Question

因此，我不熟悉JavaScript中的OOP，并且无法理解为什么在以下代码中得到“未定义”的原因，请帮忙：

function Vehicle(energyType, energy) {
	this.energyType = energyType;
	this.energy = energy;
}

function Car(energyType, energy) {
	Vehicle.call(energyType, energy);

	this.doors = 4;
}

Car.prototype = Object.create(Vehicle.prototype);

Vehicle.prototype.run = function() {
	console.log(`This vehicle is running on ${this.energyType}.`);
}

const c = new Car('gas', 80);

c.run();

当您运行代码时，即使我说汽车具有瓦斯能源类型，它也会显示“此车正在未定义的状态下运行”？

Answer 1

当.call时，第一个参数应该是您要调用的函数引用的this。所以，

Vehicle.call(energyType, energy);

使用{em> one 参数调用Vehicle，其中this的值最初是变量energyType的值（强制为对象，因为{ {1}}必须是一个对象，而不是原始对象。如果您在this内console.log，可以看到以下内容：

Vehicle

更改为：

function Vehicle(energyType, energy) {
  console.log('this', this);
  console.log('energyType', energyType);
  console.log('energy', energy);
	this.energyType = energyType;
	this.energy = energy;
}

function Car(energyType, energy) {
	Vehicle.call(energyType, energy);

	this.doors = 4;
}

Car.prototype = Object.create(Vehicle.prototype);

Vehicle.prototype.run = function() {
	console.log(`This vehicle is running on ${this.energyType}.`);
}

const c = new Car('gas', 80);

c.run();

Vehicle.call(this, energyType, energy);

Answer 2

它缺少this。

Vehicle.call(this, energyType, energy);

Answer 3

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/call

function.call（thisArg，arg1，arg2，...）

通过this中的Vehicle.call(this,energyType, energy);

function Vehicle(energyType, energy) {
	this.energyType = energyType;
	this.energy = energy;
}

function Car(energyType, energy) {
	Vehicle.call(this,energyType, energy);

	this.doors = 4;
}

Car.prototype = Object.create(Vehicle.prototype);

Vehicle.prototype.run = function() {
	console.log(`This vehicle is running on ${this.energyType}.`);
}

const c = new Car('gas', 80);

c.run();

调用原型成员时，JavaScript返回“ undefined”

3 个答案: