Rest Parameters in TypeScript (original) (raw)

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[]). JavaScript `

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

TypeScript `

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

TypeScript `

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

TypeScript ``

// 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.