调用原型成员时，JavaScript返回'undefined'

Question

所以我在JavaScript中不熟悉OOP并且无法理解为什么我在下面的代码中得到“未定义”，请帮助：

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();

当你运行代码时，它说“这辆车正在运行未定义”，尽管我说汽车有汽油能量类型......？

Answer 1

当你.call时，第一个参数应该是你希望被调用函数引用的this。所以，

Vehicle.call(energyType, energy);

导致用一个参数调用Vehicle，其中this值最初是energyType变量（强制转换为一个对象，因为this必须是一个对象，而不是一个原始对象）。如果你在console.log里面Vehicle，你可以看到这个：

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();

改成：

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();

Answer 2

它缺少一个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，...）

在this通过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();