Altcademy - a Forbes magazine logo Best Coding Bootcamp 2023

What is let in JavaScript

Understanding JavaScript Variables: A Deep Dive into 'let'

In this blog post, we're going to explore one of the most fundamental concepts of JavaScript - the 'let' keyword. If you're just starting out in JavaScript and programming in general, don't worry, we'll break this down into bite-sized, digestible pieces.

What is 'let'?

Let's start with the basics. In JavaScript, as in many other programming languages, we use variables to store data. Think of variables as little boxes where you can keep things for later use. Now, 'let' is one of the keywords you can use to create these boxes. For example:

let myAge = 25;

In this line of code, we've created a variable called 'myAge' and we've stored the number 25 in it.

'let' versus 'var' and 'const'

Before we go further into 'let', it's important to know that JavaScript has two other keywords to declare variables - 'var' and 'const'. So, what's the difference between them? Well, 'let' and 'const' were introduced in a later version of JavaScript (ES6) to address some of the problems with 'var'.

The main differences are related to scope and reassignment.

What is Scope?

In simple terms, scope is where a variable is available for use in your code. Imagine you're in a big library. Not all books are accessible from all places in the library. Some books are in the science section, others are in the fiction section. In the same way, not all variables are accessible from all parts of the code. This is what we call variable scope.

Variables declared with 'var' are either globally scoped (accessible from everywhere) or function scoped (accessible only within a function). On the other hand, 'let' is block scoped. A block is basically the area within curly brackets {}.

Let's look at an example:

if (true) {
  let name = 'Alice';
  console.log(name); // 'Alice'
console.log(name); // Error: name is not defined

In this example, 'name' is only accessible within the if block. Once we're out of the block, 'name' is no longer defined.

Can 'let' Variables be Reassigned?

Yes, unlike 'const' variables, 'let' variables can be reassigned. That means you can change what's inside the box. Here's an example:

let weather = 'sunny';
console.log(weather); // 'sunny'

weather = 'rainy';
console.log(weather); // 'rainy'

In this example, we first assign the value 'sunny' to the variable 'weather'. Later in the code, we change this value to 'rainy'. With 'let', that's perfectly fine. However, if we tried to do this with 'const', JavaScript would give us an error.

Conclusion: Embrace the Flexibility of 'let'

Think of 'let' as your flexible friend in JavaScript. It's like a reusable grocery bag. You can put items in it, take them out, and put new items in. It's also respectful of boundaries - it won't let variables leak out of blocks and cause confusion in your code.

But remember, with great power comes great responsibility. The flexibility of 'let' means you have to keep track of your variable assignments and be careful not to unintentionally change values. So, use it wisely and 'let' it help you write cleaner, more efficient code.

Next time, when you see 'let' in a piece of JavaScript code or when you use it yourself, you'll know exactly what it's doing and why it's there. Happy coding!