JavaScriptBasics

JavaScript Variables (var, let, const)

Learn JavaScript variables, how to declare them using var, let, and const, understand scope, redeclaration rules, and best practices used in real-world development.

What are Variables in JavaScript?Link to this section

Variables are used to store data values in JavaScript. These values can be numbers, text, objects, or any other type of data that a program needs to work with. A variable allows you to save a value in memory and reuse or modify it throughout your program.

Declaring Variables in JavaScriptLink to this section

JavaScript provides three ways to declare variables:
  • var
  • let
  • const
Each keyword behaves differently and is used in different situations.

Using var :Link to this section

The var keyword was the original way to declare variables in JavaScript.

Characteristics of var

- Function-scoped - Can be redeclared - Can be updated - Hoisted to the top of its scope

warning

Using var can lead to unexpected bugs due to its loose scoping behavior. It is not recommended in modern JavaScript development.

Using let :Link to this section

The let keyword was introduced in ES6 to solve problems associated with var.

Characteristics of let

- Block-scoped - Cannot be redeclared in the same scope - Can be updated - Safer and more predictable than var

tip

Use let when you know the variable value will change later.

Using const :Link to this section

The const keyword is used to declare variables whose values should not be reassigned.

Characteristics of const

- Block-scoped - Cannot be redeclared - Cannot be reassigned - Must be initialized at declaration

note

<b>const</b> does not make objects immutable. It only prevents reassignment of the variable reference.

Difference Between var, let, and const :Link to this section

Scope

- var → Function scope - let → Block scope - const → Block scope

Reassignment

- var → Allowed - let → Allowed - const → Not allowed

Redeclaration

- var → Allowed - let → Not allowed - const → Not allowed

Variable Naming RulesLink to this section

JavaScript variable names:
  • Must start with a letter, underscore (_), or dollar sign ($)
  • Cannot start with a number
  • Cannot use JavaScript keywords
  • Are case-sensitive

Best Practices for VariablesLink to this section

  • Prefer const by default
  • Use let only when reassignment is required
  • Avoid var in modern JavaScript
  • Use meaningful and descriptive variable names

    • tip

    Clean variable naming improves code readability and maintainability in large projects.
    Check Your Understanding
    Question 1 of 5

    Which keyword is block-scoped?

    Variable Declaration

    easy

    Declare a variable to store your name using const and another variable to store your age using let....

    Fix the Error

    easy

    Find and fix the issue in the following code:...

    const score; score = 100;

    Scope Understanding

    medium

    Write a small program where a variable declared using letis only accessible inside an if block....

    Naming Practice

    medium

    Create three variables with meaningful names to store:...

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