Using startsWith & endsWith to narrow type

[LinusU]

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 🚀

[RyanCavanaugh]

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]

Fair enough, thanks for taking the time to explain

[alexgleason]

Here is a declaration that does it:

interface String {
  startsWith<P extends string>(searchString: P): this is `${P}${string}`;
  startsWith<P extends string>(searchString: P, position: 0): this is `${P}${string}`;
}

I created an MR to add this to the ts-reset package: mattpocock/ts-reset#161