JavaScript Lesson 9: Objects

Objects store key-value pairs and are the most fundamental data structure in JavaScript. Everything in JS is an object or acts like one.

Creating Objects

const person = {
  name: "Alice",
  age: 30,
  city: "London",
  isStudent: false
};

// Access values
console.log(person.name);      // Alice (dot notation)
console.log(person["age"]);    // 30 (bracket notation)

// Bracket notation for dynamic keys
const key = "city";
console.log(person[key]);      // London

Modifying Objects

const car = { brand: "Toyota", year: 2020 };

// Add a property
car.color = "blue";

// Update a property
car.year = 2024;

// Delete a property
delete car.color;

// Check if property exists
console.log("brand" in car);  // true
console.log("color" in car);  // false

Object Methods

const player = {
  name: "Alice",
  score: 0,
  level: 1,
  
  addPoints(points) {
    this.score += points;
  },
  
  levelUp() {
    this.level++;
    console.log(`${this.name} is now level ${this.level}!`);
  },
  
  getInfo() {
    return `${this.name} | Score: ${this.score} | Level: ${this.level}`;
  }
};

player.addPoints(100);
player.levelUp();
console.log(player.getInfo());
// Alice is now level 2!
// Alice | Score: 100 | Level: 2

Object Utilities

const scores = { alice: 95, bob: 87, charlie: 92 };

// Get all keys
Object.keys(scores)    // ["alice", "bob", "charlie"]

// Get all values
Object.values(scores)  // [95, 87, 92]

// Get key-value pairs
Object.entries(scores) // [["alice",95], ["bob",87], ...]

// Merge objects
const defaults = { theme: "dark", lang: "en" };
const user = { lang: "fr", name: "Alice" };
const merged = { ...defaults, ...user };
// { theme: "dark", lang: "fr", name: "Alice" }