GFE Custom EventEmitter

Author: pertrai1

README.md

@@ -7,7 +7,7 @@ A comprehensive collection of coding challenges from multiple platforms for lear
 | Platform                        | Focus Area                   | Problems Solved |
 | ------------------------------- | ---------------------------- | --------------- |
 | [LeetCode](#leetcode)           | Data Structures & Algorithms | 169             |
-| [GreatFrontEnd](#greatfrontend) | Frontend Engineering         | 10              |
+| [GreatFrontEnd](#greatfrontend) | Frontend Engineering         | 11              |
 
 ## Platforms
 

greatfrontend/gfe-75/README.md

@@ -4,7 +4,7 @@ The GFE 75 is GreatFrontEnd's curated collection of 75 essential frontend coding
 
 ## 📊 Progress Tracker
 
-### Status: 5 / 75 problems completed
+### Status: 6 / 75 problems completed
 
 ## ✅ Completed Problems
 
@@ -14,6 +14,7 @@ The GFE 75 is GreatFrontEnd's curated collection of 75 essential frontend coding
 - [Flatten](./javascript-utilities/flatten/) - Medium
 - [Throttle](./javascript-utilities/throttle/) - Medium
 - [Deep Clone](./javascript-utilities/deep-clone/) - Medium
+- [EventEmitter](./javascript-utilities/event-emitter/) - Medium
 
 ### DOM Manipulation
 

greatfrontend/gfe-75/javascript-utilities/README.md

@@ -10,6 +10,7 @@ Essential JavaScript utility functions that are commonly asked in frontend inter
 - [Flatten](./flatten/) - Medium
 - [Throttle](./throttle/) - Medium
 - [Deep Clone](./deep-clone/) - Medium
+- [EventEmitter](./event-emitter/) - Medium
 
 ### To Do 📝
 

greatfrontend/gfe-75/javascript-utilities/event-emitter/README.md

@@ -0,0 +1,27 @@
+# Event Emitter
+
+In the observer pattern (also commonly known as the publish-subscribe model), we can observe/subscribe to events emitted by publishers and execute code whenever an event happens.
+
+Implement an [`EventEmitter` class](https://nodejs.org/api/events.html#class-eventemitter) similar to the one in [Node.js](https://nodejs.org/api/events.html) that follows such an observer pattern.
+
+Example usage of the EventEmitter class:
+
+```javascript
+const emitter = new EventEmitter();
+
+function addTwoNumbers(a, b) {
+  console.log(`The sum is ${a + b}`);
+}
+emitter.on('foo', addTwoNumbers);
+emitter.emit('foo', 2, 5);
+// > "The sum is 7"
+
+emitter.on('foo', (a, b) => console.log(`The product is ${a * b}`));
+emitter.emit('foo', 4, 5);
+// > "The sum is 9"
+// > "The product is 20"
+
+emitter.off('foo', addTwoNumbers);
+emitter.emit('foo', -3, 9);
+// > "The product is -27"
+```