動態方法名稱

在命名方法時，還可以評估表示式，類似於使用 [] 訪問物件屬性的方法。這對於擁有動態屬性名稱很有用，但通常與符號一起使用。

let METADATA = Symbol('metadata');

class Car {
  constructor(make, model) {
    this.make = make;
    this.model = model;
  }
  
  // example using symbols
  [METADATA]() {
    return {
      make: this.make,
      model: this.model
    };
  }

  // you can also use any javascript expression

  // this one is just a string, and could also be defined with simply add()
  ["add"](a, b) {
    return a + b;
  }

  // this one is dynamically evaluated
  [1 + 2]() {
    return "three";
  }
}

let MazdaMPV = new Car("Mazda", "MPV");
MazdaMPV.add(4, 5); // 9
MazdaMPV[3](); // "three"
MazdaMPV[METADATA](); // { make: "Mazda", model: "MPV" }