TypeScript unknown vs any: validate an API response

The problem starts outside your program

Imagine an API is supposed to return a number of credits, but a broken integration returns "20" as text. Declaring that response as any lets the rest of your program call methods and read properties without a useful compiler objection. Declaring it unknown forces the next piece of code to prove what it received. Neither declaration changes the actual response.

Make the check match the business rule

If credits must be a nonnegative safe integer, a typeof check alone is incomplete. It accepts fractional numbers and values outside the integer range that JavaScript can represent exactly. Choose the whole contract and return a result the caller can handle.

function credits(value: unknown): number | null {
  return typeof value === "number"
    && Number.isSafeInteger(value) && value >= 0
    ? value : null;
}
// credits(20) → 20
// credits("20") → null
// credits(-1) → null

Avoid silent conversions

Number(value) can be appropriate in a parser, but it is a different contract: an empty string and false convert to zero. If a user form accepts decimal text, write and test that parsing rule separately. Do not let a convenient conversion decide what your API accepts.

Test the boundary, then simplify the interior

Test zero, a positive integer, a fraction, a numeric string, null and a boolean. Once validation succeeds, pass the validated value deeper into the application with its known type. Keep error handling at the boundary. The check takes constant time and space for this scalar input; recursively validating a larger object depends on its size.