Lesson 6

Constants

Constants are variables whose values cannot be changed once assigned.

Constants are declared using the const keyword. Once a value is assigned to a constant, it cannot be reassigned.

1const PI = 3.14159;2const MAX_USERS = 100;3const APP_NAME = "Master DSA";

Must be initialized at declaration.
Value cannot be reassigned.
Use UPPER_SNAKE_CASE for constants.
Improves code reliability and readability.

```
const PI = 3.14159;
```
```
const MAX_LIMIT = 1000;
```
```
const APP_NAME = "Master DSA";
```
```
const isLoggedIn = true;
```

Use meaningful names in UPPER_SNAKE_CASE for values that don’t change.

```
PI = 3.14159;
```
// Missing const
```
const MAX = 10;MAX = 20;
```
// Reassignment

const userName = "Vivek";userName = "Kumar";

// Reassignment

Mathematical values
like PI, E, GRAVITY
Configuration values
like API_URL, TIMEOUT
Limits and sizes
like MAX_USERS, MAX_FILE_SIZE

Modify the code and click Run to see the result.

JavaScript

1// Declare some constants
2const PI = 3.14159;
3const MAX_USERS = 100;
4const APP_NAME = "Master DSA";
5
6// Try changing the value of PI below
7// PI = 3.14;  // Uncomment and see the error
8
9console.log(PI);
10console.log(MAX_USERS);
11console.log(APP_NAME);

Output

Your output will appear here...

Use const by default. Only use let when you know the value will change later.

const

let