[MobX] MobX fundamentals: deriving computed values and managing side effects with reactions

Derivations form the backbone of MobX and come in two flavors: computed values are values that can be derived from the state automatically. And reactions can be used to manage side effects, such as drawing the user interface. In this lesson you will learn how these concepts relate to each other and how they are optimized automatically by MobX.

const {observable, computed} = mobx;
const {observer} = mobxReact;
const {Component} = React;
const DevTools = mobxDevtools.default;

const t = new class Temperature {
  @observable unit = "C";
  @observable temperatureCelsius = 25;

  @computed get temperatureKelvin() {
    console.log("calculating Kelvin")
    return this.temperatureCelsius * (9/5) + 32
  }
   
  @computed get temperatureFahrenheit() {
    console.log("calculating Fahrenheit")
    return this.temperatureCelsius + 273.15
  }
   
  @computed get temperature() {
    console.log("calculating temperature")
    switch(this.unit) {
      case "K": return this.temperatureKelvin + "ºK"
      case "F": return this.temperatureFahrenheit + "ºF"
      case "C": return this.temperatureCelsius + "ºC"
    }
  }
}
   
const App = observer(({ temperature }) => (
  <div>
    {temperature.temperature}
    <DevTools />
  </div>
))

ReactDOM.render(
  <App temperature={t} />,
  document.getElementById("app")
)

 If 'unit' or 'temperateureCelsius' changed, it will automaticlly trigger the corresponding @computed function to run based on current state

JS Bin on jsbin.com