Objects

Objects are collections of key-value pairs. They model real-world entities and are the foundation of almost everything in JavaScript — including arrays, functions, and the DOM.

Creating Objects

const person = {
  name: "Alice",
  age: 30,
  isEmployed: true,
};

Each key-value pair is called a property. Keys are strings (or Symbols), and values can be any type — including other objects or functions.

Accessing Properties

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

const key = "isEmployed";
console.log(person[key]);       // true   (dynamic key lookup)

Use dot notation by default. Use bracket notation when the key is stored in a variable or contains special characters.

Adding, Updating, and Deleting

const car = { make: "Toyota", model: "Corolla" };

car.year = 2020;           // add a new property
car.model = "Camry";       // update an existing property
delete car.year;            // remove a property

Methods

When a property's value is a function, it is called a method:

const calculator = {
  value: 0,
  add(n) {
    this.value += n;
    return this;
  },
  subtract(n) {
    this.value -= n;
    return this;
  },
  result() {
    return this.value;
  },
};

console.log(calculator.add(10).subtract(3).result()); // 7

The this keyword inside a method refers to the object the method was called on.

Destructuring Objects

const { name, age } = person;
console.log(name);  // "Alice"
console.log(age);   // 30

// Rename while destructuring
const { name: fullName } = person;
console.log(fullName);  // "Alice"

// Default values
const { role = "user" } = person;
console.log(role);  // "user" (property does not exist on person)

Spread and Rest with Objects

const defaults = { theme: "light", language: "en", fontSize: 14 };
const userPrefs = { theme: "dark", fontSize: 16 };

// Merge objects — later properties win
const config = { ...defaults, ...userPrefs };
// { theme: "dark", language: "en", fontSize: 16 }

// Copy an object
const copy = { ...person };

Iterating Over Objects

const scores = { Alice: 95, Bob: 87, Carol: 92 };

// for...in — iterates over keys
for (const key in scores) {
  console.log(key + ": " + scores[key]);
}

// Object.keys, Object.values, Object.entries
Object.keys(scores);    // ["Alice", "Bob", "Carol"]
Object.values(scores);  // [95, 87, 92]
Object.entries(scores); // [["Alice", 95], ["Bob", 87], ["Carol", 92]]

Checking for Properties