Unexpected error "Variable is used before being assigned."

[amur-tiger]

Bug Report

🔎 Search Terms

typeof, void, used before being assigned

🕗 Version & Regression Information

Tested with 4.8.4, 4.9.4, nightly, down to 3.3.3 via Playground

⏯ Playground Link

https://www.typescriptlang.org/play?ts=4.9.4#code/MYewdgzgLgBAZmGBeGAKAlALhtATgSzAHMYAfGANxHwBNkA+GAbwFgAoGGAei5xAFsApjGABDADbAAruNFR84dgF8A3O1CRYFCVOEoEGNWw3QYuQRBmwUrDjCgBPAA6DsjlyDiUdepCgBEUmA0gnCEgjT+MAD8MP4AglHY-gBC-gA07Jza4rrYObqZbKrsQA

💻 Code

In this code, fn is a user-supplied function with a generic result (here string) that may also return nothing at all. void is used so return undefined does not have to be written out.

const fn = (): string | void => {
  // some calculation
};
const value = fn();
const result = {
  type: typeof value === "undefined" ? "A" : "B",
  value: value,
};

🙁 Actual behavior

In line 7: TS2454: Variable 'value' is used before being assigned.

Interestingly, the error does not occur when line 6, the typeof check, is commented out.
The error can also be avoided by writing value === undefined instead or casting value to string | undefined.

The error is unexpected because the variable was clearly defined in line 4.

🙂 Expected behavior

I would expect no error, or an error trying to use a void variable. Having to cast it to undefined would be fine.

[fatcerberus]

You shouldn’t use void in a union with other types; it’s essentially a singularity and causes weird things to happen. Write string | undefined if that’s what you mean. See #42709.

To clarify: As far as the type system is concerned void doesn’t really mean undefined. It means “I don’t know what this value is and it doesn’t matter because nobody should ever access it.”

[amur-tiger]

👍 As I thought, void is a bit special. Though as I wrote, it's handy for the callback supplier, so I'll keep it and add a cast or something. I just found the error message rather interesting.

[fatcerberus]

Keep in mind that the check for ret === undefined is technically unsound because () => T is assignable to () => void for all T. This is why string | void is suspect.

[github-actions]

This issue has been marked as 'Not a Defect' and has seen no recent activity. It has been automatically closed for house-keeping purposes.