Downlevel for (this.#x of ...) / for (this.#x in ...) emits __classPrivateFieldGet(...) as the loop target #65413

🔎 Search Terms

private field for-of downlevel, for-in private name, __classPrivateFieldGet for-of, Invalid left-hand side in for-loop, es2021 private field loop

🕗 Version & Regression Information

Not a regression. Only affects targets that lower #private (es2021 and below); --target es2022 is correct.

• Fails: 5.9.3, 6.0.3, 7.0.1-rc, 7.0.2, 7.1.0-beta, 7.1.0-dev.20261106.1, 7.1.0-dev.20261109.1

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=6.0.3&target=4#code/PTAEAEBcEMCcHMCmkBcoCiBlATABgIwDsAUAMYA20AzlaAIKgDexooAxAB5rQB2AnqAC8oXAG4WoWAFceACgD2aeQCMAVolKQAlEwmsAZvNihZkABYBLKgDpOoeftABtfPgC6OxgF89oQ8dNLGzsLHntPH1ZWWGQpWDDzK1sOcVYfH1J5Hip5ckRrcnl4WR5EAHd6WS1raTlGUABrNHxQLy0tcSA

(Playground, 6.0.3, target ES2017: the .JS tab shows for (__classPrivateFieldGet(this, _A_x, "f") of [11]) { } with 0 errors. The 7.1 nightly in Playground v2 emits the same.)

💻 Code

class A {
  #x: any = 0;
  run(o: object) {
    for (this.#x of [11]) {}
    for (this.#x in o) {}
    return this.#x;
  }
}
console.log(new A().run({ k: 1 }));

🙁 Actual behavior

tsc --target es2017 repro.ts reports nothing and emits:

        for (__classPrivateFieldGet(this, _A_x, "f") of [11]) { }
        for (__classPrivateFieldGet(this, _A_x, "f") in o) { }

node repro.js fails to load the file:

ReferenceError: Invalid left-hand side in for-loop

🙂 Expected behavior

A valid assignment target, and node repro.js prints k as it does with --target es2022. For destructuring ([this.#x] = [11]) tsc already emits ({ set value(_b) { __classPrivateFieldSet(...); } }).value; the for...in/for...of initializer, when it is not a variable declaration, needs the same treatment.

Additional information about the issue

Private accessors (set #x(v)) behave the same way. The whole module fails to parse, so nothing in the file runs.

This was found with an AI-assisted differential testing tool; the repros and outputs above were re-run on the listed versions.