What is a generic?
- TypeScript course
- Intermediate
- Lesson 15/89
- 4 min
Learn functions with type parameters.
What is a generic?
A generic lets a function or class take a type as a parameter. The letter inside angle brackets <T> is a type parameter that is replaced by the real type when called. The same function then works type-safely with different types.
function ilk<T>(dizi: T[]): T {
return dizi[0];
}
const s = ilk<number>([1, 2, 3]);
const m = ilk<string>(["a", "b"]);Without generics
Without generics you would write a separate function per type: ilkSayi, ilkMetin, etc. With a single ilk<T> the same function works for all types and the compiler still catches wrong usage. This keeps type safety while reducing duplication.
Practice Return the last element
Write a generic function son<T> with parameter dizi: T[] and return type T. It should return the last element of the array. Then print the result of son([10, 20, 30]) with console.log.
Starter code
// son<T>(dizi: T[]): T fonksiyonunu yaz
// dizinin son elemanını döndür
// son([10, 20, 30]) sonucunu console.log ile yazdırSample solution
function son<T>(dizi: T[]): T {
return dizi[dizi.length - 1];
}
console.log(son([10, 20, 30]));Expected output
30
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 What does T represent in the function below?
function ilk<T>(dizi: T[]): T- Type parameter Correct answer
- Fixed number type
- A variable name
- An interface name
Explanation: T is a type parameter; it is replaced by a real type such as number or string when called. It is not a variable name or an interface name.
2 Add the type parameter
function kimlik<____>(x: T): T {
return x;
}function kimlik<T>(x: T): T {
return x;
}Explanation: Since the body uses T, the type parameter must also be T. Writing U or Type leaves T in the body undefined; t is a different identifier and due to case sensitivity T still remains undefined.
3 Generic parts
| Term | Meaning |
|---|---|
<T> |
Type parameter declaration |
ilk<number>(...) |
Calling with a type argument |
T[] |
Type parameter used as array |
function ilk |
Generic function name |