The reason why you need to call the super-constructor from a derived class
constructor is due to where ES6 allocates instances – they are allocated by/in
the base class (this is necessary so that constructors can be subclassed that
have exotic instances, e.g. `Array`):
```js
// Base class
class A {
// Allocate instance here (done by JS engine)
constructor() {}
}
// Derived class
class B extends A {
constructor() {
// no `this` available, yet
super(); // receive instance from A
// can use `this` now
}
}
// Derived class
class C extends B {
constructor() {
// no `this` available, yet
super(); // receive instance from B
// can use `this` now
}
}
```
If you do not call `super()`, you only get into trouble if you access `this` in
some manner. Two examples:
```js
// Derived class
class B1 extends A {
constructor() {
// No super-constructor call here!
// ReferenceError: no `this` available
this.foo = 123;
}
}
// Derived class
class B2 extends A {
constructor() {
// No super-constructor call here!
// ReferenceError: implicit return (=access) of `this`
}
}
```
Therefore, there are two ways to avoid typing super-constructor calls.
First, you can avoid accessing `this` by explicitly returning an object from
the derived class constructor. However, this is not what you want, because the
object created via `new B()` does not inherit `A`’s methods.
```js
// Base class
class A {
constructor() {}
}
// Derived class
class B extends A {
constructor() {
// No super-constructor call here!
return {}; // must be an object
}
}
```
Second, you can let JavaScript create default constructors for you:
```js
// Base class
class A {
}
// Derived class
class B extends A {
}
```
This code is equivalent to:
```js
// Base class
class A {
constructor() {}
}
// Derived class
class B extends A {
constructor(...args) {
super(...args);
}
}
```
> On 10 Apr 2015, at 22:51, Jacob Parker <[email protected]> wrote:
>
> Why was this a requirement? I have a class, we’ll call a, which I want to
> extend from b, but I don’t want to call the constructor. I just want to
> inherit a few functions from it.
--
Dr. Axel Rauschmayer
[email protected]
rauschma.de
_______________________________________________
es-discuss mailing list
[email protected]
https://mail.mozilla.org/listinfo/es-discuss
