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Template type fails to infer a match #48430
Description
Bug Report
🔎 Search Terms
Template literal type narrowing fails string 2345
🕗 Version & Regression Information
- This changed between versions 4.0.5 and 4.1.5 because that is when the feature was introduced.
⏯ Playground Link
Playground link with relevant code
💻 Code
type EventName<T extends string> = `${T}Changed`; //From feature announcement: https://github.com/microsoft/TypeScript/pull/40336
const functionRequiresEventString = function(narrowedString: EventName<string>) {
console.log('The following string is an event string per the original example: ' + narrowedString);
}
const nameEvent = function(whatChanged: string) {
functionRequiresEventString('Functionality Changed');
//Next line fails: Argument of type 'string' is not assignable to parameter of type '`${string}Changed`'.ts(2345)
functionRequiresEventString(whatChanged + 'Changed');
}🙁 Actual behavior
Error: Argument of type 'string' is not assignable to parameter of type '${string}Changed'.ts(2345) (but with backticks inside the quotes in the type).
🙂 Expected behavior
No error: The concatenated string ends in 'Changed' and should pass typechecking.
Related Issues
This is a minimal example using the first example of what should be supported, from the feature announcement, so I don't think reporting an issue in the announced functionality is a duplicate of this issue looking for gaps in what that didn't cover, and that thread is now being ignored anyway. This looks a lot like this issue reported 9/1/20 as a comment on the merged PR but is simpler than this issue around numbers. It’s also simpler than what this commenter sought around a fixed prefix.
Activity
whzx5byb commentedon Mar 25, 2022
Concatenation with + won't produce a template literal type. You must use template strings like
functionRequiresEventString(`${whatChanged}Changed`)wbt commentedon Mar 25, 2022
whzx5byb Thanks for the helpful reply. I think it mainly calls out a shortcoming in the minimal example above.
A slightly more realistic case that can't be covered by that is here:
On the line
return stringInthere's an error that this variable isn't assignable to that narrower string type.whzx5byb commentedon Mar 25, 2022
wbt
The return type of
endsWithis not a type predicate, so it doesn't narrow type as you expected. Related: #46958wbt commentedon Mar 26, 2022
Thanks for the reply; this does look like it boils down to a combination of the comment above and #46958, both of which are unexpected. I'll use casting as a workaround.