LET VS VAR VS CONST – KEY DIFFERENCES IN JAVASCRIPT WITH EXAMPLES

Let vs Var vs Const – Key Differences in JavaScript with Examples

Let vs Var vs Const – Key Differences in JavaScript with Examples

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Understanding the differences between let, var, and const is crucial for any JavaScript developer. These three keywords are used to declare variables, but each behaves differently in terms of scope, hoisting, and reassignability. If you’re learning JavaScript or transitioning from older code bases, knowing when and why to use let, var, or const will help you write cleaner, more predictable, and efficient code.


In this detailed guide, we’ll explore the key differences between let vs var vs const in JavaScript, backed with simple explanations and practical examples. Whether you're a beginner trying to avoid common mistakes or an experienced developer brushing up your skills, this article will help you understand the most important concepts.






What Is var in JavaScript?


The var keyword is the original way to declare variables in JavaScript. It has function scope, meaning it is accessible throughout the entire function in which it is declared. However, var comes with hoisting behavior, where declarations are moved to the top of their scope before code execution. This can lead to unexpected bugs, especially in larger codebases.



Example:



javascript






console.log(x); // undefined var x = 5; console.log(x); // 5


Even though x is logged before being declared, JavaScript doesn’t throw an error. Instead, it hoists the declaration (not the initialization), which can confuse new developers.






What Is let in JavaScript?


Introduced in ES6, the let keyword allows you to declare variables with block scope. Unlike var, let is not hoisted in the same way and cannot be accessed before it's declared in the block. It is ideal for cases where the variable value is expected to change and should be limited to a specific block.



Example:



javascript






if (true) { let y = 10; console.log(y); // 10 } // console.log(y); // ReferenceError: y is not defined


This shows that let confines the variable to the block it was declared in, making the code more modular and less prone to side effects.






What Is const in JavaScript?


Also introduced in ES6, const is used to declare block-scoped constants. Once a const variable is assigned a value, it cannot be reassigned. However, if the value is an object or an array, the contents can still be modified.



Example:



javascript






const z = 20; // z = 30; // TypeError: Assignment to constant variable const obj = { name: "John" }; obj.name = "Jane"; // This is allowed


Use const when you want to protect your variable from reassignment, which leads to fewer bugs and better code safety.






Let vs Var vs Const – The Key Differences













































Feature var let const
Scope Function-scoped Block-scoped Block-scoped
Hoisting Yes (initializes to undefined) No (temporal dead zone) No (temporal dead zone)
Reassignment Yes Yes No
Redeclaration Yes No No
Use Case Legacy code, function-wide scope Loops, block-limited logic Constants, fixed values







When to Use var, let, or const?




  • Use var: Only when working with older JavaScript code or specific scenarios where function-level scope is required (rare today).




  • Use let: When the variable’s value will change, such as in loops, conditionals, and intermediate computations.




  • Use const: As your default choice. It ensures variables are not reassigned accidentally and promotes better coding practices.







Best Practices




  1. Prefer const by default: Use const unless you know the variable’s value needs to change.




  2. Use let for mutable values: Use let only when reassignment is necessary.




  3. Avoid var: Its function scope and hoisting behavior can cause unexpected issues, making your code less predictable.







Final Thoughts


Understanding the distinctions between let, var, and const helps you write more robust, secure, and readable JavaScript code. While all three can be used to declare variables, choosing the right one is about controlling the scope and preventing bugs.


By mastering let vs var vs const in JavaScript, you’ll be able to write modern ES6+ code confidently. Always aim for clarity, maintainability, and best practices when choosing your variable declarations. Stick with const when possible, use let when needed, and avoid var unless absolutely necessary for backward compatibility.

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