test('is able to propagate and map certain events', function(t) {
-  t.plan(2)
+test('is able to propagate and map certain events', t => {
+  t.plan(6)
   const ee1 = new EventEmitter()
   const ee2 = new EventEmitter()
   // propagate only event-1 and event-2, leaving out
@@ -123,23 +123,46 @@ test('is able to propagate and map certain events', function(t) {
     ee2
   )
 
-  ee2.on('other-event-1', function() {
+  ee2.on('other-event-1', () => {
     t.ok(true, 'event 1 received')
   })
 
-  ee2.on('other-event-2', function(a, b, c) {
+  ee2.on('other-event-2', (a, b, c) => {
     t.ok(true, 'event 2 received')
   })
 
-  ee2.on('event-3', function(a, b, c) {
-    t.ok(false, 'event 3 should not have been received')
+  ee2.on('event-3', (a, b, c) => {
+    t.fail('event 3 should not have been received')
+  })
+
+  t.true(ee1.emit('event-1'))
+  t.true(ee1.emit('event-2'))
+  t.false(ee1.emit('event-3'))
+
+  p.end()
+
+  t.false(ee1.emit('event-1'))
+})
+
+test('is able to propagate a single event', t => {
+  t.plan(5)
+  const ee1 = new EventEmitter()
+  const ee2 = new EventEmitter()
+  const p = propagate('event-1', ee1, ee2)
+
+  ee2.on('event-1', () => {
+    t.ok(true, 'event 1 received')
+  })
+
+  ee2.on('event-2', (a, b, c) => {
+    t.fail('event 3 should not have been received')
   })
 
-  ee1.emit('event-1')
-  ee1.emit('event-2')
-  ee1.emit('event-3')
+  t.true(ee1.emit('event-1'))
+  t.false(ee1.emit('event-2'))
 
   p.end()
 
-  ee1.emit('event-1')
+  t.false(ee1.emit('event-1'))
+  t.false(ee1.emit('event-2'))
 })