JavaScript Variables: var, let, const & More

Table of Contents

What is a Variable?

A variable is a named container for storing data in memory, which can be accessed and modified later. It acts as a reference to a value.

Variable  javascript
var a = 10; // value 10 is stored in a

Why variables are used?

Store Data : Hold values like numbers, strings, objects, etc
Reusability : Use the same value multiple times.
Dynamic Manipulation : Change values during program execution.

Variable declaration

There are generally 4 ways to decalre a variable

var keyword. example : var num = 7;
let keyword. example : let num = 7;
const keyword. example : const num = 7;
without_any_keyword. example : num = 7;

var keyword

var keyword can be used to declare or initialize a variable. Variable created with var keyword are

Function-scoped : Limited to the function in which it is declared

Code  javascript
function test() {
    var c = 10;
    console.log(c); // Output: 10
}
console.log(c); // ReferenceError (c is function-scoped)

Hoisted with undefined

Code  javascript
console.log(a) // Hoisted with undefined Output: undefined

var a // variable declaration using var 
a = 2 // initializing value

Can be reassigned & redeclared

Code  javascript
var a // variable declaration using var 
a = 2 // initializing value
console.log(a) //Output: 2

var b = 7 // variable declaration & initialization 
var b = 3 // redeclaration+initialization 
b = 9 // reinitialization
console.log(b)  // Output: 9

let keyword

let keyword can be used to declare or initialize a variable within a block scope. Variable created with let keyword are

Hoisted but not initialized (in Temporal Dead Zone)
- TDZ is the phase between variable creation and initialization where accessing the variable results in a ReferenceError. This applies to let and const but not var.
  Code javascript
```
console.log(a) // TDZ Output: ReferenceError: a is not defined 
let a // variable declaration using let 
```
- During memory allocation (hoisting phase), let and const variables are placed in a separate "uninitialized" memory space.
- If accessed before initialization, JavaScript throws a ReferenceError.

can be reassigned but cann't redeclared

Code  javascript
let a // variable declaration using let 
a = 2 // initializing value
a = 5 // reinitializing value
console.log(a) //Output: 5

let b = 7 // variable declaration & initialization 
let b = 3 // Error : The symbol "b" has already been declared

const keyword

It is used to create constant(Immutable) variables.It is same as let but must be initialized and cannot be reassigned.

Code  javascript
const a = 5 // assignment
console.log(a) // Output: 5

a=8 // Error: Cannot assign to "a" because it is a constant
const a = 7 // Error: The symbol "a" has already been declared

Variable without any keywords

It is strongly recommonded to use a keyword while creating a variable, but if you donn't use any then JavaScript implicitly declares the variable in the global scope (even inside functions).

Code  javascript
function test() {
    x = 7; // varable initialization
}
test();
console.log(x); // Output: 7 (x becomes a global variable)

It doesn't get hoisted (assigned at runtime). In strict mode it throws ReferenceError

Best Practices for Variables

Use const by default unless you need to reassign the variable.
Use let if reassignment is needed but avoid var.
Avoid using variables without declaration keywords to prevent accidental global scope leaks.

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