Functions, Parameters, and Return Values

Functions are reusable blocks of code designed to perform a specific task. They prevent code repetition and keep your projects modular and maintainable.

1. Defining Functions

In JavaScript, there are two primary ways to define a standard function: Function Declarations and Function Expressions.

A. Function Declarations

Defined using the function keyword followed by the function name.

function greet(name) {
  return `Hello, ${name}!`;
}
console.log(greet("Alice")); // "Hello, Alice!"

Function declarations are hoisted, meaning you can call them before they are declared in the code.

B. Function Expressions

Defined by assigning a function (often anonymous) to a variable.

const add = function(a, b) {
  return a + b;
};
console.log(add(5, 3)); // 8

Function expressions are not hoisted; you cannot call them before the line of declaration.

2. Parameters vs. Arguments

• Parameters: The variable placeholders listed in the function definition.
• Arguments: The real values passed to the function when it is called.

// 'x' and 'y' are parameters
function multiply(x, y) {
  return x * y;
}

// 4 and 5 are arguments
multiply(4, 5); 

3. Parameter Defaults and Rest Parameters

A. Default Parameters (ES6)

You can set fallback values for parameters in case they are missing or passed as undefined:

function welcomeUser(username = "Guest") {
  console.log(`Welcome back, ${username}!`);
}
welcomeUser();         // "Welcome back, Guest!"
welcomeUser("Alice");  // "Welcome back, Alice!"

B. Rest Parameters (...)

If you don't know how many arguments a function will receive, you can pack them into a single array parameter using the Rest operator (...):

function sumAll(...numbers) {
  // 'numbers' is a real array
  return numbers.reduce((sum, num) => sum + num, 0);
}
console.log(sumAll(1, 2, 3, 4)); // 10

Note: The rest parameter must always be the last parameter in the list.

4. Return Values

A function execution stops as soon as it hits the return statement. The value after the return keyword is sent back to the caller.

function checkAge(age) {
  if (age < 18) {
    return "Minor";
  }
  return "Adult"; // Only runs if age >= 18
}

If a function does not contain a return statement, it returns undefined by default.

In the next guide, we will learn about Arrow Functions and how they handle the this context differently.