Variables and Data Types in JavaScript
In JavaScript, variables are used to store data values. JavaScript is dynamically typed, meaning you don't need to declare the type of a variable explicitly. Variables can hold different types of data, such as numbers, strings, booleans, objects, and more.
Declaring Variables
Variables in JavaScript can be declared using let, const, or var.
let age = 25; // Mutable variable
const name = "Alice"; // Immutable variable
var isStudent = true; // Older way (avoid in modern code)
Common Data Types
- Number: Represents numeric values (e.g.,
10,3.14). - String: Represents text data (e.g.,
"Hello",'JavaScript'). - Boolean: Represents true or false values.
- Object: Represents key-value pairs (e.g.,
{ name: "Alice", age: 25 }). - Array: Represents an ordered collection of elements (e.g.,
[1, 2, 3]). - Undefined: Represents a variable that has not been assigned a value.
- Null: Represents an intentional absence of value.
Example:
let age = 25;
const name = "Alice";
let isStudent = true;
let scores = [90, 85, 95];
let person = { name: "Alice", age: 25 };
console.log("Name:", name);
console.log("Age:", age);
console.log("Is Student:", isStudent);
Typeof Operator
The typeof operator is used to determine the type of a variable.
console.log(typeof age); // Output: number
console.log(typeof name); // Output: string
console.log(typeof isStudent); // Output: boolean
console.log(typeof scores); // Output: object
console.log(typeof person); // Output: object
console.log(typeof undefinedVar); // Output: undefined
Type Conversion
JavaScript automatically converts types in certain situations, but you can also explicitly convert types.
// Implicit Type Conversion
let num = "10";
let result = num + 5; // Output: "105" (string concatenation)
// Explicit Type Conversion
let numAsNumber = Number(num); // Convert string to number
let resultAsNumber = numAsNumber + 5; // Output: 15
Template Literals
Template literals allow you to embed expressions inside strings using backticks (`).
let greeting = `Hello, my name is ${name} and I am ${age} years old.`;
console.log(greeting); // Output: Hello, my name is Alice and I am 25 years old.
Constants and Immutability
Variables declared with const cannot be reassigned, but their properties can be modified if they are objects or arrays.
const person = { name: "Alice", age: 25 };
person.age = 26; // Allowed
// person = { name: "Bob", age: 30 }; // Error: Assignment to constant variable