never type
- TypeScript course
- Beginner
- Lesson 27/89
- 4 min
Learn unreachable code with never.
The never type
The never type represents values that can never occur. A function that always throws an error or runs an infinite loop has the return type never. This type marks unreachable code.
function patla(mesaj: string): never {
throw new Error(mesaj);
}never indicates that the function does not end normally.
never and unreachable code
never also marks unreachable code paths. If a function ends with throw, the code after throw never runs. This code is of type never and is flagged by the compiler.
function patla(): never {
throw new Error("durdur");
console.log("buraya hiç gelinmez"); // never
}never helps catch logic errors in the program's flow.
Practice never function and error catching
Define a function named patla with a mesaj parameter of type string and return type never. The function should throw new Error(mesaj). Use try-catch to call patla("sistem hatası") and print the caught error's message with console.log.
Starter code
// patla adında bir fonksiyon tanımla
// mesaj parametresi string, dönüş türü never olsun
// fonksiyon new Error(mesaj) fırlatsın
// try-catch ile çağır ve hatanın mesajını yazdırSample solution
function patla(mesaj: string): never {
throw new Error(mesaj);
}
try {
patla("sistem hatası");
} catch (e) {
console.log("yakalandi: " + e.message);
}Expected output
yakalandi: sistem hatası
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Which function has the <code>never</code> return type?
- function patla(msg: string): never { throw new Error(msg); } Correct answer
- function selam(): void { console.log("hi"); }
- function al(): string { return "x"; }
- function topla(a: number, b: number): number { return a + b; }
Explanation: The <code>patla</code> function, which always throws, does not end normally; its return type is <code>never</code>. The others either return a value or end normally.
2 Add the never return type
function patla(mesaj: string): ____ { throw new Error(mesaj); }function patla(mesaj: string): never { throw new Error(mesaj); }Explanation: Since the function always throws an error, it does not return normally; the correct return type is <code>never</code>.
3 never concepts
| Term | Meaning |
|---|---|
never |
Value that never occurs |
throw new Error() |
Function does not end normally |
never dönüş türü |
Function does not return |
Sonsuz döngü |
Fits the never type |