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Mastering JavaScript Design Patterns - Second Edition by Simon Timms

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Observer

The observer pattern is perhaps the most used pattern in the JavaScript world. The pattern is used especially with modern single pages applications; it is a big part of the various libraries that provide Model View View-Model (MVVM) functionality. We'll explore those patterns in some detail in Chapter 7, Reactive Programming.

It is frequently useful to know when the value on an object has changed. In order to do so you could wrap up the property of interest with a getter and setter:

class GetterSetter {
  GetProperty() {
    return this._property;
  }
  SetProperty(value) {
    this._property = value;
  }
}

The setter function can now be augmented with a call to some other object which is interested in knowing that a value has changed:

SetProperty(value) ...

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