Summary - Type annotation

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.

Exercise 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.

Show example solution

function dikdortgenAlani(genislik: number, yukseklik: number): number {
    return genislik * yukseklik;
}
console.log(dikdortgenAlani(5, 3));

Test yourself

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; }
- function hesapla(a, b) { return a + b; }
Show answer

function hesapla(a: number, b: number): number { return a + b; } — In a full type annotation, both parameters and the return type must be written explicitly; <code>a: number, b: number): number</code> does this.

Add the return type

function dikdortgenAlani(genislik: number, yukseklik: number)____ { return genislik * yukseklik; }

Show answer

function dikdortgenAlani(genislik: number, yukseklik: number): number { return genislik * yukseklik; }

Annotation summary

let x: number

Variable annotation
function f(a: number)

Parameter annotation
function f(): number

Return type annotation
: type

Type declaration

Next lesson: Basic inference

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