Summary: in this tutorial, you will learn about the TypeScript rest parameters and how to use them to represent an indefinite number of arguments as an array.
A rest parameter allows a function to accept zero or more arguments of the specified type. In TypeScript, the rest parameters follow these rules:
- A function has only one rest parameter.
- The rest parameter appear last in the parameter list.
- The type of the rest parameter is an array type.
Rest parameters with a single type
To declare a rest parameter, you prefix the parameter name with three dots (…) and use the array type as the type annotation:
function fn(...rest: type[]) {
//...
}
Code language: JavaScript (javascript)
The following example shows how to use the rest parameter:
function getTotal(...numbers: number[]): number {
let total = 0;
numbers.forEach((num) => total += num);
return total;
}
Code language: JavaScript (javascript)
In this example, the getTotal()
calculates the total of numbers passed into it.
Since the numbers parameter is a rest parameter, you can pass one or more numbers to calculate the total:
console.log(getTotal()); // 0
console.log(getTotal(10, 20)); // 30
console.log(getTotal(10, 20, 30)); // 60
Code language: JavaScript (javascript)
Rest parameters with multiple types
TypeScript allows you to handle rest parameters with multiple types using union types. For example:
function combine(...args: (number | string)[]): [number, string] {
let total = 0;
let str = '';
args.forEach((arg) => {
if (typeof arg === 'number') {
total += arg;
} else if (typeof arg === 'string') {
str += arg;
}
});
return [total, str];
}
const [total, str] = combine(3, 'Happy', 2, 1, ' New Year');
console.log({ total });
console.log({ str });
Output:
{ total: 6 }
{ str: 'Happy New Year' }
Code language: CSS (css)
In this example, we define a combine()
function that takes a variable number of arguments using rest parameters which can be either numbers or strings:
...args: (number | string)[]
The function does two things:
- Sum up all the numbers
- Concatenates all the strings.
Then, it returns a tuple containing the total sum of the numbers and the concatenated string.
Summary
- Use rest parameters to allow a function to accept a variable number of arguments with the same or different types.
- Use
...args type[]
syntax to define rest parameters with the same type. - Use
...args (type1 | type2 ) []
syntax to define rest parameters with different types.