function members of primitives/builtins are not read-only

[nielstron]

🔎 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.

[RyanCavanaugh]

What runtime are you using? This is valid in default DOM contexts. If you're using a custom environment that freezes these prototypes, you'd need to customize your lib appropriately

[nielstron]

This happens both with node v20.16 and in the linked playground on MacOS. But it also does not throw an error in the browser console (Firefox).

[nielstron]

I am not aware of having changed any lib or using any custom environments...

[nmain]

Setting a property on a primitive is legal (but silently ignored) in sloppy contexts, and throws an error with "use strict"; or any implied strict context such as a module.

[RyanCavanaugh]

Ah, right. The type system doesn't have any notion of "valid in strict mode" vs not (we can only differentiate when this would be the case syntactically). Given that it's pretty hard to make this mistake unless you're intentionally trying to, I don't think we'd undertake the fairly invasive effort to change it unless there was a more compelling demonstration of how this comes up in practice.

[nielstron]

Sounds fine to me. Do you happen to know if this behavior is documented anywhere? "frozen builtins" did not yield any results upon search

[nielstron]

looks like LLM spam - this is entirely unrelated to my issue