Using observer pattern in web development

May 4, 2024

Design patterns are among the essential skills for a programmer. Mastering the application of design patterns not only enhances their understanding of programming but also elevates their value in the industry. In this article, I'll share one of the relatively common design patterns and how to apply it in frontend web development - the Observer pattern.

The Observer pattern (sometimes referred to as the pubsub pattern) is a behavioral design pattern. According to the definition from Wikipedia:

In software design and engineering, the observer pattern is a software design pattern in which an object, named the subject, maintains a list of its dependents, called observers, and notifies them automatically of any state changes, usually by calling one of their methods.

In essence, the observer pattern helps us address the issue of notifying other components about changes in a particular component. This minimizes the dependency between components in the system, making it easier to extend the system without altering much of the source code.

In this article, I'll apply the observer pattern to structure a basic todo application, including functionalities such as adding tasks, updating tasks, and deleting tasks.

Project Structure

Firstly, let's create a new project named todo-app using Vite:

$ npm create vite@latest todo-app -- --template vanilla-ts

After removing unnecessary files, the project structure will look like this:

├── index.html
├── package.json
├── public
│   └── vite.svg
├── src
│   ├── main.ts
│   └── vite-env.d.ts
├── tsconfig.json
└── yarn.lock

3 directories, 7 files

Implementing the Observer Pattern

Firstly, we'll create an Observer class to manage observers. Each observer will have an update method to receive notifications from the subject. Additionally, we'll create an Observable object with three methods:

  • registerObserver: Adds an observer to the listener list.
  • unregisterObserver: Removes an observer from the listener list.
  • notifyObservers: Triggers listeners when there's an update.
class Observer {
  update() {
    throw new Error("Method not implemented.");

class Observable {
  private observers: Observer[];

  constructor() {
    this.observers = [];

  registerObserver(observer: Observer) {

  unregisterObserver(observer: Observer) {
    this.observers = this.observers.filter((o) => o !== observer);

  notifyObservers() {
    this.observers.forEach((o) => o.update());

Applying the Observer Pattern to the Todo Application

In the case of the todo application, the UI displayed in the browser is the Observer, and the data model underneath the application is the Observable. Any changes in this data model will be reflected in the UI.

We'll create a TaskModel inheriting from the Observable class, which will contain all the logic to maintain the list of tasks for the application.

class TaskModel extends Observable {
  private tasks: Task[];

  constructor() {
    this.tasks = [];

  addTask(task: Omit<Task, "id">) {
      id: uuidV4(),

  getTasks() {
    return this.tasks;

  updateTask(task: Task) {
    this.tasks = => ( === ? task : t));

The TaskView contains logic to display data on the browser's UI. Additionally, TaskView will pass events from the UI back to the data model when the user interacts with the UI.

class TaskView extends Observer {
  private model: TaskModel;
  private container: HTMLUListElement;

  constructor(model: TaskModel, container: HTMLUListElement) {
    this.model = model;
    this.container = container;

  update() {
    this.container.innerHTML = "";
    this.model.getTasks().forEach((task) => {
      const li = document.createElement("li");
      li.textContent =;
      li.onclick = () => {
          completed: !task.completed,
      task.completed && ( = "line-through");

Finally, we initialize TaskModel and TaskView in main.ts, and attach them to the browser's DOM:

document.addEventListener("DOMContentLoaded", () => {
  const form = document.getElementById("todo-form") as HTMLFormElement;
  const input = document.getElementById("todo-input") as HTMLInputElement;
  const todoContainer = document.getElementById("todo-list") as HTMLUListElement;

  const model = new TaskModel();
  const view = new TaskView(model, todoContainer);

  form.addEventListener("submit", (event) => {
      name: input.value,
      completed: false,
    input.value = "";
    return false;

Kết luận

By using the observer pattern, we have successfully separated the application logic from the UI presentation, making the code more readable and maintainable.

Furthermore, the observer pattern is also utilized in major frameworks such as React, Angular, and Vue to manage state and UI rendering.

Link github: todo-app