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Using startsWith & endsWith to narrow type #46958
Description
lib Update Request
Configuration Check
My compilation target is ES2020 and my lib is ES2020.
Missing / Incorrect Definition
startsWith & endsWith doesn't narrow the the type, it just returns a boolean.
Sample Code
interface BuiltinPaymentMethod {
id: `builtin-${string}`
// ...
}
// ...
declare const id: string
if (id.startsWith('builtin-')) {
// Here I want `id` to be typed as `builtin-${string}`
}Documentation Link
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/startsWith
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/endsWith
Pull Request
#46959 🚀
Activity
RyanCavanaugh commentedon Nov 30, 2021
I don't think this is a good fit for the general library. Two main concerns: Changing a function from not-overloaded to overloaded has fairly pervasive effects in areas like contextual typing, generic inference, and how exactly it can be invoked when the method operand is a union. The other is that template string literals are inherently combinatorially explosive, so a call like
startsWith(union of 24) && endsWith(union of 24)produces a union of 576 elements, which is bad from a performance and comprehensibility perspective.Thankfully since this is an additional overload rather than a replacement, you can add it to your own project through declaration merging if those trade-offs are a good fit for your use cases.
LinusU commentedon Dec 1, 2021
Fair enough, thanks for taking the time to explain
alexgleason commentedon Aug 10, 2023
Here is a declaration that does it:
I created an MR to add this to the ts-reset package: mattpocock/ts-reset#161
PatrykKuniczak commentedon Mar 18, 2025
There still no place to implement it inside TS?