Indexing with keyof

Learn safe property access with keyof.

Indexing with keyof

The type obtained with keyof can be used for indexed access in square brackets. Kisi[keyof Kisi] gives the union of all property value types of Kisi. This enables safe property access on an object; a wrong key is a compile error.

interface Kisi {
  ad: string;
  yas: number;
}
type DegerTuru = Kisi[keyof Kisi];
// string | number

Safe property access

If a function takes a k: keyof Kisi parameter, the o[k] access in the body is type-safe. The compiler knows k is a valid property name. With return type Kisi[keyof Kisi], the union of all possible value types is returned.

Exercise Fetch a value by key

Define interface Kisi with ad: string and yas: number. Write function al(o: Kisi, k: keyof Kisi): Kisi[keyof Kisi] with body return o[k];. Assign { ad: "Ela", yas: 28 } to a variable kisi. Print the result of al(kisi, "ad") with console.log.

Show example solution

interface Kisi {
  ad: string;
  yas: number;
}
function al(o: Kisi, k: keyof Kisi): Kisi[keyof Kisi] {
  return o[k];
}
const kisi = { ad: "Ela", yas: 28 };
console.log(al(kisi, "ad"));

Test yourself

What does T become in the code below?
```
interface Kisi { ad: string; yas: number; }
type T = Kisi[keyof Kisi];
```
- "ad" | "yas"
- Kisi
- any
- string | number
Show answer

string | number — Kisi[keyof Kisi] gives the union of all property value types. Kisi's properties are string and number, so the result is string | number. It does not give the key names.

Indexed access type

interface Kisi { ad: string; yas: number; }
function al(o: Kisi, k: keyof Kisi): Kisi[____] {
  return o[k];
}

Show answer

interface Kisi { ad: string; yas: number; }
function al(o: Kisi, k: keyof Kisi): Kisi[keyof Kisi] {
  return o[k];
}

keyof indexing parts

keyof Kisi

Kisi's property names
Kisi[keyof Kisi]

Union of all value types
o[k]

Safe property access
al(kisi, "ad")

Fetch a value by key

Next lesson: Summary - keyof and typeof

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