Understanding var, let, and const in JavaScript: Differences and Best Practices

One of the most fundamental yet frequently misunderstood topics in JavaScript is the difference between var, let, and const. While they all declare variables, their behaviors differ in ways that can significantly affect the clarity and reliability of your code. In this post, we’ll break down their key differences, explore when you might intentionally use var, and offer best practices for modern JavaScript development.

🔍 Key Differences at a Glance

🧠 Understanding Each Declaration

var – Function Scope & Hoisting

function testVar() {
  console.log(a); // undefined (hoisted)
  var a = 5;
}

• Declared variables are hoisted to the top of their function and initialized as undefined.
• Scope is limited to the function, not to {} blocks.

let – Block Scope & Safer Hoisting

function testLet() {
  // console.log(b); // ReferenceError (TDZ)
  let b = 10;
}

• Variables are block-scoped (e.g., inside if, for, while blocks).
• Not accessible before declaration due to the temporal dead zone (TDZ).

const – Immutable Bindings

const c = 20;
c = 30; // ❌ TypeError

• Similar scope and hoisting as let.
• Must be initialized during declaration.
• Note: You can still mutate objects or arrays declared with const:

const user = { name: "Dan" };
user.name = "Daniel"; // ✅ allowed

🤔 When Would You Intentionally Use var?

Although var has mostly been replaced by let and const, there are still a few use cases:

✅ Maintaining Legacy Code

Older JavaScript codebases often use var. When modifying or extending such code, sticking with var may help maintain consistency.

✅ Function-Level Hoisting

In rare cases, you might want a variable that exists throughout the entire function regardless of block scope.

function hoistMe() {
  hoisted(); // works due to hoisting

  function hoisted() {
    console.log("I’m hoisted!");
  }
}

✅ Global Variables (Not Recommended)

Using var in the global scope adds the variable to the window object in browsers:

var globalVar = "I’m global";
console.log(window.globalVar); // "I’m global"

This is generally discouraged in modern development.

✅ Best Practices

• Use const by default. It makes your code safer by preventing reassignment.
• Use let when you need to reassign values, such as in loops or conditional logic.
• Avoid var unless you have a specific reason, like supporting legacy environments or working in function scope on purpose.

📌 Summary

Modern JavaScript encourages clean, predictable, and block-scoped variable declarations. Understanding the nuances of var, let, and const is essential for writing robust and maintainable code. Stick to const and let in your everyday work, and use var sparingly when it truly serves a purpose.

Up next: We’ll explore how let and const behave in loops, and how closures interact with variable declarations. Stay tuned!