tj.crowder at farsightsoftware.com (2017-01-06T10:41:28.593Z)
On Thu, Jan 5, 2017 at 5:31 PM, James Treworgy <jamietre at gmail.com> wrote:
I can't address your questions about "why" (I wasn't plugged into the
discussions around it), but addressing this:
> This has come into play lately for me, as an DI container we use that
> does exactly this doesn't work with ES6 classes (and as far as I can
> tell, there's no way to make it work, other than having devs no longer
> use class syntax).
Can you clarify what prevents it from being made to work? I'm probably
missing the point you're making there. For instance, this does some
brain-dead DI (injecting an argument in the constructor) by dynamically
extending the class:
```js
// The class we'll do DI on
class Original {
constructor($foo) {
this.foo = $foo;
}
run(num) {
const result = this.foo.fooMethod(num);
console.log(`num is ${num}, result is ${result}`);
}
}
// Brain-dead di function
const di = (cls, Foo) => {
const o = {
[cls.name]: class extends cls {
constructor(...args) {
super(new Foo(), ...args);
}
}
};
return o[cls.name];
};
// Ues a class that's been DI'd
const use = Original => {
new Original().run(42);
};
// Use it in dev
use(di(Original, class Foo {
fooMethod(num) {
return num * 2;
}
}));
// Use it in production
use(di(Original, class Foo {
fooMethod(num) {
return num / 2;
}
}));
```
That outputs
num is 42, result is 84
num is 42, result is 21
...because of the different injected `Foo`s. (This is obviously a
simplistic example.)
Separately, there are some tools you can use, such as
[`Reflect.construct`][1], but granted that does create an instance. For
instance, if for some reason you wanted to extend a class *without* using
`class`:
```js
class A {
amethod() {
console.log("amethod");
}
}
function B() {
const t = Reflect.construct(A, [], B);
return t;
}
B.prototype = Object.create(A.prototype);
B.prototype.constructor = B;
B.prototype.bmethod = function() {
console.log("bmethod");
};
const b = new B();
b.amethod(); // "amethod"
b.bmethod(); // "bmethod"
console.log(b instanceof A); // true
console.log(b instanceof B); // true
```
Of course, that cheats a bit with that `return t;`. :-)
There are probably some tools that should be added to the list. For
instance, there's [this proposal][2] for `Reflect.isCallable` and
`Reflect.isConstructor`). And my `bmethod` above isn't really a method, so
it wouldn't be able to use `super`; in theory one could argue for a
`Reflect.makeMethod` (but use cases are limited, given `class` syntax). New
tools can be added if persuasive use cases come up (and people step forward
to define them and get a champion on board).
But circling back, I could be well wide of the mark above. If you can give
us more specifics about use cases that aren't supported, we can probably do
better helping with them.
[1]: http://www.ecma-international.org/ecma-262/7.0/index.html#sec-reflect.construct
[2]:
https://github.com/caitp/TC39-Proposals/blob/master/tc39-reflect-isconstructor-iscallable.md
-- T.J.
On Thu, Jan 5, 2017 at 5:31 PM, James Treworgy <jamietre at gmail.com> wrote: I can't address your questions about "why" (I wasn't plugged into the discussions around it), but addressing this: > This has come into play lately for me, as an DI container we use that > does exactly this doesn't work with ES6 classes (and as far as I can > tell, there's no way to make it work, other than having devs no longer > use class syntax). Can you clarify what prevents it from being made to work? I'm probably missing the point you're making there. For instance, this does some brain-dead DI (injecting an argument in the constructor) by dynamically extending the class: ```js // The class we'll do DI on class Original { constructor($foo) { this.foo = $foo; } run(num) { const result = this.foo.fooMethod(num); console.log(`num is ${num}, result is ${result}`); } } // Brain-dead di function const di = (cls, Foo) => { const o = { [cls.name]: class extends cls { constructor(...args) { super(new Foo(), ...args); } } }; return o[cls.name]; }; // Ues a class that's been DI'd const use = Original => { new Original().run(42); }; // Use it in dev use(di(Original, class Foo { fooMethod(num) { return num * 2; } })); // Use it in production use(di(Original, class Foo { fooMethod(num) { return num / 2; } })); ``` That outputs num is 42, result is 84 num is 42, result is 21 ...because of the different injected `Foo`s. (This is obviously a simplistic example.) Separately, there are some tools you can use, such as [`Reflect.construct`][1], but granted that does create an instance. For instance, if for some reason you wanted to extend a class *without* using `class`: ```js class A { amethod() { console.log("amethod"); } } function B() { const t = Reflect.construct(A, [], B); return t; } B.prototype = Object.create(A.prototype); B.prototype.constructor = B; B.prototype.bmethod = function() { console.log("bmethod"); }; const b = new B(); b.amethod(); // "amethod" b.bmethod(); // "bmethod" console.log(b instanceof A); // true console.log(b instanceof B); // true ``` Of course, that cheats a bit with that `return t;`. :-) There are probably some tools that should be added to the list. For instance, there's [this proposal][2] for `Reflect.isCallable` and `Reflect.isConstructor`). And my `bmethod` above isn't really a method, so it wouldn't be able to use `super`; in theory one could argue for a `Reflect.makeMethod` (but use cases are limited, given `class` syntax). New tools can be added if persuasive use cases come up (and people step forward to define them and get a champion on board). But circling back, I could be well wide of the mark above. If you can give us more specifics about use cases that aren't supported, we can probably do better helping with them. [1]: http://www.ecma-international.org/ecma-262/7.0/index.html#sec-reflect. construct [2]: https://github.com/caitp/TC39-Proposals/blob/master/tc39-reflect-isconstructor-iscallable.md -- T.J. On Thu, Jan 5, 2017 at 5:31 PM, James Treworgy <jamietre at gmail.com> wrote: > Hi - I am brand new to this list, I find myself here because of a > confounding issue related to ES6 classes vs. traditional constructors. > Forgive me if this is something that's been hashed out in times past. I > looked around for discussion online and couldn't find anything more than > the observation that the spec prohibits invoking it - not really any > discussion. Probably a failing of google more than anything else, so if > there's some discussion that I should read to catch up please point me > there. > > Here's my issue. The ES6 spec prohibits invoking class constructors > without "new". This makes such functions a special case, e.g. > > class Test() {} > > // typeof Test === 'function' // yep > // Test.prototype.constructor === Test // yep > > // Test() => nope ... TypeError: Class constructor Test cannot be invoked > without 'new' > // Test.call() ... nope > // Test.apply() ... nope > > This has some interesting consequences. It means testing something for > typeof "function" no longer guarantees it can be invoked without error. > Also "function.toString()" can now return something that isn't actually a > legal function definiton (since it returns the whole class as text). There > seems to be no method, through various javascript reflection/invocation > techniques or otherwise, to invoke a class constructor except by creating > an instance of the class. > > For tool-builders the consequences of this are significant. It's no longer > possible to create something that can extend/wrap/act on a prototype by > intercepting it's construction process, as it was before with plain ES5 > constructors. So classes are fundamentally different than prototype > contructors in how we can use them, far more than syntactic sugar. This has > come into play lately for me, as an DI container we use that does exactly > this doesn't work with ES6 classes (and as far as I can tell, there's no > way to make it work, other than having devs no longer use class syntax). > > This seems a strange design decision. Even conventional OO languages like > C# have the capability to reflect on classes and access the constructor > directly as a function. It seems to fly in the face of the basic > openness/dyanamic nature of JavaScript, and more signficantly, creates a > kind of backward incompatibility since a function is no longer just a > function. > > I'm wondering whether I'm missing some mechanism for legally accessing a > class constructor as a function (other than parsing the output of > toString() and eval!) -- and generally thoughts on this aspect of the ES6 > specification. > > Thank you! > > > _______________________________________________ > es-discuss mailing list > es-discuss at mozilla.org > https://mail.mozilla.org/listinfo/es-discuss > > -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://mail.mozilla.org/pipermail/es-discuss/attachments/20170105/baaf2cd5/attachment-0001.html>