Fix decorator in typescript targeting ES3

[andykog]

When tsc target is set to ES3 (default), it won't add 3'rd argument for decorator https://github.com/Microsoft/TypeScript/blob/v1.8.10/src/compiler/emitter.ts#L5707
Then it won't use Object.defineProperty https://github.com/Microsoft/TypeScript/blob/v1.8.10/src/compiler/emitter.ts#L364

I don't know, whether we should care about this setup, because mobx itself is not usable under ES3 environment, but this fixes problem on updating from mobx 2.2.2

[andykog]

Fiddle, representing the problem: https://jsfiddle.net/Sl1v3r/r3oLj9fz/

[mweststrate]

Hey @andykog, what will this fix? If target is ES3, then Object.defineProperty should not be available in the first place right? I guess people should just not compile for ES3. So maybe we should just throw an exception here?

[andykog]

I mean some people can forget to specify typescript target, i thought, it would be nice if mobx still works. Fixing jsfiddle is also nice.
And it fixes Reflect.decorate (#333)

[mweststrate]

Ha sorry yeah read this in the wrong other :) Thanks for sorting this out!
I'll run some more tests but I think this will do.

Op ma 20 jun. 2016 om 16:32 schreef Andy Kogut notifications@github.com:

In src/utils/decorators.ts
#343 (comment):

@@ -48,6 +48,11 @@ export function createClassPropertyDecorator(
}
}
};

•       if (arguments.length < 3) {
  

•           // Typescript target is ES3, so it won't define property for us
  

•           Object.defineProperty(target, key, descriptor);
  

I mean some people can forget to specify typescript target, i thought, it
would be nice if mobx still works. Fixing jsfiddle is also nice.
And it fixes Reflect.decorate (#333
#333)

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[coveralls]

Coverage decreased (-0.1%) to 93.195% when pulling 76d3a88 on andykog:fixTypescriptES3decorator into 4aff829 on mobxjs:master.

[mweststrate]

Thanks again for sorting this out!