Summary - Generic constraints

Write a property picker with constrained generics.

Summary - Generic constraints

Generic constraints limit what a type parameter can do. T extends Adli forces specific properties; K extends keyof T makes property names safe; <T = string> gives the type parameter a default. Combining the three you can write type-safe property selectors and helpers.

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

Exercise Type-safe property selector

Write the generic function sec<T, K extends keyof T>(o: T, k: K): T[K] with body return o[k];. Assign { kullanici: "ali", puan: 90 } to a variable kayit. Print sec(kayit, "kullanici") and sec(kayit, "puan") on separate lines.

Show example solution

function sec<T, K extends keyof T>(o: T, k: K): T[K] {
  return o[k];
}
const kayit = { kullanici: "ali", puan: 90 };
console.log(sec(kayit, "kullanici"));
console.log(sec(kayit, "puan"));

Test yourself

You want to access x.uzunluk in a generic function body. Which constraint is needed?
- T extends Uzunluklu, Uzunluklu = { uzunluk: number }
- K extends keyof T
- T = string
- None; no constraint is needed
Show answer

T extends Uzunluklu, Uzunluklu = { uzunluk: number } — To access a property the type must have it; you declare this with extends. keyof bounds a property name, default only gives a fallback type; neither grants access.

Property selector signature

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

Show answer

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

Match constraint kinds

T extends Adli

Forces a specific property
K extends keyof T

Bounds a property name
<T = string>

Gives a default type
T[K]

Value type of property K

Next lesson: Partial

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