What is the difference between let and var in JavaScript?

Table of Contents

• Introduction
• Key Differences
  • 1. Scope
  • 2. Hoisting
  • 3. Redeclaration
• Conclusion

Introduction

In JavaScript, let and var are both used to declare variables, but they have distinct differences that affect their behavior in terms of scope, hoisting, and redeclaration. Understanding these differences is essential for writing clean and efficient code.

Key Differences

1. Scope

• var: Variables declared with var have a function scope or global scope, meaning they are accessible anywhere within the function or globally if declared outside any function.

• let: Variables declared with let have a block scope, meaning they are only accessible within the block (enclosed by {}) in which they are declared.

2. Hoisting

Both var and let are hoisted to the top of their containing scope, but their behavior differs:

• var: The variable is hoisted and initialized with undefined. You can access it before its declaration without an error, although its value will be undefined.

• let: The variable is hoisted but remains uninitialized, leading to a "temporal dead zone." Accessing it before its declaration results in a ReferenceError.

3. Redeclaration

• var: You can redeclare a variable declared with var within the same scope without any errors.

• let: Redeclaring a variable with let in the same scope results in a SyntaxError.

Conclusion

The differences between let and var in JavaScript significantly impact variable behavior, especially concerning scope, hoisting, and redeclaration rules. Using let is generally recommended for modern JavaScript development due to its block scope and prevention of unintended redeclaration, leading to clearer and more predictable code. Understanding these differences is essential for writing effective JavaScript programs.