TypeScript incorrectly narrows boolean type to true for variable updated within Map<...>.get(...)?.forEach(...) block.

[paulkim3151]

🔎 Search Terms

"type narrowing", "map of set"

🕗 Version & Regression Information

• This is the behavior in every version I tried (including v5.6.3 and 5.7.0-beta in playground)

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.6.3#code/MYewdgzgLgBAtgQwA4HkBmBlAplAXDAWWQB5oAnASzAHMAaGbKUqSmgPjZgF4YwsB3QsgAUASgDcAKESpMOAHQQcwgEQBrLAE8V9PoMaqEK0RMmSANjhhoQIbjBYBXLFJnpG86svVbjAfnkbMgBRBGAAC2Exbk4Ab0kYa1t7NARzJSkAX1NJUEgQS3lzEGphGxBTIA

💻 Code

const mapOfSet: Map<string, Set<string>> = new Map();
mapOfSet.set("key", new Set("a"));

let foo = true;
mapOfSet.get("key")?.forEach(() => {
  foo = false;
});

console.log(foo); 

🙁 Actual behavior

The foo variable is expected to be of type boolean because it may change to false within the .forEach(...) block. However, TypeScript incorrectly narrows foo's type to true, assuming it will not change.

🙂 Expected behavior

TypeScript should maintain the boolean type for the foo variable, as it can change to false inside the .forEach(...) block when the map's value is accessed and iterated over.

Additional information about the issue

No response

[MartinJohns]

Duplicate of #9998.