Typed foundations practice

Opening your lesson

Alder Payments, a fictional internal tools team, is replacing loosely checked utility functions with explicit contracts. The tasks revisit numeric parameters, closed states, readonly arrays, unknown values, object guards, and generic lookup. The runtime fixtures use fresh records, but the type contracts still have to reject the same unsafe calls. Passing output tests alone isn't enough when a helper accidentally accepts a missing field or returns a wider type than its caller can use. Keep the boundary validation separate from the typed helper that follows it. An empty successful value should remain successful, and a missing field should follow its documented fallback.

function increment(quantity: number): number { return quantity + 1; }
function solve(quantities: number[]): number[] {
return quantities.map(increment);
}

Choose a method from the unit, calculate a small example by hand, and compare the returned value with the requested shape.

Choose your character

Your Work Gym account