If you've ever heard people talking about "declaring," "defining," or "initializing" variables in JavaScript and felt confused — you’re not alone! These words sound similar, but they mean different things.

🟢 Declaration

When you declare a variable, you're telling JavaScript to create a placeholder for it in memory. However, at this point, it doesn’t yet hold a value.

let a;
var b;

Here, a and b are declared but not given any values yet.

🟡 Initialization

When you initialize a variable, you assign it a value for the first time.

let a;
a = 10; // Initialization

You can also declare and initialize at the same time:

let x = 5; // Declaration + initialization

For const, you must initialize immediately:

const y = 20; // Declaration + initialization required

🔵 Definition

In JavaScript, the term definition is sometimes used informally to describe the combination of declaration and initialization together:

let name = "JavaScript"; // Declared and initialized (defined)

For functions, "definition" means providing both the name and its body (implementation):

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

⚖️ Quick Recap Table

Term	Meaning	Example
Declaration	Reserve a name in memory	`let a;`
Initialization	Assign first value	`a = 10;`
Definition	Declare and assign together	`let b = 5;`

💬 Conclusion

Declare first, initialize later — possible with var and let.
Must initialize immediately with const.

🔥 Declaring vs Defining vs Initializing Variables in JavaScript