Closure scope leak in for loops

[oblador]

Bug Description

Hermes uses var semantics for let/const variables, leading to scope leaking in for loops.

• I have run gradle clean and confirmed this bug does not occur with JSC

Hermes version: 0.11.0
React Native version (if any): N/A
OS version (if any): macOS
Platform (most likely one of arm64-v8a, armeabi-v7a, x86, x86_64): arm64-v8a

Steps To Reproduce

const keys = ['a', 'b', 'c'];
const obj = {};
for (let i = 0; i < keys.length; i++) {
  const key = keys[i];
  obj[key] = () => key;
}
console.log([obj.a(), obj.b(), obj.c()]);

Hermes output: ['c', 'c', 'c'] 🛑

The Expected Behavior

JSC output: ['a', 'b', 'c'] ✅
Node output: ['a', 'b', 'c'] ✅

[neildhar]

Hi @oblador, Hermes does not support block scoping, and we instead rely on babel transforms to lower code like this into something that Hermes can correctly run. Please see #575 for more details.

[oblador]

Wouldn't it be better to not support the let/const syntax alltogether?