JavaScript Design Patterns - Behavioral - Observer

Theobserverpattern 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 theregister()method. When something is updated, thenotifyAll()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.

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