Constraints on interface members in typescript generics

Question

I have a method, that should accepts any object, as long as all its fields are strings or numbers

I made this, which works great with duck typing

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Answer 1

T's constraint can refer to T (with some restrictions), so you can use a mapped type to generate a constraint that has the same fields as T:

function interpolateParams(
    route: string, 
    params: T) : string { /*...*/ }

Beware that trickery with circular type parameter constraints can sometimes cause problems, though this scenario will likely be OK.