In JavaScript, there are different ways to declare variables. Two modern keywords you'll use are 

. Think of these as different types of wires:

 creates a wire that can be reconnected to different values later.

 creates a wire that is fixed to one value and cannot be changed once set.

, it is an older way to declare variables and has quirks that can lead to errors. While it's good to know that 

 exists since you might encounter it, it's best to use let and const.

, you can change (reconnect) the value later. For example:

, once you assign a value, the wire remains fixed—it cannot be reconnected. For example:

This is useful for values that should remain constant throughout your program.

 exists but is less predictable due to reasons not important right now. Suffice it to say that 

 is legacy and modern JavaScript code favors 

Create a variable called temperature using 

Print the final value using console.log(temperature).

index.js

index.test.js

Reconnect the Wire with let

Keeping the Wire Fixed with const

## Let and Const

In JavaScript, there are different ways to declare variables. Two modern keywords you'll use are **let** and **const**. Think of these as different types of wires:

- **let** creates a wire that can be reconnected to different values later.
- **const** creates a wire that is fixed to one value and cannot be changed once set.

While you might also encounter **var**, it is an older way to declare variables and has quirks that can lead to errors. While it's good to know that **var** exists since you might encounter it, it's best to use let and const.

---

## Using let

When you use **let**, you can change (reconnect) the value later. For example:

```javascript
let score = 10;
score = 15;
console.log(score); // prints 15
```

@embed ee1ed11a13bb

---

## Using const

With **const**, once you assign a value, the wire remains fixed—it cannot be reconnected. For example:

```javascript
const birthYear = 1990;
// Attempting to reassign will cause an error:
// birthYear = 2000;
console.log(birthYear); // prints 1990
```

This is useful for values that should remain constant throughout your program.

@embed c8fdb0627ef7

---

## When to Use let vs. const

- Use **let** when you know the value might change.
- Use **const** when the value should stay the same.
- **var** exists but is less predictable due to reasons not important right now. Suffice it to say that **var** is legacy and modern JavaScript code favors **let** and **const**.

---

## Exercises

@exercise 0bd2113e6ed8

@exercise bec2b70c0d46


- Luo muuttuja nimeltä temperature käyttäen **let**-avainsanaa ja aseta sen arvoksi 25.
- Vaihda sen arvoksi 30.
- Tulosta lopullinen arvo käyttämällä console.log(temperature).

Kytke vaijeri uudelleen käyttämällä letiä

- Luo muuttuja nimeltään **pi** käyttäen **const** ja aseta sen arvoksi 3.14.
- Tulosta muuttujan **pi** arvo käyttäen console.log(pi).

Luo kiinteä vaijeri constilla

Let vs. Const: Declaring Variables in JavaScript

Learn programming with JavaScript, arguably the most important and versatile programming language in today's world, at least for web development.

Welcome to Hakatemia’s beginner-friendly JavaScript course, where you will learn the fundamentals of programming with JavaScript. In today’s world, JavaScript is one of the most popular and versatile programming languages—especially in web development, where it holds a unique position: no other programming language can run both in the browser and on the server. For this reason, JavaScript has become extremely popular and is used for developing user interfaces (React, Vue, Angular, Svelte, etc.), back-end systems (NodeJS/Express, HAPI, etc.), and full-stack solutions (NextJS, NuxtJS, etc.).

These JavaScript frameworks will be covered in later Hakatemia courses, but before we get there, we must first master the basics of JavaScript programming. This course is designed precisely for that purpose.

Throughout the course, you’ll learn core programming concepts such as variables, operators, conditional statements, functions, arrays, and objects. Each module is divided into small, manageable segments with plenty of practical exercises.

This course is being released as a beta—the first part of Hakatemia’s comprehensive web development curriculum—and it is the first course that uses Hakatemia’s new programming exercise environment. Please provide feedback using the button at the bottom of the page or on Discord if you encounter any issues!

Let vs. Const: Declaring Variables in JavaScript

Let and Const

Learn to hack — start here