keyof constraint

Learn the property name constraint with keyof.

The keyof constraint

keyof T gives the union of all property names of T. Writing K extends keyof T ensures K is a valid property name of T. So al(obje, "ad") is safe; a non-existent property name is a compile error.

function al<T, K extends keyof T>(o: T, k: K): T[K] {
  return o[k];
}
const u = { ad: "Ayşe", yas: 30 };
console.log(al(u, "ad"));  // Ayşe

The T[K] indexed access type

T[K] gives the value type of property K on T. The signature al<T, K extends keyof T>(o: T, k: K): T[K] ensures the returned value has exactly that property's type. So al(u, "yas") returns number while al(u, "ad") returns string; both are known at compile time.

Exercise Fetch a property

Write the generic function al<T, K extends keyof T>(o: T, k: K): T[K] with body return o[k];. Assign { ad: "Ece", yas: 25 } to a variable kisi. Print al(kisi, "ad") and al(kisi, "yas") on separate lines.

Show example solution

function al<T, K extends keyof T>(o: T, k: K): T[K] {
  return o[k];
}
const kisi = { ad: "Ece", yas: 25 };
console.log(al(kisi, "ad"));
console.log(al(kisi, "yas"));

Test yourself

What does K extends keyof T guarantee?
```
function al<T, K extends keyof T>(o: T, k: K): T[K]
```
- That K is a number
- That T is an array
- That T has property K
- That K is a valid property name of T
Show answer

That K is a valid property name of T — keyof T gives all property names of T; K extends keyof T guarantees K is one of those names. It is about K's name, not T's structure.

Bound K with keyof

function al<T, K extends ____>(o: T, k: K): T[K] {
  return o[k];
}

Show answer

function al<T, K extends keyof T>(o: T, k: K): T[K] {
  return o[k];
}

keyof constraint parts

keyof T

Union of T's property names
K extends keyof T

Bounds K to a property name
T[K]

Value type of property K
al(u, "ad")

Call with a valid property name

Next lesson: Default type parameter

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