TypeScript Rest Parameters

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[]) {
   //...
}

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;
}

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

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' }

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.