fix: add handler only once

Author: monochromer

src/event-emitter.js

@@ -21,7 +21,11 @@ class EventEmitter {
    * @param {function} fn event handler
    */
   on(type, fn) {
-    (this._events[type] = this._events[type] || []).push(fn);
+    const events = this._events[type] || (this._events[type] = []);
+    if (events && events.some(handler => handler === fn)) {
+      return;
+    };
+    events.push(fn);
     return this;
   }
 

test/index.js

@@ -38,6 +38,15 @@ describe('Event Emitter', () => {
       assert.property(ee._events, eventName);
       assert.deepEqual(ee._events[eventName], [fn1, fn2]);
     });
+
+    it('should add the same handler only once', () => {
+      const fn = () => {};
+      const eventName = 'event';
+      ee.on(eventName, fn);
+      ee.on(eventName, fn);
+      assert.property(ee._events, eventName);
+      assert.deepEqual(ee._events[eventName], [fn]);
+    });
   });
 
   describe('unsubscribe handlers', () => {