Parameter annotation

Learn to type function parameters.

Parameter annotation

By giving function parameters explicit types, you ensure that only values of the correct type can be passed. Each parameter gets : type after its name. Functions without parameter type annotations have parameters that are implicitly any.

function topla(a: number, b: number) {
  return a + b;
}

Parameter annotations prevent passing the wrong type of data to a function.

Multiple parameter annotations

Each parameter gets its own type; : type is written after each comma-separated name. This lets each argument be checked separately at the call site.

function bilgi(isim: string, yas: number, aktif: boolean) {
  console.log(isim, yas, aktif);
}

Giving each of multiple parameters a separate type clarifies the function's contract.

Exercise Write a function with typed parameters

Define a function named topla with a and b parameters of type number. The function should return a + b. Print the result of calling topla(3, 7) with console.log.

Show example solution

function topla(a: number, b: number) {
    return a + b;
}
console.log(topla(3, 7));

Test yourself

What does <code>function selam(isim: string)</code> guarantee?
- The isim parameter is optional
- The function returns a string
- The function is constant
- Only string values can be passed as isim
Show answer

Only string values can be passed as isim — <code>isim: string</code> means the parameter accepts only strings; passing a number or boolean causes a compile error.

Add the parameter type

function kare(kenar: ____) { return kenar * kenar; }

Show answer

function kare(kenar: number) { return kenar * kenar; }

Parameter annotation concepts

function f(a: number)

Parameter type declaration
isim: string

String parameter
yas: number

Number parameter
Parametre annotation

Giving a type to each parameter

Next lesson: Return type annotation

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