In programming, we often repeat the same logic multiple times.

For example, imagine calculating the sum of two numbers in different places in a program.

Without functions:

let result1 = 10 + 5;
let result2 = 20 + 8;
let result3 = 7 + 3;

This approach works, but it quickly becomes repetitive.

JavaScript solves this problem using functions.

Functions allow us to write reusable blocks of code that can be executed whenever needed.

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What is a Function?

A function is a reusable block of code designed to perform a specific task.

Example:

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

Here:

• add is the function name

• a and b are parameters

• the function returns the sum

Calling the function:

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

Output

8

Instead of repeating logic, we can call the function multiple times.

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Why Do We Need Functions?

Functions help us:

• avoid repeating code

• organize programs better

• make code easier to read

• reuse logic in multiple places

Example:

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

greet("Gagan");
greet("Rahul");
greet("Amit");

Output

Hello Gagan
Hello Rahul
Hello Amit

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Function Declaration

A function declaration defines a function using the function keyword.

Syntax

function functionName(parameters) {
  // code
}

Example

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

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

Output

20

Function declarations are the most common way beginners start writing functions.

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Function Expression

A function expression stores a function inside a variable.

Syntax

let variableName = function(parameters) {
  // code
};

Example

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

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

Output

20

The function works the same way, but it is stored inside a variable.

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Declaration vs Expression

Feature	Function Declaration	Function Expression
Syntax	Uses function name()	Stored in a variable
Hoisting	Can be called before definition	Cannot be called before definition
Usage	Common for general functions	Useful when functions are treated like values

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Example Comparison

Function declaration:

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

greet();

Function expression:

let greet = function() {
  console.log("Hello");
};

greet();

Both produce the same result.

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Understanding Hoisting (High Level)

In JavaScript, some declarations are moved to the top of their scope during execution.

This behavior is called hoisting.

Because of hoisting, function declarations can be called before they appear in the code.

Example:

sayHello();

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

This works even though the function appears later.

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Function Expression and Hoisting

Function expressions behave differently.

Example:

sayHello();

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

This will produce an error because the function is not available before the variable assignment.

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Simple Function Execution Flow

4

Execution steps:

1. Function is defined

2. Function is called

3. Parameters receive values

4. Code inside the function runs

5. Result is returned

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When to Use Each Type

Use function declarations when:

• writing general reusable functions

• defining main program logic

• organizing code clearly

Use function expressions when:

• assigning functions to variables

• passing functions as values

• creating functions inside other functions

For beginners, function declarations are usually easier to understand.

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Practical Example

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

console.log(add(10, 20));

Output

30

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Practice Assignment

1 Write a Function Declaration

Create a function that multiplies two numbers.

Example:

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

console.log(multiply(4, 6));

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2 Write the Same Logic Using Function Expression

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

console.log(multiply(4, 6));

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3 Call the Functions Before Defining

Try this with the declaration:

multiply(3,4);

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

Now try the same with function expression and observe the behavior.