Declaring a Function

A function is defined using the function keyword, followed by a name, parentheses (), and a block of code {}.

function greet() {
    console.log("Hello, welcome to JavaScript!");
  }

To execute a function, it must be called by its name with parentheses:

greet(); // Outputs: Hello, welcome to JavaScript!

Function Parameters and Arguments

Functions can accept parameters (input values) and use them within their block.

function greetUser(name) {
    console.log("Hello, " + name + "!");
  }
  
greetUser("Alice"); // Outputs: Hello, Alice!

• Parameters are placeholders in the function definition.

• Arguments are actual values passed to the function when called.

A function can have multiple parameters:

function addNumbers(a, b) {
    console.log(a + b);
  }
  
addNumbers(5, 10); // Outputs: 15

Return Statement

A function can return a value using the return keyword.

function multiply(x, y) {
    return x * y;
  }
  
let result = multiply(4, 5);
console.log(result); // Outputs: 20

• Once return is executed, the function stops running.

• Returned values can be stored in variables for later use.

Function Expressions

Functions can be stored in variables as expressions.

const square = function(num) {
    return num * num;
  };
  
console.log(square(6)); // Outputs: 36

• Function expressions do not have a name (anonymous functions).

• The function is called using the variable name.

Arrow Functions (ES6)

Arrow functions provide a shorter syntax for writing functions.

const multiply = (a, b) => a * b;
console.log(multiply(3, 4)); // Outputs: 12

• If the function has only one statement, {} and return can be omitted.

• If there is only one parameter, () can be omitted:

const greet = name => console.log("Hello, " + name);
greet("John"); // Outputs: Hello, John

Default Parameters

Functions can have default parameter values that apply when no argument is provided.

function greet(name = "Guest") {
    console.log("Hello, " + name);
}
  
greet(); // Outputs: Hello, Guest
greet("Alice"); // Outputs: Hello, Alice

Rest Parameters (...)

The rest parameter syntax allows a function to accept multiple arguments as an array.

function sum(...numbers) {
  let total = 0;
    for (let num of numbers) {
        total += num;
    }
    return total;
  }
  
console.log(sum(1, 2, 3, 4, 5)); // Outputs: 15

• The ...numbers parameter gathers all arguments into an array.

• Useful when the number of arguments is unknown.

Callback Functions

A callback is a function passed as an argument to another function and executed later.

function processUser(name, callback) {
    console.log("Processing user: " + name);
    callback();
  }
  
function done() {
      console.log("User processed successfully.");
  }

processUser("Alice", done);

• processUser calls done after printing a message.

• Callbacks enable asynchronous programming and event handling.

Immediately Invoked Function Expressions (IIFE)

An IIFE runs immediately after it is defined.

(function() {
    console.log("This function runs immediately!");
})();

• Wrapped in parentheses to execute instantly.

• Avoids polluting the global scope.

Conclusion

Functions are a fundamental part of JavaScript, allowing code reuse and modularity. The next section will explore objects and arrays, which are essential for organizing and managing data.