Lacking support for ES6 block scoping

[tido64]

Bug Description

There is a bug in Object.defineProperty that always returns the last set property regardless of what property is being accessed.

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

Hermes version: 0.8.1
React Native version: n/a
OS version: n/a
Platform: arm64-v8a, x86_64

Steps To Reproduce

Run the following code example with hermes-engine-cli and node, and compare the output:

const target = {
  a: "a",
  b: "b",
  c: "c",
};

const copy = {};
let keys = "";
for (const prop in target) {
  keys += " " + prop;
  Object.defineProperty(copy, prop, {
    get: () => target[prop],
    enumerable: true,
  });
}

// I don't know how to output to console so I'm just using a throw 😛
throw new Error(`copy: a = ${copy.a}, b = ${copy.b}, c = ${copy.c} (keys:${keys})`);

Output:

Uncaught Error: copy: a = c, b = c, c = c (keys: a b c)
    at global (repro.js:18:16)

The Expected Behavior

This should throw copy: a = a, b = b, c = c, as Node does:

Error: copy: a = a, b = b, c = c (keys: a b c)
    at Object.<anonymous> (/~/repro.js:18:7)
    at Module._compile (internal/modules/cjs/loader.js:1072:14)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:1101:10)
    at Module.load (internal/modules/cjs/loader.js:937:32)
    at Function.Module._load (internal/modules/cjs/loader.js:778:12)
    at Function.executeUserEntryPoint [as runMain] (internal/modules/run_main.js:76:12)
    at internal/main/run_main_module.js:17:47

[tido64]

cc @Huxpro, @mganandraj, @JasonVMo

[ljharb]

Hermes’ behavior is how for-in and for loops behave, but i believe for-of loops specifically address this (ie, the issue is correct)

[kelset]

hey @ljharb could I just ask you to rewrite your comment

Hermes’ behavior is how for-in and for loops behave, but i believe for-of loops specifically address this (ie, the issue is correct)

in a bit of a more verbose/n00b-friendly fashion? I've read it I think 5 times now but I still don't understand what you are saying 😓

[tmikov]

This is not related to Object.defineProperty(). Hermes does not fully support ES6 or ES6 block scoping in particular (nor does it claim to). FWIW, we don't support const either (it is treated as an alias of var), but that does not affect this example.

You need to run the source through the regular Babel pipeline, resulting in the following, which does execute correctly:

var target = {
  a: "a",
  b: "b",
  c: "c"
};
var copy = {};
var keys = "";

var _loop = function _loop(prop) {
  keys += " " + prop;
  Object.defineProperty(copy, prop, {
    get: function get() {
      return target[prop];
    },
    enumerable: true
  });
};

for (var prop in target) {
  _loop(prop);
}

[tido64]

Thanks for the explanation and workaround, @tmikov. That pushed me in the right direction. 😄

[tmikov]

FWIW, we have had ES6 block scoping in the works for a very long time, but we are finding it difficult to integrate since the changes are pervasive and it is very disruptive to the entire project (compiler, interpreter, debugger, affects almost every single compiler test, etc).