Generic Constraints

A plain <T> accepts any type, which means you cannot use any of its properties. Add a constraint with extends to require a shape: <T extends { length: number }> accepts strings, arrays and anything else with a length. <K extends keyof T> limits a key to the real keys of T, which is how you write a type-safe property getter.

How it reads

• The constraint is what makes a.length legal inside longest
• K extends keyof T rejects keys the objects do not have
• T[K][] is "an array of whatever type that property has"

Cloud tip: Start generic functions unconstrained, then add exactly the constraint the body needs and no more.