Variables & Data Types

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1. Variable Declaration

        
let name = 'John';      // Can be reassigned

const PI = 3.14159;     // Cannot be reassigned

var oldWay = 'avoid';   // Old way, don't use

Click "Run Demo" to see output...

2. Data Types

        
// Primitive types

let str = 'Hello';       // String

let num = 42;            // Number

let bool = true;         // Boolean

let nothing = null;      // Null

let notDefined;          // Undefined

Click "Run Demo" to see output...

3. Template Literals

        
let firstName = 'John';

let lastName = 'Doe';

let age = 25;

// Old way (concatenation)

'Hello, ' + firstName + ' ' + lastName

// New way (template literals)

`Hello, ${firstName} ${lastName}! You are ${age} years old.`

Click "Run Demo" to see output...

4. Type Checking

        
typeof 'hello'      // "string"

typeof 42           // "number"

typeof true         // "boolean"

typeof undefined    // "undefined"

typeof null         // "object" (JS quirk!)

typeof []           // "object"

typeof {}           // "object"

Click "Run Demo" to see output...

5. Arrays

        
let fruits = ['apple', 'banana', 'orange'];

let numbers = [1, 2, 3, 4, 5];

fruits[0]        // 'apple' (first element)

fruits.length    // 3

fruits.push('mango')  // Add to end

Click "Run Demo" to see output...

6. Objects

        
let person = {

    name: 'John',

    age: 25,

    isStudent: true,

    greet: function() {

        return `Hello, I'm ${this.name}`;

    }

};

Click "Run Demo" to see output...

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