Summary - keyof and typeof

Write a safe settings manager with the operators.

Summary - keyof and typeof

keyof gives a type's keys, typeof gives a value's type. Combining them: derive a type from an object with typeof, take its keys with keyof, and write functions that fetch values safely via indexed access. This is ideal for structures like a settings manager.

const ayarlar = { tema: "koyu", dil: "tr" };
type AyarTip = typeof ayarlar;
type AyarAnahtar = keyof AyarTip;
function al(k: AyarAnahtar): AyarTip[AyarAnahtar] {
  return ayarlar[k];
}

Exercise Settings manager

Assign the object { tema: "koyu", dil: "tr" } to a variable ayarlar. Define type AyarTip as typeof ayarlar. Define type AyarAnahtar as keyof AyarTip. Write function al(k: AyarAnahtar): AyarTip[AyarAnahtar] with body return ayarlar[k];. Print the result of al("tema") with console.log.

Show example solution

const ayarlar = { tema: "koyu", dil: "tr" };
type AyarTip = typeof ayarlar;
type AyarAnahtar = keyof AyarTip;
function al(k: AyarAnahtar): AyarTip[AyarAnahtar] {
  return ayarlar[k];
}
console.log(al("tema"));

Test yourself

Which order enables safe property access from a settings object?
- indexed access, keyof, typeof
- typeof for type, keyof for keys, indexed access for value
- keyof for keys, typeof for type, indexed access
- typeof, indexed access, keyof
Show answer

typeof for type, keyof for keys, indexed access for value — First typeof derives a type from the object, then keyof takes its keys, and finally indexed access fetches the value safely. Any other order breaks the type-level inference.

Indexed access return type

const ayarlar = { tema: "koyu", dil: "tr" };
type AyarTip = typeof ayarlar;
type AyarAnahtar = keyof AyarTip;
function al(k: AyarAnahtar): ____ {
  return ayarlar[k];
}

Show answer

const ayarlar = { tema: "koyu", dil: "tr" };
type AyarTip = typeof ayarlar;
type AyarAnahtar = keyof AyarTip;
function al(k: AyarAnahtar): AyarTip[AyarAnahtar] {
  return ayarlar[k];
}

Match the operators

typeof ayarlar

Derives a type from a value
keyof AyarTip

Gives a type's keys
AyarTip[AyarAnahtar]

Value type of a key
al("tema")

Safe value fetch

Next lesson: What is a mapped type?

We Are Enhancing Your Experience

Cookie Preferences

Strictly Necessary Cookies

Analytical Cookies