Exported mutable variable in namespace with setter function does not mutate value

[abrenneke]

Describe the bug

If an export from a namespace is mutable, and a function in the namespace mutates the value, the exported value is not mutated.

Input code

namespace A {
    export let a = 5;

    export function setA(value: number) {
        a = value;
    }
}

Config

{
  "jsc": {
    "parser": {
      "syntax": "typescript",
      "tsx": false
    },
    "target": "es5",
    "loose": false,
    "minify": {
      "compress": false,
      "mangle": false
    }
  },
  "module": {
    "type": "commonjs"
  },
  "minify": false,
  "isModule": true
}

Playground link

https://play.swc.rs/?version=1.2.218&code=H4sIAAAAAAAAA8tLzE0tLkhMTlVwVKjmUgCC1IqC%2FKIShZzUEoVEBVsFU2suZOG00rzkksz8PIXi1BJHjbLEnNJUK4W80tyk1CJNqAEgANIJlrQGC9Vy1QIA8jab8GkAAAA%3D&config=H4sIAAAAAAAAA0WNSwrDMAxE76J1tt3kDj2ESZXg4B8aBWqM7165OGQnzedNoxMbrY2KE7CMCzWp%2B9JKWgtjE1%2BUFlKYtLsA7vY4OVgtwniZF3IGT3eh6JPf6yBtORZh4LFcOsKd7AaK%2BXMNof3HDGiVmNMJ6g9olj3eM61ycf8BnwbyVboAAAA%3D

Expected behavior

tsc output correctly mutates the value:

"use strict";
var A;
(function (A) {
    A.a = 5;
    function setA(value) {
        A.a = value;
    }
    A.setA = setA;
})(A || (A = {}));

Actual behavior

Mutates the variable internal to the namespace but not the exported value from the namespace.

"use strict";
var A;
(function(A) {
    var setA = function setA(value) {
        a = value;
    };
    var a = A.a = 5;
    A.setA = setA;
})(A || (A = {}));

Version

1.2.218

Additional context

No response

[kwonoj]

Afaik this is expected by we try to model similar approach to babel where we compile each module as isolated.

https://babeljs.io/docs/en/babel-plugin-transform-typescript

Q: Why doesn't Babel allow export of a var or let?

A: The TypeScript compiler dynamically changes how these variables are used depending on whether or not the value is mutated. Ultimately, this depends on a type-model and is outside the scope of Babel. A best-effort implementation would transform context-dependent usages of the variable to always use the Namespace.Value version instead of Value, in case it was mutated outside of the current file. Allowing var or let from Babel (as the transform is not-yet-written) is therefore is more likely than not to present itself as a bug when used as-if it was not const

Babel raises compile time exception though, while we do not fail but does not allow those behavior.

I may wrong, so will leave issue opened until getting second eye on this.

babel repl for ref: https://babeljs.io/repl/#?browsers=defaults%2C%20not%20ie%2011%2C%20not%20ie_mob%2011&build=&builtIns=false&corejs=3.21&spec=false&loose=false&code_lz=HYQwtgpgzgDiDGEAEBBJBvAUEnSIA8YB7AJwBckAbCCkJAXiQFYBuTbXA48pAMwFdg8MgEsiwJFBooAFADcQlfhABcSYPzAAjCCQCUGDrlx1GCpRDbGAvpmtA&debug=false&forceAllTransforms=false&shippedProposals=false&circleciRepo=&evaluate=false&fileSize=false&timeTravel=false&sourceType=module&lineWrap=true&presets=env%2Creact%2Cstage-2%2Ctypescript&prettier=false&targets=&version=7.18.9&externalPlugins=&assumptions=%7B%7D

[abrenneke]

Yeah that makes sense, babel doesn't even try to compile it so 🤷 - swc does compile it so I was surprised when a call simply didn't work at all, would rather it fail instead of code just running differently though.

[kdy1]

It's consistent with behavior of tsc with isolatedModules: true

[swc-bot]

This closed issue has been automatically locked because it had no new activity for a month. If you are running into a similar issue, please create a new issue with the steps to reproduce. Thank you.