Introduction

In JavaScript, variables are fundamental building blocks that allow you to store and manipulate data throughout your code. Whether you're tracking user input, managing state, or simply holding a value to use later, variables are indispensable in any JavaScript application. As JavaScript has evolved, so too have the ways in which we define these variables.

Today, there are three primary ways to declare a variable in JavaScript: var, let, and const. Each of these keywords offers distinct behaviors, and understanding when to use each one is crucial for writing clean, efficient, and bug-free code.

In this blog, we'll explore the differences between javascript var vs let vs const, compare their usage, and provide practical code examples to illustrate the best practices for each. By the end, you'll have a clear understanding of how to choose the right variable declaration for your needs, helping you write better JavaScript code.

Understanding var

var was the original way to declare variables in JavaScript and was a staple for many years. However, as JavaScript evolved, the limitations and issues with var led to the introduction of let and const in ES6 (ECMAScript 2015).
A key characteristic of var is that it is function-scoped, meaning it’s only accessible within the function where it’s declared. If declared outside a function, it becomes a global variable. This function scope differs from the block-scoping provided by let and const.

Another important feature of var is hoisting, where variable declarations are moved to the top of their scope during execution. This allows you to reference a var variable before it is declared, but its value will be undefined until the assignment happens. While hoisting can be convenient, it often leads to confusion and subtle bugs, especially in larger codebases.

Example of Hoisting:

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

In this example, even though x is logged before it's declared, the code doesn't throw an error. Instead, it outputs undefined due to hoisting. JavaScript treats the code as if the var x declaration was moved to the top of its scope.

Issues with var: Accidental Globals and Re-declaration

One of the common pitfalls with var is the accidental creation of global variables. If you forget to use var in a function, JavaScript will create a global variable, which can lead to unexpected behavior.

function setValue() {
    value = 10; // Accidentally creates a global variable
}
setValue();
console.log(value); // Outputs: 10 (global variable created unintentionally)

Another issue is that var allows re-declaration within the same scope, which can lead to hard-to-track bugs:

var x = 10;
var x = 20;
console.log(x); // Outputs: 20

Here, the variable x is re-declared and assigned a new value, potentially overwriting the previous value without any warning.

When to Use var

In modern JavaScript let vs var vs const , var is generally discouraged in favor of let and const, which offer better scoping and prevent many common issues. However, var might still be applicable in legacy codebases where refactoring is not an option, or in certain scenarios where function-level scoping is explicitly desired.

Understanding let

let is a block-scoped variable declaration introduced in ES6 (ECMAScript 2015). Unlike var, which is function-scoped, let is confined to the block in which it is defined, such as within a loop or an if statement. This block scoping helps prevent errors and makes your code more predictable by limiting the variable's accessibility to the specific block where it is needed.

The main difference between function scope and block scope is that function-scoped variables (var) are accessible throughout the entire function in which they are declared, while block-scoped variables (let) are only accessible within the specific block, such as a loop or a conditional statement, where they are defined. This behavior of let can help avoid issues that arise from variables being unintentionally accessible outside their intended scope.

Example of let in a Loop:

for (let i = 0; i < 3; i++) {
    console.log(i); // Outputs 0, 1, 2
}
console.log(i); // ReferenceError: i is not defined

In this example, i is only accessible within the loop due to block scoping.

Comparison with var:

if (true) {
    var x = 10;
    let y = 20;
}
console.log(x); // Outputs 10 (function-scoped)
console.log(y); // ReferenceError: y is not defined (block-scoped)

Here, x is accessible outside the if block due to var's function scope, while y is not accessible outside the block due to let's block scope.

Understanding const

const is another block-scoped variable declaration introduced in ES6, similar to let. However, const is used to declare variables that are intended to remain constant throughout the program. The key difference between const and let is immutability: once a const variable is assigned a value, it cannot be reassigned. This makes const ideal for values that should not change, ensuring that your code is more predictable and less prone to errors.

However, it’s important to understand that const enforces immutability on the variable binding, not the value itself. This means that while you cannot reassign a const variable, if the value is an object or array, the contents of that object or array can still be modified.

Example with Primitive Values

const myNumber = 10;
myNumber = 20; // Error: Assignment to constant variable.

In this example, trying to reassign the value of myNumber results in an error because const does not allow reassignment.

Example with Objects/Arrays

const myArray = [1, 2, 3];
myArray.push(4); // Allowed
console.log(myArray); // Output: [1, 2, 3, 4]

const myObject = { name: "John" };
myObject.name = "Doe"; // Allowed
console.log(myObject); // Output: { name: "Doe" }

Here, even though the myArray and myObject variables are declared with const, their contents can be modified. The const keyword only ensures that the variable itself cannot be reassigned, not that the data inside the object or array is immutable.

When to Use const

Best practices in modern JavaScript suggest using const by default for most variables. This approach helps prevent unintended variable reassignment and makes your code more reliable. You should only use let when you know that a variable's value will need to be reassigned. By adhering to this principle, you can reduce bugs and improve the overall quality of your code.

Comparing var, let, and const

Key Differences:

Code Examples

Example of Scope:

function scopeTest() {
    if (true) {
        var a = 1;
        let b = 2;
        const c = 3;
    }
    console.log(a); // Outputs 1 (function-scoped)
    console.log(b); // ReferenceError: b is not defined (block-scoped)
    console.log(c); // ReferenceError: c is not defined (block-scoped)
}
scopeTest();

In this example, var is function-scoped, so a is accessible outside the if block. However, let and const are block-scoped, so b and c are not accessible outside the block they were defined in.

Example of Hoisting:

console.log(varVar); // Outputs undefined
console.log(letVar); // ReferenceError: Cannot access 'letVar' before initialization
console.log(constVar); // ReferenceError: Cannot access 'constVar' before initialization

var varVar = "var";
let letVar = "let";
const constVar = "const";

Here, var is hoisted and initialized as undefined, so it can be referenced before its declaration without causing an error. However, let and const are hoisted but not initialized, resulting in a ReferenceError if accessed before their declarations.

Example of Re-declaration

var x = 10;
var x = 20; // No error, x is now 20

let y = 10;
let y = 20; // Error: Identifier 'y' has already been declared

const z = 10;
const z = 20; // Error: Identifier 'z' has already been declared

With var, re-declaring the same variable is allowed, and the value is updated. However, let and const do not allow re-declaration within the same scope, leading to an error if you try to do so.

Example of Immutability:

const myArray = [1, 2, 3];
myArray.push(4); // Allowed
console.log(myArray); // Output: [1, 2, 3, 4]

myArray = [4, 5, 6]; // Error: Assignment to constant variable

In this case, const prevents reassignment of the variable myArray, which would result in an error. However, the contents of the array can still be modified, such as adding a new element.

Best Practices

In modern JavaScript, the consensus among developers is to use const and let in place of var to ensure code that is more predictable, maintainable, and less prone to bugs. Here are some best practices to follow:

1. Use const by Default Whenever possible, use const to declare variables. Since const ensures that the variable cannot be reassigned, it makes your code easier to understand and prevents accidental modifications. By defaulting to const, you signal to other developers (and yourself) that the value should remain constant throughout the code's execution.
2. Use let Only When Reassignment is Necessary If you know that a variable's value will need to change, use let. let allows for reassignment while still providing the benefits of block-scoping, which helps avoid issues that can arise from variables leaking out of their intended scope.
3. Avoid var in Modern JavaScript In modern JavaScript, it’s best to avoid using var altogether. var's function-scoping, hoisting, and the ability to be redeclared can lead to unpredictable behavior, especially in larger codebases. The only time you might need to use var is when maintaining or working with legacy code that relies on it.

4. Sample Refactor: Converting var to let and const
   Here’s a simple example of refactoring older JavaScript code that uses var to a more modern approach with let and const.

   Before Refactoring:
   

   function calculateTotal(prices) {
   var total = 0;
   for (var i = 0; i < prices.length; i++) {
       var price = prices[i];
       total += price;
   }
   var discount = 0.1;
   var finalTotal = total - (total * discount);
   return finalTotal;
   }
   

   After Refactoring:
   

   function calculateTotal(prices) {
   let total = 0;
   for (let i = 0; i < prices.length; i++) {
       const price = prices[i]; // price doesn't change within the loop
       total += price;
   }
   const discount = 0.1; // discount remains constant
   const finalTotal = total - (total * discount); // finalTotal doesn't change after calculation
   return finalTotal;
   }
   

   In the refactored version, total is declared with let since its value changes throughout the function. price, discount, and finalTotal are declared with const because their values are not reassigned after their initial assignment. This refactoring makes the function more robust and easier to reason about, reducing the likelihood of accidental errors.

Common Pitfalls and How to Avoid Them

When working with var, let, and const, developers often encounter common pitfalls that can lead to bugs or unexpected behavior. Understanding these pitfalls and knowing how to avoid them is crucial for writing clean, reliable code.

Accidental Global Variables with var

One of the most common mistakes with var is accidentally creating global variables. This happens when a var declaration is omitted inside a function or block, causing the variable to be attached to the global object.

function calculate() {
    total = 100; // No var/let/const declaration, creates a global variable
}
calculate();
console.log(total); // Outputs 100, but total is now global!

How to Avoid:
Always use let or const to declare variables. This ensures that the variable is scoped to the block or function in which it is defined, preventing unintended global variables.

Hoisting Confusion with var

var is hoisted to the top of its scope, but only the declaration is hoisted, not the assignment. This can lead to confusing behavior if you try to use the variable before it is assigned.

console.log(name); // Outputs undefined
var name = "Alice";

How to Avoid:
Use let or const, which are also hoisted but not initialized. This prevents variables from being accessed before they are defined, reducing the chance of errors.

Re-declaration with var

var allows for re-declaration within the same scope, which can lead to unexpected overwrites and bugs, especially in larger functions.

var count = 10;
var count = 20; // No error, but original value is lost

How to Avoid:
Avoid using var. Use let or const instead, which do not allow re-declaration within the same scope. This ensures that variable names are unique and helps prevent accidental overwrites.

Misunderstanding const with Objects and Arrays

Many developers assume that const makes the entire object or array immutable, but in reality, it only prevents reassignment of the variable. The contents of the object or array can still be modified.

const person = { name: "Alice" };
person.name = "Bob"; // Allowed, object properties can be modified

person = { name: "Charlie" }; // Error: Assignment to constant variable

How to Avoid: Understand that const applies to the variable binding, not the value itself. If you need a truly immutable object or array, consider using methods like Object.freeze() or libraries that enforce immutability.

Scope Misconceptions with let and const

Developers may incorrectly assume that variables declared with let or const are accessible outside of the block they were defined in, similar to var.

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

Always be aware of the block scope when using let and const. If you need a variable to be accessible in a wider scope, declare it outside the block.

By understanding these common pitfalls and using var, let, and const appropriately, you can avoid many of the issues that commonly arise in JavaScript development. This leads to cleaner, more maintainable, and less error-prone code.

Conclusion

In this blog, we've explored the key differences between var, let, and const—the three primary ways to define variables in JavaScript. We've seen how var is function-scoped and hoisted, but its quirks can lead to unintended behavior. On the other hand, let and const, introduced in ES6, offer block-scoping and greater predictability, making them the preferred choices for modern JavaScript development.

For further reading and to deepen your understanding of JavaScript variables, check out the following resources:

MDN Web Docs: var

MDN Web Docs: let

MDN Web Docs: const

Understanding when and how to use var, let, and const is crucial for writing clean, efficient, and bug-free code. By defaulting to const, using let only when necessary, and avoiding var in new code, you can avoid many common pitfalls and improve the maintainability of your projects.