Merge pull request #7 from shubhamgautam89/pattern/observer

observer patternn

patterns/observer/observer.js

@@ -0,0 +1,23 @@
+/*
+ * With the observer pattern, we can subscribe certain objects, the observers, to another object, called the observable. Whenever an event occurs, the observable notifies all its observers!
+ */
+
+class Observable {
+  constructor() {
+    this.observers = [];
+  }
+
+  subscribe(f) {
+    this.observers.push(f);
+  }
+
+  unsubscribe(f) {
+    this.observers = this.observers.filter((subs) => subs !== f);
+  }
+
+  notify(data) {
+    this.observers.forEach((observer) => observer(data));
+  }
+}
+
+export default Observable;

patterns/observer/observer.test.js

@@ -0,0 +1,37 @@
+import Observable from "./observer";
+
+describe("Observable", () => {
+  let observable;
+  beforeEach(() => {
+    observable = new Observable();
+  });
+
+  test("subscribes a function and notiofies it", () => {
+    const mockFunction = jest.fn();
+    observable.subscribe(mockFunction);
+    observable.notify("test-data");
+
+    expect(mockFunction).toHaveBeenCalledWith("test-data");
+  });
+
+  test("unsubscirbes a function and does not notify it", () => {
+    const mockFunction = jest.fn();
+    observable.subscribe(mockFunction);
+    observable.unsubscribe(mockFunction);
+
+    observable.notify("test-data");
+
+    expect(mockFunction).not.toHaveBeenCalled();
+  });
+
+  test("notifies only subscribed function", () => {
+    const mockFunction1 = jest.fn();
+    const mockFunction2 = jest.fn();
+
+    observable.subscribe(mockFunction1);
+    observable.notify("test-data");
+
+    expect(mockFunction1).toHaveBeenCalledWith("test-data");
+    expect(mockFunction2).not.toHaveBeenCalled();
+  });
+});