Function Declaration vs Function Expression: What’s the Difference?

When writing programs, we often need to repeat the same logic multiple times. Instead of rewriting the same code again and again, we can place that logic inside a function.

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

Think of a function like a small machine:

• You give it some inputs
• It processes them
• It gives you an output Why Functions Are Useful

Functions help us:

• Reuse code
• Organize programs into smaller pieces
• Make code easier to read
• Avoid repetition

For example, imagine we want to add two numbers many times in our program.

Without a function:

console.log(2 + 3)
console.log(10 + 5)
console.log(7 + 8)

With a function:

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

console.log(add(2, 3))
console.log(add(10, 5))
console.log(add(7, 8))

Function Declaration Syntax

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

function functionName(parameters) {
  // code to run
}
function greet(name) {
  return "Hello " + name
}

console.log(greet("Alice"))

Here:

• greet is the function name
• name is a parameter
• The function returns a greeting message

*Function Expression Syntax
*
A function expression stores a function inside a variable.

const variableName = function(parameters) {
  // code
}

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

console.log(greet("Alice"))

This works the same as a function declaration, but the function is assigned to a variable.

*What is Hoisting? *

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

Function Declarations Are Hoisted

sayHello()

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

Even though the function is written after the call, JavaScript already knows about it.

Function Expressions Are NOT Hoisted
This will cause error:

sayHello()

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

Because the variable sayHello does not exist yet when the function is called.
When Should You Use Each Type?
Use Function Declarations When:

1. The function should be available anywhere in the file
2. You want simpler syntax
3. The function represents a core part of your program

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

Use Function Expressions When:

1. You want to store functions in variables
2. You want more control over when functions are created
3. You are writing modern JavaScript patterns

Example:

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