Variables and Data Types

Understanding variables and data types is fundamental to programming. Variables are containers for storing data values, and data types define the kind of value a variable can hold and the operations that can be performed on it.

Variables

In programming, a variable is a symbolic name given to a memory location where a value can be stored. You can think of it like a labeled box where you can put information.

Declaring and Initializing Variables

Before you can use a variable, you typically need to declare it. Initialization is the process of assigning an initial value to a variable.

// Declaring a variable named 'userName'
let userName;

// Initializing the variable with a value
userName = "Alice";

// Declaring and initializing in one step
const userAge = 30;
var score = 100; // Older way of declaring variables, generally prefer let/const
            

Variable Naming Rules

• Variable names must start with a letter, an underscore (_), or a dollar sign ($).
• Variable names cannot start with a number.
• Variable names can only contain alphanumeric characters and underscores (A-z, 0-9, and _).
• Variable names are case-sensitive (userName and username are different variables).
• Reserved words (like if, for, function) cannot be used as variable names.

Data Types

Data types specify the type of data that a variable can hold. Different programming languages have different sets of data types, but most common ones include:

Primitive Data Types

These are the most basic data types:

1. String

Represents a sequence of characters, typically enclosed in quotes.

let greeting = "Hello, world!";
let message = 'This is a message.';
            

2. Number

Represents numeric values. This includes integers (whole numbers) and floating-point numbers (numbers with decimal points).

let count = 42;
let price = 19.99;
let temperature = -5;
            

3. Boolean

Represents a logical value, which can be either true or false.

let isActive = true;
let isComplete = false;
            

4. Undefined

A variable that has been declared but not yet assigned a value has the type undefined.

let notAssigned; // notAssigned is undefined
            

5. Null

Represents the intentional absence of any object value. It is an assigned value, unlike undefined.

let emptyValue = null;
            

6. Symbol (ES6+)

A unique and immutable primitive value, often used as identifiers for object properties.

const id = Symbol('uniqueId');
            

7. BigInt (ES11+)

For representing integers of arbitrary precision, larger than the maximum safe integer representable by the Number type.

const bigNumber = 1234567890123456789012345678901234567890n;
            

Composite (or Reference) Data Types

These are more complex types that can hold collections of data:

1. Object

A collection of key-value pairs. Keys are typically strings, and values can be any data type, including other objects.

let person = {
    firstName: "John",
    lastName: "Doe",
    age: 25
};
            

2. Array

An ordered list of values. Elements are accessed by their index (starting from 0).

let colors = ["red", "green", "blue"];
let numbers = [1, 2, 3, 4, 5];
            

Type Coercion

Type coercion is the process of automatically converting one data type to another. This can happen implicitly during operations.

For example:

let num = 5;
let str = "10";
let result = num + str; // Coerced to string: "510"

let anotherResult = num * str; // Coerced to number: 50
            

Checking Data Types

You can use the typeof operator to determine the data type of a variable.

let myVar = "Hello";
console.log(typeof myVar); // Output: "string"

let myNum = 123;
console.log(typeof myNum); // Output: "number"

let myBool = false;
console.log(typeof myBool); // Output: "boolean"

let myObj = { a: 1 };
console.log(typeof myObj); // Output: "object"

let myArray = [1, 2];
console.log(typeof myArray); // Output: "object" (Note: typeof Array is 'object')

console.log(typeof null); // Output: "object" (A known quirk in JavaScript)
            

To specifically check if something is an array, you can use Array.isArray().

console.log(Array.isArray(myArray)); // Output: true