Function parameters are the names listed in a function's definition, allowing you to pass data into the function. They act as placeholders for values that will be provided when the function is called.

Basic Syntax

In JavaScript, you define function parameters within parentheses after the function name:

function functionName(parameter1, parameter2) {
    // Function body
}

Using Parameters

Parameters enable functions to work with different inputs. Here's a simple example:

function greet(name) {
    console.log("Hello, " + name + "!");
}

greet("Alice"); // Output: Hello, Alice!
greet("Bob");   // Output: Hello, Bob!

Default Parameters

JavaScript allows you to set default values for parameters. These values are used when an argument is not provided or is undefined.

function greetWithDefault(name = "Guest") {
    console.log("Welcome, " + name + "!");
}

greetWithDefault();        // Output: Welcome, Guest!
greetWithDefault("Alice"); // Output: Welcome, Alice!

Rest Parameters

The rest parameter syntax allows a function to accept an indefinite number of arguments as an array. It's denoted by three dots (...) followed by the parameter name:

function sum(...numbers) {
    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

Parameter Destructuring

You can use destructuring in function parameters to extract values from objects or arrays passed as arguments:

function printUserInfo({ name, age }) {
    console.log(`Name: ${name}, Age: ${age}`);
}

const user = { name: "Alice", age: 30, country: "USA" };
printUserInfo(user); // Output: Name: Alice, Age: 30

Best Practices

Use descriptive parameter names to improve code readability.
Limit the number of parameters to maintain function simplicity.
Consider using object parameters for functions with many options.
Validate parameters to ensure your function receives the expected data types.

Related Concepts

To deepen your understanding of JavaScript functions, explore these related topics:

By mastering function parameters, you'll be able to create more flexible and reusable JavaScript code. Practice with different parameter types and experiment with various function designs to enhance your programming skills.