[Bug]: the order of exports is incorrect when converting ESM to CJS

[evanw]

💻

• Would you like to work on a fix?

How are you using Babel?

Programmatic API (babel.transform, babel.parse)

Input code

export let a = 1
export let c = 3
export let b = 2

Configuration file name

No response

Configuration

import * as babel from '@babel/core';
import esm2cjs from '@babel/plugin-transform-modules-commonjs';
output = babel.transformSync(input, { plugins: [esm2cjs] }).code;

Current and expected behavior

Babel generates this code when converting ESM to CommonJS:

"use strict";

Object.defineProperty(exports, "__esModule", {
  value: true
});
exports.b = exports.c = exports.a = void 0;
let a = 1;
exports.a = a;
let c = 3;
exports.c = c;
let b = 2;
exports.b = b;

However, the order of exports is incorrect and does not match what native ESM does. For example, ESM code that uses Object.keys() on this file will behave differently when converted to CommonJS. The order should be ['a', 'b', 'c'] but Babel gives an order of ['a', 'c', 'b'] instead. According to the specification, exports must be sorted:

Table 29 — Internal Slots of Module Namespace Exotic Objects

Internal Slot	Type	Description
[[Module]]	Module Record	The Module Record whose exports this namespace exposes.
[[Exports]]	List of String	A List containing the String values of the exported names exposed as own properties of this object. The list is ordered as if an Array of those String values had been sorted using Array.prototype.sort using SortCompare as comparefn.

Environment

My package.json file:

{
  "dependencies": {
    "@babel/core": "7.15.5",
    "@babel/plugin-transform-modules-commonjs": "7.15.4"
  }
}

Possible solution

I believe Babel should generate exports.c = exports.b = exports.a = void 0; instead of exports.b = exports.c = exports.a = void 0; here.

Additional context

No response

[babel-bot]

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