JavaScript Variables: let vs const vs var

JavaScript Simplified: A Beginner's Guide to Mastering Interactive Web Development

Post 4 of 30: Understanding Variables in JavaScript

In this post, we’ll explore what variables are, how to declare them, and how they work in JavaScript.

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What is a Variable?

A variable is a way to store a value in JavaScript so we can use it later. Think of a variable like a labeled box where you can keep things and retrieve them when needed.

For example, instead of writing "John" multiple times in your code, you can store it in a variable and use it anywhere.

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Declaring Variables in JavaScript

JavaScript provides three ways to declare variables:

• var (older method, not recommended)
• let (modern and recommended for changeable values)
• const (for values that should not change)

1. Using let (Recommended)

let name = "John";
console.log(name);

Output:

John

Here, we:

• Created a variable called name
• Assigned it the value "John"
• Used console.log() to print the value of name

2. Using const (For Constant Values)

const PI = 3.1416;
console.log(PI);

Output:

3.1416

• const is used for values that should not change.
• Once assigned, you cannot reassign a new value to PI.

3. Using var (Older Method – Avoid Using)

var age = 25;
console.log(age);

Output:

25

• var was commonly used before let and const, but it has scoping issues, so use let or const instead.

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Changing Variable Values

With let, you can change a variable’s value, but with const, you cannot.

Example with let:

let city = "New York";
console.log(city); // Output: New York

city = "Los Angeles"; // Changing the value
console.log(city); // Output: Los Angeles

Example with const (This will cause an error)

const country = "USA";
console.log(country);

country = "Canada"; // ❌ This will cause an error
console.log(country);

Error: Uncaught TypeError: Assignment to constant variable.

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Variable Naming Rules

When naming variables, follow these rules:

✔️ Can contain letters, numbers, $, and _

✔️ Must start with a letter, $, or _ (not a number)

✔️ Case-sensitive (name and Name are different)

✔️ Cannot be a reserved keyword (like let, console, function, etc.)

Examples of Valid Variable Names:

let name = "John";
console.log(name);

Examples of Invalid Variable Names:

John

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Practical Exercise: Storing and Changing Values

Try this in your script.js file:

const PI = 3.1416;
console.log(PI);

Expected Output:

3.1416

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Next Steps

Now that we understand how variables work, the next step is to explore data types in JavaScript—including numbers, strings, booleans, and more!

Stay tuned for the next post! ?

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Pro Tip:

? Use let when you expect the value to change.

? Use const when the value should stay the same.

? Avoid var unless you specifically need it.

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