第二个参数Object.create

Question

我的期望是我用新的toString方法覆盖了Vehicle的toString方法.然而,这似乎不起作用,我不知道为什么.基于这篇文章看起来应该是https://strongloop.com/strongblog/an-introduction-to-javascript-es6-classes/(向下滚动到Class Extending)

function Vehicle(make, year) {
  this.make = make;
  this.year = year;
}

Vehicle.prototype.toString = function() {
  return this.make + ' ' + this.year;
};

var vehicle = new Vehicle('Toyota Corolla', 2009);

function Motorcycle(make, year) {
  Vehicle.apply(this, [make, year]);
}

Motorcycle.prototype = Object.create(Vehicle.prototype, {
  toString: function() {
    return 'Motorcycle ' + this.make + ' ' + this.year;
  }
});

Motorcycle.prototype.constructor = Motorcycle;

var motorcycle = new Motorcycle('harley', 2010);
console.log(motorcycle.toString()); //TypeError

Answer 1

作为第二个参数给出的属性对象Object.create应该包含属性描述符,而不仅仅是值.这纠正了问题:

Motorcycle.prototype = Object.create(Vehicle.prototype, {
  toString: {
    configurable: true, enumerable: true, writable: true, 
    value: function() {
      return 'Motorcycle ' + this.make + ' ' + this.year;
    }
  }
});

另请参阅MDN参考Object.create.