Does the same apply to subclassing Error?

I've been trying to do that but it doesn't seem to work as expected, in
particular passing the message as argument to `super` doesn't seem to do
anything.

For example:

``` javascript
class Foo extends Error {
  constructor(message) {
    super(message);
  }
}

var err1 = new Foo("arg");
console.log(err1); // plain Foo object
console.log(err1.message); // "" (empty string!?)

var err2 = new Error("arg");
console.log(err2); // Error object
console.log(err2.message); // "arg"
```

The above is true both with Traceur and Babel.

https://google.github.io/traceur-compiler/demo/repl.html#class%20Foo%20extends%20Error%20%7B%0A%20%20constructor(message)%20%7B%0A%20%20%20%20super(message)%3B%0A%20%20%7D%0A%7D%0A%0Avar%20err1%20%3D%20new%20Foo(%22arg%22)%3B%0Aconsole.log(err1)%3B%20%2F%2F%20plain%20Foo%20object%0Aconsole.log(err1.message)%3B%20%2F%2F%20%22%22%20(empty%20string!%3F)%0A%0Avar%20err2%20%3D%20new%20Error(%22arg%22)%3B%0Aconsole.log(err2)%3B%20%2F%2F%20Error%20object%0Aconsole.log(err2.message)%3B%20%2F%2F%20%22arg%22
https://babeljs.io/repl/#?experimental=true&evaluate=true&loose=false&spec=false&code=class%20Foo%20extends%20Error%20%7B%0A%20%20constructor(message)%20%7B%0A%20%20%20%20super(message)%3B%0A%20%20%7D%0A%7D%0A%0Avar%20err1%20%3D%20new%20Foo(%22arg%22)%3B%0Aconsole.log(err1)%3B%20%2F%2F%20plain%20Foo%20object%0Aconsole.log(err1.message)%3B%20%2F%2F%20%22%22%20(empty%20string!%3F)%0A%0Avar%20err2%20%3D%20new%20Error(%22arg%22)%3B%0Aconsole.log(err2)%3B%20%2F%2F%20Error%20object%0Aconsole.log(err2.message)%3B%20%2F%2F%20%22arg%22

On Sat, 11 Apr 2015 at 03:30 Axel Rauschmayer <axel at rauschma.de> wrote:

> No engine has implemented subclassing of `Array`, yet:
> http://kangax.github.io/compat-table/es6/#Array_is_subclassable
>
> And, as Sebastian mentioned, you can’t transpile it, because it depends on
> the cooperation of `Array`: it becomes the base constructor and allocates
> an exotic array instance (with special handling for `length` etc.) whose
> prototype is `new.target`.
>
> Axel
>
>
> On 11 Apr 2015, at 03:57, Garrett Smith <dhtmlkitchen at gmail.com> wrote:
>
> On 4/10/15, Axel Rauschmayer <axel at rauschma.de> wrote:
>
> 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`):
>
> Can you please explain how extending Array works. Also what is the
> optional identifier optional for in ClassExpression:
>
> var myArray = (new class B extends Array {
>   constructor() {
>     super(1,2,3,4,5);
>  }
> });
> alert(myArray.length); // it's 0 in Babel.
>
> --
> Garrett
> @xkit
> ChordCycles.com
> garretts.github.io
> personx.tumblr.com
>
>
> --
> Dr. Axel Rauschmayer
> axel at rauschma.de
> rauschma.de
>
>
>
> _______________________________________________
> es-discuss mailing list
> es-discuss at mozilla.org
> https://mail.mozilla.org/listinfo/es-discuss
>

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Does the same apply to subclassing Error?

I've been trying to do that but it doesn't seem to work as expected, in
particular passing the message as argument to `super` doesn't seem to do
anything.

For example:

``` javascript
class Foo extends Error {
  constructor(message) {
    super(message);
  }
}

var err1 = new Foo("arg");
console.log(err1); // plain Foo object
console.log(err1.message); // "" (empty string!?)

var err2 = new Error("arg");
console.log(err2); // Error object
console.log(err2.message); // "arg"
```

The above is true both [with Traceur](https://google.github.io/traceur-compiler/demo/repl.html#class%20Foo%20extends%20Error%20{%0A%20%20constructor%28message%29%20{%0A%20%20%20%20super%28message%29%3B%0A%20%20}%0A}%0A%0Avar%20err1%20%3D%20new%20Foo%28%22arg%22%29%3B%0Aconsole.log%28err1%29%3B%20%2F%2F%20plain%20Foo%20object%0Aconsole.log%28err1.message%29%3B%20%2F%2F%20%22%22%20%28empty%20string!%3F%29%0A%0Avar%20err2%20%3D%20new%20Error%28%22arg%22%29%3B%0Aconsole.log%28err2%29%3B%20%2F%2F%20Error%20object%0Aconsole.log%28err2.message%29%3B%20%2F%2F%20%22arg%22) and [Babel](https://babeljs.io/repl/#?experimental=true&evaluate=true&loose=false&spec=false&code=class%20Foo%20extends%20Error%20{%0A%20%20constructor%28message%29%20{%0A%20%20%20%20super%28message%29%3B%0A%20%20}%0A}%0A%0Avar%20err1%20%3D%20new%20Foo%28%22arg%22%29%3B%0Aconsole.log%28err1%29%3B%20%2F%2F%20plain%20Foo%20object%0Aconsole.log%28err1.message%29%3B%20%2F%2F%20%22%22%20%28empty%20string!%3F%29%0A%0Avar%20err2%20%3D%20new%20Error%28%22arg%22%29%3B%0Aconsole.log%28err2%29%3B%20%2F%2F%20Error%20object%0Aconsole.log%28err2.message%29%3B%20%2F%2F%20%22arg%22).

Does the same apply to subclassing Error?

I've been trying to do that but it doesn't seem to work as expected, in
particular passing the message as argument to `super` doesn't seem to do
anything.

For example:

``` javascript
class Foo extends Error {
  constructor(message) {
    super(message);
  }
}

var err1 = new Foo("arg");
console.log(err1); // plain Foo object
console.log(err1.message); // "" (empty string!?)

var err2 = new Error("arg");
console.log(err2); // Error object
console.log(err2.message); // "arg"
```

The above is true both [with Traceur](https://google.github.io/traceur-compiler/demo/repl.html#class%20Foo%20extends%20Error%20%7B%0A%20%20constructor(message)%20%7B%0A%20%20%20%20super(message)%3B%0A%20%20%7D%0A%7D%0A%0Avar%20err1%20%3D%20new%20Foo(%22arg%22)%3B%0Aconsole.log(err1)%3B%20%2F%2F%20plain%20Foo%20object%0Aconsole.log(err1.message)%3B%20%2F%2F%20%22%22%20(empty%20string!%3F)%0A%0Avar%20err2%20%3D%20new%20Error(%22arg%22)%3B%0Aconsole.log(err2)%3B%20%2F%2F%20Error%20object%0Aconsole.log(err2.message)%3B%20%2F%2F%20%22arg%22) and [Babel](https://babeljs.io/repl/#?experimental=true&evaluate=true&loose=false&spec=false&code=class%20Foo%20extends%20Error%20%7B%0A%20%20constructor(message)%20%7B%0A%20%20%20%20super(message)%3B%0A%20%20%7D%0A%7D%0A%0Avar%20err1%20%3D%20new%20Foo(%22arg%22)%3B%0Aconsole.log(err1)%3B%20%2F%2F%20plain%20Foo%20object%0Aconsole.log(err1.message)%3B%20%2F%2F%20%22%22%20(empty%20string!%3F)%0A%0Avar%20err2%20%3D%20new%20Error(%22arg%22)%3B%0Aconsole.log(err2)%3B%20%2F%2F%20Error%20object%0Aconsole.log(err2.message)%3B%20%2F%2F%20%22arg%22).