The observer pattern allows for the definition of one-to-many dependency between objects so that all its dependents are notified and updated automatically when one object changes state.

In this example, we are creating a simple class product that other classes can observe registering about changes in the register() method. When something is updated, the notifyAll() method will communicate with all the observers about these changes.

class ObservedProduct {
  constructor() {
    this.price = 0;
    this.actions = [];
  }

  setBasePrice(val) {
    this.price = val;
    this.notifyAll();
  }

  register(observer) {
    this.actions.push(observer);
  }

  unregister(observer) {
    this.actions.remove.filter(function (el) {
      return el !== observer;
    });
  }

  notifyAll() {
    return this.actions.forEach(
      function (el) {
        el.update(this);
      }.bind(this)
    );
  }
}

class Fees {
  update(product) {
    product.price = product.price * 1.2;
  }
}

class Profit {
  update(product) {
    product.price = product.price * 2;
  }
}

export { ObservedProduct, Fees, Profit };

A complete example is here 👉 https://stackblitz.com/edit/vitejs-vite-kyucyd?file=main.js

Conclusion

Use this pattern when changes to the state of one object may require changing other objects, and the actual set of objects is unknown beforehand or changes dynamically.

I hope you found it helpful. Thanks for reading. 🙏

Let's get connected! You can find me on:

• Medium: https://medium.com/@nhannguyendevjs/
• Dev: https://dev.to/nhannguyendevjs/
• Hashnode: https://nhannguyen.hashnode.dev/
• Linkedin: https://www.linkedin.com/in/nhannguyendevjs/
• X (formerly Twitter): https://twitter.com/nhannguyendevjs/
• Buy Me a Coffee: https://www.buymeacoffee.com/nhannguyendevjs