Summary - Type annotation
- TypeScript course
- Beginner
- Lesson 33/89
- 4 min
Write a calculation function with explicit types.
Summary - Type annotation
Type annotations make your code explicit: variables get : type, parameters get : type, and the return type goes after the parameter list. Together they form a fully typed signature.
function alan(genislik: number, yukseklik: number): number {
return genislik * yukseklik;
}Explicit types make code easier to read and maintain.
Practice Fully typed calculation function
Define a function named dikdortgenAlani with genislik and yukseklik parameters of type number and return type number. The function should return genislik * yukseklik. Print the result of calling dikdortgenAlani(5, 3) with console.log.
Starter code
// dikdortgenAlani adında bir fonksiyon tanımla
// genislik ve yukseklik parametreleri number olsun
// dönüş türü number olsun
// genislik * yukseklik değerini döndür
// dikdortgenAlani(5, 3) sonucunu yazdırSample solution
function dikdortgenAlani(genislik: number, yukseklik: number): number {
return genislik * yukseklik;
}
console.log(dikdortgenAlani(5, 3));Expected output
15
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Which is a function with full type annotations?
- function hesapla(a: number, b) { return a + b; }
- function hesapla(a, b): number { return a + b; }
- function hesapla(a: number, b: number): number { return a + b; } Correct answer
- function hesapla(a, b) { return a + b; }
Explanation: In a full type annotation, both parameters and the return type must be written explicitly; <code>a: number, b: number): number</code> does this.
2 Add the return type
function dikdortgenAlani(genislik: number, yukseklik: number)____ { return genislik * yukseklik; }function dikdortgenAlani(genislik: number, yukseklik: number): number { return genislik * yukseklik; }Explanation: The function returns <code>genislik * yukseklik</code>, which is a number, so the return type must be <code>: number</code>. <code>: string</code>, <code>: void</code> and <code>: boolean</code> are wrong types.
3 Annotation summary
| Term | Meaning |
|---|---|
let x: number |
Variable annotation |
function f(a: number) |
Parameter annotation |
function f(): number |
Return type annotation |
: type |
Type declaration |