What Are Functions?

A function is a reusable block of code that performs a specific task.

Instead of writing the same code multiple times, you can place it inside a function and call it whenever you need it.

Think of a function like a machine:

Input → Function → Output

Example: adding two numbers.

function add(a, b) {
  return a + b;
}

console.log(add(3, 5));

Output:

8

Here:

• add is the function name

• a and b are parameters

• The function returns the result

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Function Declaration

A function declaration defines a function using the function keyword followed by a name.

Syntax

function functionName(parameters) {
  // code to execute
}

Example

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

greet("Rahul");

Output:

Hello Rahul

In this example:

• greet is the function

• "Rahul" is passed as an argument

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Function Expression

A function expression stores a function inside a variable.

Syntax

const functionName = function(parameters) {
  // code
};

Example

const greet = function(name) {
  console.log("Hello " + name);
};

greet("Riya");

Output:

Hello Riya

Here:

• The function does not need a name

• It is stored inside the variable greet

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Declaration vs Expression (Side-by-Side)

Function Declaration

function add(a, b) {
  return a + b;
}

Function Expression

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

Both do the same job, but they behave differently in certain situations.

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Understanding Hoisting (Simple Explanation)

Hoisting means JavaScript moves certain declarations to the top of the code before execution.

This affects function declarations and function expressions differently.

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Function Declaration and Hoisting

You can call the function before it is defined.

Example:

sayHello();

function sayHello() {
  console.log("Hello!");
}

This works because function declarations are hoisted.

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Function Expression and Hoisting

Function expressions cannot be used before they are defined.

Example:

sayHello();

const sayHello = function() {
  console.log("Hello!");
};

This will cause an error because the variable exists but the function is not assigned yet.

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Comparison Table

Feature	Function Declaration	Function Expression
Syntax	function name(){}	const name = function(){}
Hoisting	✅ Fully hoisted	❌ Not hoisted
Use before definition	✅ Yes	❌ No
Stored in variable	❌ No	✅ Yes
Flexibility	Basic	More flexible

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When Should You Use Each?

Use Function Declarations When:

• You want a simple reusable function

• You want to call the function before its definition

• Writing basic program logic

Example:

function calculateTotal(price, tax) {
  return price + tax;
}

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Use Function Expressions When:

• You want to store functions in variables

• You want to pass functions as arguments

• Working with callbacks or advanced patterns

Example:

const calculateTotal = function(price, tax) {
  return price + tax;
};

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Simple Execution Flow of a Function Call

Program Starts
      ↓
Function is Defined
      ↓
Function is Called
      ↓
Code Inside Function Runs
      ↓
Result is Returned

Example:

function multiply(a, b) {
  return a * b;
}

console.log(multiply(4, 5));

Output:

20

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Practice Assignment 🧠

Try implementing the following yourself.

1️⃣ Write a Function Declaration

Create a function that multiplies two numbers.

function multiply(a, b) {
  return a * b;
}

console.log(multiply(4, 3));

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2️⃣ Write the Same Logic Using Function Expression

const multiply = function(a, b) {
  return a * b;
};

console.log(multiply(4, 3));

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3️⃣ Try Calling Functions Before Definition

Experiment with this:

console.log(add(2, 3));

function add(a, b) {
  return a + b;
}

Now try this with a function expression:

console.log(add(2, 3));

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

Observe the difference and understand how hoisting works.