Support for Reflect.getMetadata() in a published Expo app


Just a quick sanity check.

Should I expect react-native to work correctly with Typescript decorators, specifically Reflect.getMetadata? I’ve enabled emitDecoratorMetadata and experimentalDecorators along with react-native-typescript-transformer.

To add a bit to this, a non-published expo app works with this (and I see the transpiled code emitted correctly), but not once published. Specifically, my decorator implementation is executed but Reflect.getMetadata always returns null in a published app.

I know the tooling here is pretty specific, but think its worth a shot to ask.


Hi @cantide5ga.

I’m not that familiar with Typescript so don’t have a great answer for this specific issue. One thing that might help is running exp with exp start --minify --no-dev. That’ll let you load the bundle as if it was a published bundle. My guess would be that something with minify is breaking it. Maybe that’ll make it easier to debug.


Excellent suggestion. I’ve cross posted this question in a few other places as well, but will keep this in parity when I figure things out.

Really appreciate it.


For some reason (maybe timing), debugging of any sort gets me a null return from the return of Reflect.getMetadata(). Digging into the object despite this, I get some good stuff - including the mangling done by minification that @jesse was alluding to.

The second issue is consequence of the first - my implementation was relying too much on the handle of the class type of the decorated property that changed when bundled. So that’s why it blew up.

I changed my implementation to work off of signatures that were spared minification (in this case, the property name). Guess I didn’t dig enough into the bundling done here, so learned some good stuff.

Thanks @jesse!

That being said, I’m afraid I’m relying too much on an implementation detail as I usually see variable names minified as well. Do you have any suggestions on configuring what is minified?


I believe RN uses uglify-js for minification. The library might allow you to configure how these different elements are minified, but I don’t think Metro (the bundler RN’s tools use) exposes a configuration interface for it. Possibly worth looking into though!


Yup, not Expo’s problem. Thanks for all the tips!

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