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Eloquent Javascript: The Secret Life of Objects

Class notation

So JavaScript classes are constructor functions with a prototype property. That is how they work, and until 2015, that was how you had to write them. These days, we have a less awkward notation.

class Rabbit {
  constructor(type) {
    this.type = type;
  }
  speak(line) {
    console.log(`The ${this.type} rabbit says '${line}'`);
  }
}

let killerRabbit = new Rabbit("killer");
let blackRabbit = new Rabbit("black");


.. Fortunately, JavaScript comes with a class called <span style="color: #000000;">Map</span> that is written for this exact purpose. It stores a mapping and allows any type of keys.

let ages = new Map();
ages.set("Boris", 39);
ages.set("Liang", 22);
ages.set("Júlia", 62);

class Temperature {
constructor(celsius) {
this.celsius = celsius;
}
get fahrenheit() {
return this.celsius * 1.8 + 32;
}
set fahrenheit(value) {
this.celsius = (value – 32) / 1.8;
}

static fromFahrenheit(value) {
return new Temperature((value – 32) / 1.8);
}
}

let temp = new Temperature(22);
console.log(temp.fahrenheit);
// → 71.6
temp.fahrenheit = 86;
console.log(temp.celsius);
// → 30