Rest parameters are a powerful feature in TypeScript that allow functions to accept an indefinite number of arguments as an array. They provide a clean and flexible way to handle variable-length argument lists.

Syntax and Usage

To declare a rest parameter, use the ellipsis (...) followed by the parameter name. This parameter must be the last in the function's parameter list.

function exampleFunction(...args: type[]): returnType {
    // Function body
}

The rest parameter is treated as an array inside the function, allowing you to use array methods and access individual elements.

Benefits of Rest Parameters

Simplify function declarations when the number of arguments is unknown
Eliminate the need for the arguments object
Provide type safety for variable-length argument lists
Improve code readability and maintainability

Examples

1. Sum Function

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

console.log(sum(1, 2, 3)); // Output: 6
console.log(sum(4, 5, 6, 7)); // Output: 22

In this example, the sum function can accept any number of arguments, which are then summed up using the reduce method.

2. Logging Function

function log(message: string, ...details: any[]): void {
    console.log(message);
    if (details.length > 0) {
        console.log("Additional details:", details);
    }
}

log("User logged in"); // Output: User logged in
log("Error occurred", "File not found", 404); // Output: Error occurred \n Additional details: ["File not found", 404]

This logging function demonstrates how rest parameters can be used alongside regular parameters. It allows for flexible logging with optional additional details.

Best Practices

Use rest parameters instead of the arguments object for better type checking and readability
Consider using Union Types with rest parameters for more specific type definitions
Combine rest parameters with Function Overloading for advanced function signatures
Be cautious with performance when dealing with very large argument lists

Related Concepts

To further enhance your understanding of TypeScript functions, explore these related topics:

By mastering rest parameters, you'll gain more flexibility in function design and improve your overall TypeScript coding skills.