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.

Comienza con el ejemplo enlazado de TypeScript Playground y reproduce las asignaciones con --strict usando tsc. Rastrea cómo se comprueban las asignaciones a miembros primitivos y, después, añade el rechazo de escrituras ilegales en builtin functions, conservando casos legales de JavaScript como array.length; añade o actualiza las pruebas del compilador para cubrir los ejemplos.