TypeScript Rest Parameters

Note: If you're new to TypeScript, check our Getting Started with TypeScript tutorial first.

In TypeScript, rest parameters allow a function to accept an indefinite number of arguments as a single array.

They are indicated by the ... symbol before the parameter name.

function addNumbers(...numbers: number[]): number{
    let sum: number = 0;
    for (let num of numbers) {
        sum += num;
    }
    return sum;
}

console.log(addNumbers(5, 10, 15)); // Output: 30
console.log(addNumbers(5, 10));  // Output: 15
console.log(addNumbers(5));  // Output: 5
console.log(addNumbers());  // Output: 0

Syntax for Defining a Rest Parameter

You can create a rest parameter using the ... symbol before the parameter name.

Since rest parameters are arrays, you also need to use the [] symbol when specifying its type.

Here's the syntax for defining a rest parameter in your function:

function functionName(...restParameterName: type[]) {
    // Function body
}

Here,

• ... - Indicates that we're defining a rest parameter.
• restParameterName - The name of the rest parameter.
• type[] - The type of the array that constitutes the rest parameter.

Example: TypeScript Rest Parameters

// Function with rest parameter 'deposits'
function studentInfo(name: string, ...marks: number[]): void {
 
    console.log(`Student Name: ${name}`);
    console.log("Marks Obtained:");

    // Print the rest parameter
    for (let mark of marks) {
        console.log(mark);
    }
    console.log();
}

// Pass a single argument to the rest parameter
studentInfo("Joe Smith", 100);

// Pass two arguments to the rest parameter
studentInfo("Jane Doe", 85, 92);

Output

Student Name: Joe Smith
Marks Obtained:
100

Student Name: Jane Doe
Marks Obtained:
85
92

Here's how the program works:

1. When studentInfo("Joe Smith", 100) is called.

• name is "Joe Smith".
• The marks[] array (i.e., the rest parameter) has only one element: 100.

2. When studentInfo("Jane Doe", 85, 92) is called.

• name is "Jane Doe".
• The marks[] array has two elements: 85 and 92.

// Function with rest parameter that can take number or string arguments
// The function returns a tuple of number and string

function addConcat(...args: (number | string)[]): [number, string] {
    let sum: number = 0;
    let message: string = "";
  
    for (let arg of args) {
        if (typeof arg === "number") {
            sum += arg;
        }
        else {
            message += arg;
        }
    }
  
    return [sum, message];
}

// Call the function
// Pass numbers and strings in random order
let [total, text]: [number, string] = addConcat(12, 2, 3, "Hasta ", 1, "la vista");

console.log(total);  // 18
console.log(text);  // Hasta la vista

Also Read:

• TypeScript Function and Function Expressions
• TypeScript Default Parameters
• TypeScript Optional Parameters