Having trouble narrowing type with Array.filter about typebox

[sinclairzx81]

@eloyrobillard Hiya,

Going to close off this issue as this all appears to be standard (albeit interesting) Ts/Js behaviors.

Feel free to reply on this thread if you have any follow up questions though.
Cheers!
S

[sinclairzx81]

@eloyrobillard Hi,

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First Bug

I think this is normal TypeScript behavior. Consider the following (without TypeBox) where there is an explicit guard for number types only, however the return value is inferred as any[].

const foos: any[] = [0, 1, 2];

const bars = foos.filter(foo => typeof foo === 'number') // any[]

However when passing a function, I believe TypeScript is able to correctly derive the return guard via Instantiation Expression. However it might be good to ping this question on the TypeScript repository (as this behavior is specific to TypeScript)

const foos: any[] = [0, 1, 2];

const check = (value: number): value is number => typeof value === 'number'

const nums = foos.filter(check) // number[]

---

Second Bug

The compiled TypeCheck<T> is an object type with internal fields that are accessed via this. As such, you won't be able to pass the Foo.Check function directly as the this context would be lost (which causes the error). However it is possible to .bind to ensure the context is retained. The following should work.

const foos: any[] = [0, 1, 2];

const Foo = TypeCompiler.Compile(Type.Number());

const nums = foos.filter(Foo.Check.bind(Foo)); // number[]

The above is a little awkward, but is standard JavaScript.

Hope this helps!
S

[eloyrobillard]

Thank you for the detailed reply