LET AND VAR CONST DIFFERENCE – A CLEAR GUIDE TO JAVASCRIPT VARIABLE DECLARATIONS

Let and Var Const Difference – A Clear Guide to JavaScript Variable Declarations

Let and Var Const Difference – A Clear Guide to JavaScript Variable Declarations

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Let and Var Const Difference – A Clear Guide to JavaScript Variable Declarations

Introduction
In JavaScript, declaring variables correctly is vital for writing efficient, bug-free code. The three main keywords used for variable declaration are var, let, and const. Each behaves differently in terms of scope, hoisting, and mutability. Understanding these differences is essential for any JavaScript developer.

1. var

  • Scope: Function-scoped

  • Hoisting: Variables are hoisted and initialized with undefined

  • Re-declaration: Allowed within the same scope

  • Use Case: Legacy code and functions

Example:

javascript
function example() { console.log(x); // undefined var x = 10; console.log(x); // 10 }

2. let

  • Scope: Block-scoped (within {})

  • Hoisting: Hoisted but not initialized (Temporal Dead Zone)

  • Re-declaration: Not allowed in the same scope

  • Use Case: For variables whose values can change

Example:

javascript
{ let y = 20; console.log(y); // 20 } // console.log(y); // Error: y is not defined

3. const

  • Scope: Block-scoped

  • Hoisting: Hoisted but not initialized (Temporal Dead Zone)

  • Re-declaration: Not allowed

  • Re-assignment: Not allowed after initial assignment

  • Use Case: For constants or variables that should not change

Example:

javascript
const z = 30; z = 40; // Error: Assignment to constant variable.

Summary Table

Feature var let const
Scope Function Block Block
Hoisting Yes (initialized to undefined) Yes (Temporal Dead Zone) Yes (Temporal Dead Zone)
Re-declaration Allowed Not allowed Not allowed
Re-assignment Allowed Allowed Not allowed

Best Practices

  • Prefer const by default to prevent accidental changes.

  • Use let when you know the variable’s value will change.

  • Avoid var to prevent bugs related to scoping and hoisting.

Conclusion

Knowing the differences between var, let, and const helps you write more reliable and maintainable JavaScript code. Use block-scoped declarations to keep variables confined to their intended context and reduce bugs.

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