Function declaration
- TypeScript course
- Beginner
- Lesson 42/89
- 4 min
Learn to define functions with types.
Function declaration
In TypeScript, a function is defined with parameter types and a return type. This provides type safety both at the call site and inside the function body.
function topla(a: number, b: number): number {
return a + b;
}Parameter types go inside the parentheses, the return type goes after them.
Calling a typed function
When calling a typed function, TypeScript checks that the arguments match the parameter types. A wrong type causes a compile error.
function topla(a: number, b: number): number {
return a + b;
}
topla(3, "metin"); // error: string not numberType checking prevents incorrect use of the function.
Practice Write a typed function
Define a function named topla with a and b parameters of type number and return type number. The function should return a + b. Print the result of calling topla(4, 5) with console.log.
Starter code
// topla adında bir fonksiyon tanımla
// a ve b parametreleri number, dönüş türü number olsun
// a + b değerini döndür
// topla(4, 5) sonucunu yazdırSample solution
function topla(a: number, b: number): number {
return a + b;
}
console.log(topla(4, 5));Expected output
9
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 In <code>function carp(x: number, y: number): number</code>, what does <code>: number</code> after the parameter list specify?
- The parameter type
- The function name
- A variable declaration
- The return type Correct answer
Explanation: The <code>: number</code> after the parameter list's closing parenthesis specifies the type of the value the function returns.
2 Add the return type
function kare(x: number): ____ { return x * x; }function kare(x: number): number { return x * x; }Explanation: <code>x * x</code> returns a number, so the return type must be <code>number</code>. <code>string</code>, <code>void</code> and <code>boolean</code> are wrong types for a number.
3 Function declaration concepts
| Term | Meaning |
|---|---|
function f(): void |
Function returning no value |
function f(a: number) |
Function with number parameter |
function f(): string |
Function returning a string |
Function declaration |
Typed function definition |