Generic Constraints

Constraints (🏁 solution)

👨‍💼 You've mastered generic constraints!

🦉 The keyof constraint pattern is incredibly powerful:

function getProperty<ObjectType, Key extends keyof ObjectType>(
	obj: ObjectType,
	key: Key,
): ObjectType[Key]

This ensures:

key must be an actual property of obj
The return type matches the property's type exactly

const user = { name: 'Alice', age: 30 }
getProperty(user, 'name') // Returns string
getProperty(user, 'age') // Returns number
getProperty(user, 'foo') // ❌ Error: 'foo' is not a key of user

This is how TypeScript's built-in Pick, Omit, and other utility types work under the hood!

06. Generics/03. Generic Constraints (🏁 solution)

Generic Constraints