*

coding_interviews/frontend/js-coding-questions/event-target/event-target.js

@@ -0,0 +1,58 @@
+/*
+Implement an EventTarget class (similar to the EventTarget interface of the DOM),
+which keeps track of event listeners and dispatches events.
+
+Your EventTarget class should have the following three methods:
+
+addEventListener
+
+- This function takes in two arguments: the name of an event as a string and a callback function,
+to be called when the event is dispatched to the target.
+- For example, target.addEventListener('click', onClick) should make it such that the onClick
+callback is called when the 'click' event is dispatched to the target.
+A target should be able to have multiple event listeners for the same event (for example, onClick1
+and onClick2, both attached to the 'click' event). However, adding the same exact event listener
+twice (with the same event and the same callback) should have no effect.
+
+removeEventListener
+
+- This function takes in the same arguments as addEventListener and removes the relevant event listener.
+- For example, target.removeEventListener('click', onClick) should undo the effects of the addEventListener
+call in the bullet point above.
+- If there's no current event listener for the passed-in arguments, removeEventListener should have no effect.
+Also, if two different callbacks have been added for the same 'click' event (e.g., onClick1 and onClick2),
+removing one shouldn't remove the other.
+
+dispatchEvent
+
+- This function takes in the name of an event as a string. If there are no event listeners for that event,
+nothing should happen. Otherwise, event listeners that do exist for that event should have their callback functions invoked.
+- For example, given the event listener added in the first bullet point and assuming it hadn't been removed,
+dispatchEvent('click') would call onClick.
+- Events can be dispatched multiple times, and each time, every associated callback should be invoked.
+*/
+
+class EventTarget {
+  constructor() {
+    this.events = new Map();
+  }
+
+  addEventListener(name, callback) {
+    this.events.set(name, [...(this.events.get(name) || []), callback]);
+  }
+
+  removeEventListener(name, callback) {
+    const removedCallback = (this.events.get(name) || []).filter(
+      (event) => event !== callback
+    );
+    this.events.set(name, removedCallback);
+  }
+
+  dispatchEvent(name) {
+    if (this.events.has(name)) {
+      for (const callback of this.events.get(name)) {
+        callback();
+      }
+    }
+  }
+}