What Are Functions?

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

Instead of writing the same logic multiple times, we place that logic inside a function and call it whenever we need it.

Example Without a Function

let result1 = 5 + 3;
let result2 = 10 + 7;
let result3 = 20 + 4;

We repeat the same logic multiple times.

Example With a Function

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

console.log(add(5, 3));
console.log(add(10, 7));
console.log(add(20, 4));

Now the logic is written once and reused multiple times.

Functions help us:

• Reuse code

• Make programs easier to read

• Organize logic better

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2. Function Declaration

A function declaration defines a function using the function keyword and a name.

Syntax

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

Example

function greet(name) {
  return "Hello " + name;
}

console.log(greet("Rahul"));

Here:

• greet is the function name

• name is the parameter

• The function returns a greeting message

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3. Function Expression

A function expression stores a function inside a variable.

Syntax

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

Example

let greet = function(name) {
  return "Hello " + name;
};

console.log(greet("Rahul"));

Here the function does not have to be named because it is stored inside the variable greet.

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4. Declaration vs Expression (Side by Side)

Function Declaration

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

Function Expression

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

Both perform the same task, but they behave differently in some situations.

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5. Basic Idea of Hoisting

In JavaScript, function declarations are hoisted.

This means they can be called before they appear in the code.

Works with Function Declaration

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

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

This works because JavaScript moves the function declaration to the top during execution.

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Does NOT Work with Function Expression

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

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

This causes an error because the variable add is not initialized yet.

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6. Key Differences

Feature	Function Declaration	Function Expression
Syntax	Uses function name()	Stored in a variable
Hoisting	Fully hoisted	Not usable before definition
Use case	General reusable functions	Often used inside variables, callbacks

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7. When to Use Each

Use Function Declaration

• When defining main reusable functions

• When you want the function available everywhere in the file

Use Function Expression

• When assigning functions to variables

• When using functions as arguments

• When working with callbacks

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8. Practice Assignment

Try the following exercise.

1. Function Declaration

Write a function that multiplies two numbers.

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

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

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2. Function Expression

Write the same logic using a function expression.

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

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

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3. Experiment with Hoisting

Try calling the functions before defining them and observe what happens.