JavaScript Variables: Understanding and Usage with Examples

Variables are fundamental to programming as they allow us to store and manipulate data. In JavaScript, variables play a crucial role in storing values of different data types. Let's explore JavaScript variables and see how they are used with examples.

Declaring Variables

In JavaScript, we can declare variables using the 'var', 'let', or 'const' keywords. Here's an example of declaring a variable using 'var':

var age = 25;

In this example, we declare a variable named 'age' and assign it the value of 25. The 'var' keyword is used for declaring variables that have function scope or global scope.

With the introduction of ES6, the 'let' and 'const' keywords were introduced. 'let' is used to declare variables with block scope, while 'const' is used to declare constants that cannot be reassigned. Here's an example using 'let':

let name = "John Doe";

In this case, we declare a variable named 'name' and assign it the value "John Doe". The variable 'name' is accessible within the block where it is declared.

Using Variables

Once we have declared a variable, we can use it in our JavaScript code. Variables can store various data types, such as numbers, strings, booleans, arrays, and objects. Here are a few examples:

var age = 25;
let name = "John Doe";
const PI = 3.14;

var isStudent = true;
var grades = [90, 85, 95];
var person = { firstName: "John", lastName: "Doe" };

In the examples above, 'age' stores a number, 'name' stores a string, 'PI' stores a constant value, 'isStudent' stores a boolean, 'grades' stores an array, and 'person' stores an object.

Modifying Variables

Variables in JavaScript can be modified by assigning new values to them. Let's see an example:

var counter = 0;
counter = counter + 1;
console.log(counter); // Output: 1

In this example, we initialize 'counter' with the value 0. Then we modify the 'counter' variable by adding 1 to its current value. The updated value of 'counter' is then printed to the console.

Variable Scope

JavaScript variables have different scopes based on how and where they are declared. Variables declared with 'var' have function scope or global scope, while variables declared with 'let' or 'const' have block scope.

function sayHello() {
  var message = "Hello!";
  console.log(message);
}

sayHello(); // Output: Hello!
console.log(message); // Error: message is not defined
  

    In this example, the 'message' variable is declared within the 'sayHello()' function. It is accessible only within that function. Trying to access it outside the function will result in an error.
  
  

    Understanding variables and their scopes is essential for writing clean and maintainable JavaScript code.