如何使用“Function.prototype.call”调用未附加到任何对象原型的函数

Question

以下代码的输出是undefined has undefined wheels，而不是Bike has 2 wheels。 Instantiated object 不能调用没有附加到任何对象的prototype 的函数吗？

var vehicle = function(name, wheels){
	name = "car",
	wheels = 4,
	fuel = "petrol"
}

function drive(name, wheels) {
	console.log(this.name + " has " + this.wheels + " wheels" );
}

var vehicle1 = new vehicle('Bike' , 2);
drive.call(vehicle1); //undefined is driven with undefined wheels 

Answer 1

很遗憾，您的代码几乎所有内容都不正确。

首先，“车辆”函数必须在this上设置属性，并且它应该使用传入的参数而不是常量：

var vehicle = function(name, wheels) {
    this.name = name,
    this.wheels = wheels,
    this.fuel = "petrol"
}

那么，你的函数“drive”应该不带任何参数：

function drive() {
  console.log(this.name + " is driven with " + this.wheels + " wheels" );
}

使用您的车辆作为this 值调用drive()：

var vehicle1 = new vehicle("Bike", 2);
drive.call(vehicle1); 

.call()的第一个参数将作为被调用函数内部this的值。

Answer 2

几个问题：

1. 您将vehicle 用作构造函数，但未将任何属性分配给this。

2. 您没有在vehicle 中使用name 或wheels 参数

3. 您将name 和wheels 参数定义为drive，但在调用它时不使用它们并且不提供它们。

4. 不是真正的问题，只是约定（正如我在your last question 中提到的那样）：如果您打算将new 与vehicle 一起使用，最初应该设置上限：Vehicle 按压倒性约定。

见 cmets：

var vehicle = function(name, wheels){
  // Actually use name and wheels and put them on `this`
  this.name = name;
  this.wheels = wheels;
}; // <== Added missing ;

function drive(/* No parameters here*/) {
	console.log(this.name + " is driven with " + this.wheels + " wheels" );
}

var vehicle1 = new vehicle('Bike' , 2);
drive.call(vehicle1);

Answer 3

你需要像这样使用this：

var vehicle = function(name, wheels){
	this.name = "car",
	this.wheels = 4,
	this.fuel = "petrol"
}

function drive(name, wheels) {
	console.log(this.name + " is driven with " + this.wheels + " wheels" );
}

var vehicle1 = new vehicle('Bike' , 2);
drive.call(vehicle1); //undefined is driven with undefined wheels