function members of primitives/builtins are not read-only

🔎 Search Terms

builtin number string member readonly

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about "Common Bugs that arent Bugs"

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.5.4&ssl=6&ssc=1&pln=1&pc=1#code/DYUwLgBAHhC8ECYDcAoKA6AbgQ2AVxAHkAzOCACgQEotcCTUVRIBPMgIgAsRhgB7dqhboAxp2wAnAIKR4wsZJmogA

💻 Code

let x = 2;
x.valueOf = (2).valueOf;

let y = "hello";
y.charAt = y.charAt;

🙁 Actual behavior

Code compiles just fine, even with --strict enabled.
Executing JavaScript fails:

can't assign to property "valueOf" on 2: not an object 

🙂 Expected behavior

Should be rejected by tsc as read-only assignment.

Additional information about the issue

Seems similar in nature to https://github.com/microsoft/TypeScript/issues/49113
Notably, array.length is allowed (because its legal in JS) but string.length is not (because it is illegal in JS). Since assigning to builtin functions is also illegal, this should be flagged by tsc.

Start with the linked TypeScript Playground example and reproduce the assignments under --strict with tsc. Trace how assignments to primitive members are checked, then add rejection for illegal writes to builtin functions while preserving legal JavaScript cases such as array.length; add or update compiler tests to cover the examples.