Add listener type check

Closes #112

EventEmitter.js

@@ -118,6 +118,16 @@
         return response || listeners;
     };
 
+    function isValidListener (listener) {
+        if (typeof listener === 'function' || listener instanceof RegExp) {
+            return true
+        } else if (listener && typeof listener === 'object') {
+            return isValidListener(listener.listener)
+        } else {
+            return false
+        }
+    }
+
     /**
      * Adds a listener function to the specified event.
      * The listener will not be added if it is a duplicate.
@@ -129,6 +139,10 @@
      * @return {Object} Current instance of EventEmitter for chaining.
      */
     proto.addListener = function addListener(evt, listener) {
+        if (!isValidListener(listener)) {
+            throw new TypeError('listener must be a function');
+        }
+
         var listeners = this.getListenersAsObject(evt);
         var listenerIsWrapped = typeof listener === 'object';
         var key;

tests/tests.js

@@ -146,6 +146,12 @@
 
             assert.strictEqual(count, 1);
         });
+
+        test('it throws if you try to add a non-function/regex listener', function () {
+            assert.throws(ee.addListener.bind(ee, 'foo', null), /listener must be a function/)
+            assert.throws(ee.addListener.bind(ee, 'foo'), /listener must be a function/)
+            assert.throws(ee.addListener.bind(ee, 'foo', 'lol'), /listener must be a function/)
+        })
     });
 
     suite('addOnceListener', function () {