Rest Parameters in TypeScript

Q: How do you declare rest parameters in TypeScript?

Declared using ... before the parameter name, e.g., function example(...params: number[]).

Q: How do I declare a rest parameter?

Use the ellipsis (...) followed by the parameter name in the function definition. For example: function example(...args: number[]) {}.

Last Updated : 22 Jan, 2025

Rest parameters in TypeScript enable functions to handle an unlimited number of arguments by grouping them into an array. They are defined using … and must be the last parameter.

Allow flexible and dynamic input handling in functions.
Simplify working with multiple arguments without specifying them individually.

Syntax

function function_name(...rest: type[]) {
  // Type of the is the type of the array. 
}

Parameters:

functionName: The name of your function.
…rest: The rest parameter that collects all additional arguments into an array.
type[]: Specifies the type of elements in the rest array (e.g., number[], string[]).

function sum(...numbers: number[]): number {
  return numbers.reduce((total, num) => total + num, 0);
}

console.log(sum(1, 2, 3));
console.log(sum(10, 20));

The sum function uses the rest parameter …numbers to collect all arguments passed to it into an array of type number[].
The reduce method is applied to the numbers array, adding all its elements together to compute the total.

Output:

6
30

Calculating the Average of Numbers

function average(...numbers: number[]): number {
    let total = 0;
    for (let num of numbers) {
        total += num;
    }
    return numbers.length === 0 ? 0 : total / numbers.length;
}

console.log("Average of the given numbers is:", average(10, 20, 30, 60));
console.log("Average of the given numbers is:", average(5, 6));
console.log("Average of the given numbers is:", average(4));

The average function uses a rest parameter …numbers to accept any number of numeric arguments.
It calculates the total sum of these numbers and returns their average.

Output:

Average of the given numbers is : 30
Average of the given numbers is : 5.5
Average of the given numbers is : 4

Concatenating Strings

function joinStrings(...strings: string[]): string {
    return strings.join(', ');
}

console.log(joinStrings("rachel", "john", "peter") + " are mathematicians");
console.log(joinStrings("sarah", "joseph") + " are coders");

The joinStrings function accepts multiple string arguments using a rest parameter.
It concatenates them into a single string, separated by commas.

Output:

rachel, john, peter are mathematicians
sarah, joseph are coders

Incorrect Usage of Rest Parameters

// Incorrect usage - will raise a compiler error
function job(...people: string[], jobTitle: string): void {
  console.log(`${people.join(', ')} are ${jobTitle}`);
}

// Uncommenting the below line will cause a compiler error
// job("rachel", "john", "peter", "mathematicians");

In this example, the rest parameter …people is not placed at the end of the parameter list.
TypeScript requires rest parameters to be the last parameter; otherwise, a compiler error occurs.

Output: Typescript compiler raised the error.

main.ts(2,14): error TS1014: A rest parameter must be last in a parameter list.

Best Practices for Using TypeScript Rest Parameters

Place Rest Parameters Last: Always define rest parameters at the end of the parameter list to ensure correct function behavior.
Use Appropriate Types: Specify the correct array type for rest parameters to maintain type safety and code clarity.
Limit to One Rest Parameter: A function should have only one rest parameter to avoid complexity and potential errors.
Avoid Overuse: Use rest parameters judiciously; overuse can lead to code that is hard to understand and maintain.