用多态替换条件语句

refactorgram

switch (bird.type) {
  case 'EuropeanSwallow':
    return "average";
  case 'AfricanSwallow':
    return (bird.numberOfCoconuts > 2) ? "tired" : "average";
  case 'NorwegianBlueParrot':
    return (bird.voltage > 100) ? "scorched" : "beautiful";
  default:
    return "unknown";

class EuropeanSwallow {
  get plumage() {
    return "average";
  }
class AfricanSwallow {
  get plumage() {
     return (this.numberOfCoconuts > 2) ? "tired" : "average";
  }
class NorwegianBlueParrot {
  get plumage() {
     return (this.voltage > 100) ? "scorched" : "beautiful";
  }