Object variable declaration conflicts on commonjs

[falsandtru]

TypeScript Version: 3.7.x-dev.20191203

Search Terms:

Code

export default {}
const Object = {};

Expected behavior:

"use strict";
Object.defineProperty(exports, "__esModule", { value: true });
{
exports.default = {};
const Object = {};
}

no error

Actual behavior:

"use strict";
Object.defineProperty(exports, "__esModule", { value: true });
exports.default = {};
const Object = {};

VM69:2 Uncaught ReferenceError: Cannot access 'Object' before initialization
    at eval (eval at <anonymous> (main-3.js:1239), <anonymous>:2:1)
    at main-3.js:1239

Playground Link: http://www.typescriptlang.org/play/index.html?module=1#code/KYDwDg9gTgLgBAE2AMwIYFcA28DeBfAKAGMIA7AZ3gHkAjAK2CPgF458BuIA

Related Issues:

[weswigham]

I'm pretty sure babel does the same thing. If you incompatibly mangle Object, then you incompatibly mangle Object.

[falsandtru]

Avoiding reference to global variables such as const { Object } = globalThis; is an effective performance tuning technique: https://falsandtru.github.io/benchmark/suites/10/

But the current behavior prevents this tuning.

[falsandtru]

If you incompatibly mangle Object, then you incompatibly mangle Object.

As an aside, I'm not sure why you wrote the same thing twice. Do you need a coffee?

[RyanCavanaugh]

At a minimum we should issue an error when we detect that the emit won't work

[falsandtru]

However, developers would feel that error strange. And the error would be enabled only with the few module types (any other similar errors dependent on module types?). Basically other (mainly native) module types don't have this limitation. Therefore this is incompatibility of module types with a very common namespace. Compilers should hide such incompatibility as much as possible. I can find no reason to avoid an easy solution that just surrounding code by curly brackets.

[falsandtru]

Node.js is a user of this technique too.

https://github.com/nodejs/node/search?l=JavaScript&q=primordials+Object

[falsandtru]

Note that you must not make import { Object } from '...' an error because it won't be a runtime error.