In TypeScript, a generic function with an index constraint

Is it possible to define an element that operates in the manner indicated here?

function fn<T, U extends keyof T, T[U] extends number>()

I am having trouble making the "T[U] extends number" portion function correctly.

Answer №1

What do you think of this solution?

function customFunction<T extends Record<U, number>, U extends keyof T>(t: T, u: U): number {
  return t[u];
}

When we specify

T extends Record<U, number>
, it means that the property T[U] is present and its type is number:

customFunction({ name: 'jane', age: 30 }, 'age'); // valid
customFunction({ name: 'jane', age: 30 }, 'name'); // invalid
customFunction({ name: 'jane', age: 30 }, 'oops'); // invalid

Is this approach satisfactory for your needs?

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