Rest params
- TypeScript course
- Beginner
- Lesson 44/89
- 4 min
Learn to take variable arguments with rest parameters.
Rest params
A rest parameter collects a variable number of arguments into a single array. Write ... before the parameter name. The rest parameter must be an array type and must always be the last parameter.
function topla(...sayilar: number[]): number {
let t = 0;
for (let s of sayilar) { t += s; }
return t;
}With ... you can pass as many arguments as you want.
Rest parameter rules
A rest parameter must always be the last parameter in the function; it comes after other parameters. You can pass zero or more arguments. Inside the function, the rest parameter is used like a regular array.
function listele(baslik: string, ...ogeler: string[]): void {
console.log(baslik);
for (let o of ogeler) { console.log(o); }
}Rest parameters let you write functions that accept a flexible number of arguments.
Practice Write a function with a rest parameter
Define a function named topla with a ...sayilar: number[] rest parameter and return type number. The function should sum all numbers with a loop and return the result. Print the result of calling topla(1, 2, 3, 4) with console.log.
Starter code
// topla adında bir fonksiyon tanımla
// ...sayilar rest parametresi number[], dönüş türü number olsun
// tüm sayıları döngü ile topla
// topla(1, 2, 3, 4) sonucunu yazdırSample solution
function topla(...sayilar: number[]): number {
let t = 0;
for (let s of sayilar) {
t += s;
}
return t;
}
console.log(topla(1, 2, 3, 4));Expected output
10
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 What does <code>...sayilar: number[]</code> do in a function parameter?
- Gives the parameter a default value
- Makes the parameter optional
- Collects multiple number arguments into an array Correct answer
- Makes the parameter readonly
Explanation: The <code>...</code> rest operator collects all passed number arguments into a single array named <code>sayilar</code>; this lets the function accept a variable number of arguments.
2 Complete the rest parameter syntax
function topla(____) { let t = 0; for (let s of sayilar) { t += s; } return t; }function topla(...sayilar: number[]) { let t = 0; for (let s of sayilar) { t += s; } return t; }Explanation: For a rest parameter, the <code>...</code> operator is written before the parameter name; <code>sayilar: number[]</code> without the mark expects a single array argument, and <code>*</code> or trailing <code>...</code> are invalid syntax.
3 Rest params concepts
| Term | Meaning |
|---|---|
...sayilar: number[] |
Rest parameter (variable arguments) |
... (parametre önünde) |
Collects arguments into an array |
Rest param |
Must be the last parameter |
topla(1, 2, 3) |
Three arguments collected by rest parameter |