How to override a base class constructor in Javascript

问题

The Udacity ES6 training has a question about overriding a base class constructor. I've got a solution but Udacity doesn't let me get away with it.

The assignment is: Create a Bicycle subclass that extends the Vehicle class. The Bicycle subclass should override Vehicle's constructor function by changing the default values for wheels from 4 to 2 and horn from 'beep beep' to 'honk honk'.

class Vehicle {
    constructor(color = 'blue', wheels = 4, horn = 'beep beep') {
        this.color = color;
        this.wheels = wheels;
        this.horn = horn;
    }

    honkHorn() {
        console.log(this.horn);
    }
}

// your code here


/* tests
const myVehicle = new Vehicle();
myVehicle.honkHorn(); // beep beep
const myBike = new Bicycle();
myBike.honkHorn(); // honk honk
*/

The solution that I come up with is:

class Bicycle extends Vehicle {
    constructor(wheels, horn){
        super(wheels, horn)
        this.wheels = 2
        this.horn = "honk honk" 
    }

    honkHorn(){
        super.honkHorn()
    }

}

But that is not good enough And I do not understand why that is. The feedback I got is:

Your Bicycles constructor doesn't set default values for color, wheels, and horn

回答1:

you should not be using

    this.wheels = 2
    this.horn = "honk honk"

when already overriding these in super constructor.

class Vehicle {
	constructor(color = 'blue', wheels = 4, horn = 'beep beep') {
		this.color = color;
		this.wheels = wheels;
		this.horn = horn;
	}

	honkHorn() {
		console.log(this.horn);
	}
}

class Bicycle extends Vehicle {
	constructor(wheels = 2, horn = 'honk honk') {
		super(undefined, wheels, horn);
	}

	honkHorn() {
		super.honkHorn()
	}

}

let by = new Bicycle();
by.honkHorn();

回答2:

class Bicycle extends Vehicle {
    constructor(wheels =2, horn= "honk honk"){
        super(undefined, wheels, horn)
    }

    honkHorn(){
        super.honkHorn()
    }

}

then for the test I added:

const yourBike = new Bicycle(3, "tring tring")

Although other options did provide the right answers for the test cases described in the question. By adding this extra test I found out that overriding a base class constructor is not possible from super nor from this.wheels (as was my first attempt).

However Udacity does not accept it......

来源：https://stackoverflow.com/questions/55760091/how-to-override-a-base-class-constructor-in-javascript

标签

javascript

constructor

overriding

es6-class