Let vs Const in JavaScript: Key Differences and Best Practices for ES6+

Learn the key differences between let and const in JavaScript. This beginner-friendly guide explains ES6+ variable declarations, best practices, and how to write cleaner, modern JavaScript code.

Introduction

With the introduction of ES6 (ECMAScript 2015), JavaScript gained new ways to declare variables: let and const. These modern declarations replaced the older var keyword in most cases, giving developers more predictable and reliable behavior when managing variables.

Understanding the differences between let and const is important for writing clean, modern JavaScript code. Although they may have similar appearances, their actions regarding reassignment and usage make them quite distinct.

What You’ll Learn

• What let and const mean in JavaScript.
• The main differences between let and const.
• When to use let vs const.
• Common mistakes beginners make with let and const.
• Best practices for using these ES6+ features in your projects.

Understanding let in JavaScript

Let’s start with let. Declaring variables that can be reallocated later is possible with the let keyword. Since it is block-scoped, it can only be found inside the block {} in which it is defined.

Example:

let count = 1;
count = 2; // Works fine, reassignment allowed

In comparison to the older var, let prevents issues such as leaking variables outside of their block.

Now that you understand let, let’s move to const, which behaves differently.

Understanding const in JavaScript

In JavaScript, const is used to declare variables whose values cannot change, and it shares the same block scope behavior as let.

Example:

const name = "Wisdom";
name = "Udo"; // Error: Assignment to constant variable

It’s important to note that const makes the binding constant, not the value itself. If an object is a variable or array, the contents can still be modified.

Example:

const numbers = [1, 2, 3];
numbers.push(4); // Works, array contents can change

With both letand constexplained, let’s look at them side by side, observing their differences.

Key Differences Between let and const

Here’s a quick comparison of how let and const differ:

• Reassignment: You can reassign values with let, but const does not allow reassignment.
• Scope: let and const apply only within their block, but var works within the whole function.
• Initialization: const must be assigned a value when declared, while let can be declared without an initial value.

By understanding these differences, you’ll know which one to apply in various contexts. Let's talk about excellent practices now.

Best Practices for Using let and const

When working on modern JavaScript projects, here are some best practices to follow:

• Use const by default. It makes your code easier to read and prevents unintended changes.
• Use let only when reassignment is necessary. For example, in loops or counters.
• Avoid var unless working with older codebases.
• Always initialize const variables when you declare them.

By applying these practices, your code will be cleaner, more secure, and easier to debug.

Conclusion

The arrival of let and const in ES6 made JavaScript easier and safer to use. With let, you can change the value of a variable later, but with const, the value stays the same once you set it. Both only work within the block of code where you write them, which helps avoid mistakes that can occur with var.

Most of the time, it’s best to use const so your code is steady and easy to follow. Use let only when you know the value will change. By doing this, you’ll be writing clean contemporary JavaScript with ease.

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