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Classes cannot extend classes that extend classes #1529

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gfwilliams opened this issue Oct 18, 2018 · 2 comments
Closed

Classes cannot extend classes that extend classes #1529

gfwilliams opened this issue Oct 18, 2018 · 2 comments

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@gfwilliams
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class A {
}
class B extends A {
}
class C extends B {
}
new A(); // ok
new B(); // ok
new C(); // stack overflow

Why? Because there's a default constructor that calls the constructor on the class that's been extended. That's in itself is fine.

The issue is that this is always set to the instance of C, so when the default constructor in B searches for the default constructor on the class extended by B, it always points to B's constructor instead.

It's an issue on any function in an extended class that uses super. I guess we might need to store another superThis variable that points to 'this' for the currently executing function.
@nebbishhacker
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I just ran into this bug :)

One workaround is to call the base class constructor explicitly:

class A { constructor() { this.foo = 42; } }
class B extends A { constructor() { A.call(this); } }
class C extends B { constructor() { B.call(this); } }
print(new C().foo); // prints 42

(Note that although this code works on Espruino, it isn't valid ES6 since it doesn't call super())

@gfwilliams
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Sent in a PM to me, this has a problem too:

class Being  { 
    constructor(theLabel) {
        this.label = theLabel;
    }

    name() {
        return this.label;
    }
}

class Animal extends Being { 
    constructor(theLabel) {
        super(theLabel);
    }
}

class Dog extends Animal { 
    constructor(theLabel) {
        super(theLabel);
    }
}

class Cat extends Animal {
    constructor(theLabel) {
        super(theLabel);
    }
 }


const c = new Cat("Felix");
const d = new Dog("Fido");

console.log("Name:" + c.name());

Before, we were adding default constructor code of if(this.__proto__.__proto__.constructor)this.__proto__.__proto__.constructor.apply(this,arguments) to the constructor if it wasn't specified, and I figured I could do what @nebbishhacker had done above and explicitly reference the extended class (which works!) and fixes the original issue.

However super isn't so easy - I'm looking into it now

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@gfwilliams @nebbishhacker