A function is a block of code designed to perform a particular task. You can define a function once and call it multiple times.

1. Function Declaration

A function declaration defines a function with a specific name and can be called after it's defined.

// Function declaration
function greet(name) {
  console.log('Hello, ' + name);
}

// Calling the function
greet('Shubhadip');  // Output: Hello, Shubhadip

• The greet function takes a parameter name and logs a greeting message.
• Function declarations are hoisted, meaning they can be called before their definition.

2. Function Expression

A function expression assigns a function to a variable. The function can be anonymous (without a name) or named.

// Function expression
const add = function(a, b) {
  return a + b;
};

// Calling the function
console.log(add(2, 3));  // Output: 5

• The function is stored in the variable add.
• Function expressions are not hoisted, so they must be called after they are defined.

3. Arrow Function (ES6)

Arrow functions provide a shorter syntax for writing functions.

// Arrow function
const multiply = (a, b) => a * b;

// Calling the function
console.log(multiply(2, 3));  // Output: 6

• Arrow functions are often more concise, especially when there’s only one expression.
• They also don't have their own this context, so they behave differently in certain situations.

4. Function with Return Statement

A function can return a value that can be used later.

function square(x) {
  return x * x;
}

console.log(square(4));  // Output: 16

• The return keyword exits the function and returns a value.