es6 `let` block-scope binding construct is particularly dangerous

[psbanka]

React-native's support for ES6 is fantastic, but one area where this could use some improvement is how let is handled.

When a developer is using Chrome for debugging, the let statement is fine and no errors are produced. However, when a dev switches to the native JS engine in the phone or emulator, you get a white-screen crash with no error messages. This can result in a very sad and painful debugging experience, that I can personally attest to 😢 .

It would be nice if the pre-compile step would throw an exception for let when bundling or to throw an error in Chrome's console. Is that possible?

[ide]

723e988 should make this less of a mystery by putting up an error box when there's a syntax error. Not the same as your suggested fix but I believe it should make debugging the root cause a lot easier.

[jtremback]

Why doesn't react-native just use babel?

[psbanka]

Anything that crashes loudly would be perfect. Sounds good 👍

[ide]

@jtremback it's partly historical and partly due to performance as I understand it. Babel/6to5 wasn't as big as it is today (or didn't even exist) when React/RN were developed, and FB's JS processing pipeline already used jstransform. The good news is that a migration to Babel is on the horizon, though someone else would have to give you the timeline for that. https://github.com/facebook/esprima/issues/111

[ide]

Babel is in master, RN is JS-hipster-approved, closing this out. tadeuzagallo@56d6ee3

[jtremback]

Is this in RN5.0?

[ide]

I expect it will ship with 0.6 because it's a bigger change and not a bug fix for 0.5.0 rc.

[jtremback]

Phew! Those 2 hours setting up and debugging Babel in my project yesterday were not wasted!

[psbanka]

Thank you!