Missing error for enum using non-number variables named `Infinity`/`NaN`

[nicolo-ribaudo]

Bug Report

🔎 Search Terms

enum number infinity nan

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about

⏯ Playground Link

Playground link with relevant code

💻 Code

{
  let Infinity = {};
  enum En {
    X = Infinity
  }
}

{
  let NaN = {};
  enum En {
    X = NaN
  }
}

{
  let SomethingElse = {};
  enum En {
    X = SomethingElse
  }
}

🙁 Actual behavior

TS reports "Type '{}' is not assignable to type 'number' as required for computed enum member values" only for X = SomethingElse

🙂 Expected behavior

TS should also report an error for X = Infinity and X = NaN.

[Andarist]

First things first - should let Infinity = {} be even considered legal? 😅

[fatcerberus]

I mean let undefined = {} is also legal so...

There's nothing special about Infinity (or undefined¹), it just happens that the ES spec provides a read-only (and nonconfigurable) global variable by that name.

Footnotes

1. Fun fact, prior to ES5, there was no global binding named undefined and strict mode wasn't yet a thing, so code could freely do stuff like undefined = 777 and blow everything up. The intention was that you'd get the value undefined "by default" by accessing a nonexistent variable, and specifically using the identifier undefined for this purpose was effectively just convention. You were meant to use null (which is actually a keyword) if you needed to explicitly assign an empty value to something. ↩

[Andarist]

Yeah, I know about those things - but since it's not-configurable you immediately get an error if u try to do it at runtime. So perhaps this should be recognized by TS as a problem? 🤔

[fatcerberus]

There's no runtime error for

{
    let undefined = 777;
    console.log(undefined);
}

Likewise for Infinity. You only get a runtime error if you try to assign to the global binding, but shadowing it is fine.

[Andarist]

Ah, got it - got an error yesterday when I executed it in the console without adding extra scope/block 👍

[nicolo-ribaudo]

Also, in module code you do not need to add the extra block. Only in scripts.