JavaScript Concept

Source: Dev.to

One of the first and most important concepts you encounter in JavaScript is variables. JavaScript provides three main ways to declare variables: var, let, and const. While they may look similar, they behave very differently.

Variable

A variable is like a container that holds a value. You give it a name so you can reference or update that value later in your code.

var – The Old Way

Key features of var

• Function scoped
• Can be redeclared
• Can be updated

Example

// Declaration
var age = 16;

// Updating
age = 18;

// Redeclaring (may overwrite values unexpectedly)
var age = 20; // <-- allowed, but can lead to bugs

let

Introduced to fix many of the problems caused by var.

Key features of let

• Block scoped
• Can be updated
• Cannot be redeclared in the same scope

Example

let score = 10;

// Updating
score = 15;

// Redeclaring in the same block causes an error
// let score = 20; // Uncaught SyntaxError: Identifier 'score' has already been declared

const

Used for fixed values that should never change.

Key features of const

• Block scoped
• Cannot be updated
• Cannot be redeclared
• Must be initialized immediately

Example

const country = "Nigeria";

// Attempting to reassign throws an error
// country = "Ghana"; // Uncaught TypeError: Assignment to constant variable.

Benefits of Using the Correct Variable Type

• Prevent bugs caused by accidental reassignments or redeclarations
• Make your code more readable and intention‑revealing
• Improve maintainability by clearly signaling mutability

Best Practice

• Use const by default – when a value should not change.
• Use let when you need to update a value.
• Avoid var in modern JavaScript unless you have a specific reason for its function‑scoped behavior.