extends constraint

Learn to constrain a type parameter with extends.

The extends constraint

With extends you put a bound on the type parameter. T extends Kisisel means T must have at least all members of Kisisel. Inside the body you can then access known properties like .ad through T; accessing an unknown property is a compile error.

interface Kisisel {
  ad: string;
}
function yaz<T extends Kisisel>(x: T): void {
  console.log(x.ad);  // safe
}

Why a bound matters

Without a constraint, you can only do things common to every type (e.g. assignment) on T; accessing .ad or .uzunluk is a compile error. Adding a bound lets the generic body safely use specific properties, improving reuse without losing type safety.

Exercise Print the length

Define interface Uzunluklu as { uzunluk: number }. Write function uzunlukYaz<T extends Uzunluklu>(x: T): void that prints the length via console.log(x.uzunluk). Then call uzunlukYaz({ uzunluk: 7 }) to print the object's length.

Show example solution

interface Uzunluklu {
  uzunluk: number;
}
function uzunlukYaz<T extends Uzunluklu>(x: T): void {
  console.log(x.uzunluk);
}
uzunlukYaz({ uzunluk: 7 });

Test yourself

Why can the function body access x.uzunluk?
```
interface Uzunluklu { uzunluk: number; }
function isle<T extends Uzunluklu>(x: T) {
  return x.uzunluk;
}
```
- T is bound via extends to Uzunluklu
- T always becomes string
- Because any is used
- T can be any type
Show answer

T is bound via extends to Uzunluklu — Because of T extends Uzunluklu the compiler knows T has the uzunluk property. Without it the access would be a compile error.

Add the constraint

interface Adli { ad: string; }
function selamla<T ____ Adli>(x: T): string {
  return "Merhaba " + x.ad;
}

Show answer

interface Adli { ad: string; }
function selamla<T extends Adli>(x: T): string {
  return "Merhaba " + x.ad;
}

extends constraint parts

interface Adli

Constraint type declaration
T extends Adli

Bound on the type parameter
x.ad

Safe access thanks to the bound
yaz<Calisan>(...)

Calling with a type satisfying the bound

Next lesson: keyof constraint

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