decoratorMetadataEmit not compatible with Babel's 6-to-5

[alexeagle]

I believe it's expected that tsc -t es6 should be externally transpilable to ES5 (eg with Babel)

See https://gist.github.com/alexeagle/24243fbb87dbd8bad6f1

Babel replaces the __decorate line this.__decorate with undefined.__decorate so that gives a runtime error. Uncaught TypeError: Cannot read property '__decorate' of undefined app_bin.js:7

Note I also tried this with Closure Compiler ES6->ES5, and had a different problem because of this line in the emit:
Object.defineProperty(App, "name", { value: "App", configurable: true });
in ES6 spec, new objects are configurable:true by default, but in mainline Chrome it is configurable:false by default so this is also a runtime error. For this case, it leads me to wonder if ES5->ES6 transpilers should be expected to translate API incompatibilities in addition to new ES6 syntax.

[mhegazy]

I believe the core of the issue is that the emit for this.__decorate is incompatible with ES6 module semantics (i.e. use of this). this is the same issue as #2811.

I do not have enough context for Object.defineProperty(App, "name", { value: "App", configurable: true }); can you elaborate.

[alexeagle]

In current chrome, here is an interactive session:

var f = function(){}
undefined
Object.getOwnPropertyDescriptor(f, "name");
Object {value: "", writable: false, enumerable: false, configurable: false}
Object.defineProperty(f, "name", {value: "name", configurable: true});
VM583:2 Uncaught TypeError: Cannot redefine property: name

[rbuckton]

@mhegazy, we have to manually assign the name of the class that has decorators due to the double-bind issue with class declarations. The problem is that the "name" property of Function has [[Configurable]]: true in ES6, but has [[Configurable]]: false in ES5 Chrome, as well as non-strict contexts in Chrome Canary with ES6 features enabled.

@alexeagle Given Chrome's update cycle, how likely would it be that they make Function#name configurable by default before ES6 support ships?

[alexeagle]

I have no idea. But for my part, as long as this is an isolated problem
with external tools doing the 6-to-5 lowering, this is a low priority issue
and can wait for at least a few weeks.

On Mon, Apr 20, 2015 at 3:16 PM Ron Buckton notifications@github.com
wrote:

@mhegazy https://github.com/mhegazy, we have to manually assign the
name of the class that has decorators due to the double-bind issue with
class declarations. The problem is that the "name" property of Function has
[[Configurable]]: true in ES6, but has [[Configurable]]: false in ES5
Chrome, as well as non-strict contexts in Chrome Canary with ES6 features
enabled.

@alexeagle https://github.com/alexeagle Given Chrome's update cycle,
how likely would it be that they make Function#name configurable by default
before ES6 support ships?

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#2836 (comment)
.

[rbuckton]

@alexeagle, I could change this from Object.defineProperty to Reflect.defineProperty, assuming the polyfill babel uses for its runtime includes Reflect.defineProperty (which I believe it does).

Reflect.defineProperty does not throw if the property has [[Configurable]]: false.

[alexeagle]

I don't think this would help us in Closure, their runtime polyfill seems
to just be a couple of function declarations.
Let's revisit if this doesn't make it into mainline Chrome soon.

On Tue, Apr 21, 2015 at 11:44 AM Ron Buckton notifications@github.com
wrote:

@alexeagle https://github.com/alexeagle, I could change this from
Object.defineProperty to Reflect.defineProperty, assuming the polyfill
babel uses for its runtime includes Reflect.defineProperty (which I
believe it does).

Reflect.defineProperty does not throw if the property has
[[Configurable]]: false.

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#2836 (comment)
.

[mhegazy]

Fixed in #2897

[jamiewinder]

FYI, I'm still seeing this on the 1.5 beta.

[mhegazy]

@jamiewinder can you provide more details?

this input:

function dec() { }

@dec
export class Foo {

}

on 1.5-beta gives me:

if (typeof __decorate !== "function") __decorate = function (decorators, target, key, desc) {
    if (typeof Reflect === "object" && typeof Reflect.decorate === "function") return Reflect.decorate(decorators, target, key, desc);
    switch (arguments.length) {
        case 2: return decorators.reduceRight(function(o, d) { return (d && d(o)) || o; }, target);
        case 3: return decorators.reduceRight(function(o, d) { return (d && d(target, key)), void 0; }, void 0);
        case 4: return decorators.reduceRight(function(o, d) { return (d && d(target, key, o)) || o; }, desc);
    }
};
function dec() { }
var Foo = (function () {
    function Foo() {
    }
    Foo = __decorate([
        dec
    ], Foo);
    return Foo;
})();

[jamiewinder]

Here's a basic reproduction, which uses TypeScript 1.5-beta, ES6 output, Babel, and SystemJS together:

index.html

<script src="system.js"></script>
<script>
    System.transpiler = "babel";
    System.babelOptions = {
        modules: "system"
    };
    System.import("app");
</script>

app.ts

import { DecoratedClass } from './decoratedclass';

var dc = new DecoratedClass();
var dummy = dc.name;

DecoratedClass.ts

import { TestDecorator } from './testdecorator';

export class DecoratedClass {
    @TestDecorator()
    public get name(): string {
        return "This is a test";
    }
}

TestDecorator.ts

export function TestDecorator() {
    return (target: any, key: string, props: PropertyDescriptor) => {
        var getter = props.get;

        props.get = function () {
            var val = getter();
            console.log(val);
            return val;
        };
    };
}

The expected result is that the 'This is a test' is written to the console (by the decorator). If I build as ES5 using a CommonJS module system, then it does exactly that. However, if I build to ES6, then I get an error:

Potentially unhandled rejection [2] TypeError: Error evaluating decoratedclass
Cannot read property '__decorate' of undefined
    at execute (http://localhost:31818/decoratedclass.js!eval:15:35)

Let me know if you need more information.