Objects

Objects

What it means

Objects are collections of key-value pairs. They’re the foundation of JavaScript’s object-oriented features. Keys are strings (or symbols), values can be any type including functions (methods).

Examples

// Creating objects
const user = {
  name: "Alice",
  age: 25,
  email: "alice@example.com",
  greet() {
    return `Hello, I'm ${this.name}`;
  }
};

// Accessing properties
console.log(user.name);          // dot notation
console.log(user["email"]);      // bracket notation

// Adding/modifying properties
user.city = "NYC";
user.age = 26;

// Deleting properties
delete user.email;

// Computed property names
const key = "role";
const admin = {
  [key]: "administrator",
  [`is${key}`]: true
};

// Object methods
const keys = Object.keys(user);           // ['name', 'age', 'city', 'greet']
const values = Object.values(user);       // ['Alice', 26, 'NYC', [Function]]
const entries = Object.entries(user);     // [['name', 'Alice'], ...]

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

// Spread operator
const updated = { ...user, verified: true };

// Object.assign (merge objects)
const merged = Object.assign({}, user, { premium: true });

Common mistake

Forgetting that objects are passed by reference:

// BAD - both variables point to same object
const obj1 = { count: 1 };
const obj2 = obj1;
obj2.count = 2;
console.log(obj1.count); // 2 (unexpected!)

// GOOD - create a copy
const obj3 = { count: 1 };
const obj4 = { ...obj3 };
obj4.count = 2;
console.log(obj3.count); // 1 (independent)

Fix: Use spread operator {...obj} or Object.assign({}, obj) to create copies.

Quick practice

1. How do you check if an object has a property?

   Answer

   `'key' in obj` or `obj.hasOwnProperty('key')`

2. What’s the difference between Object.keys() and Object.values()?

   Answer

   `Object.keys()` returns array of property names; `Object.values()` returns array of values.

3. Extract name and age from const user = { name: "Bob", age: 30, city: "LA" } using destructuring.

   Answer

   `const { name, age } = user;`