Objects

Key-value pairs. The fundamental building block of JavaScript.

Creating Objects

Key-value pairs inside curly braces.

// Object literal (most common):
const user = {
  name: "Alice",
  age: 28,
  isAdmin: false,
};

// Empty object:
const empty = {};

// Shorthand — variable names become keys:
const name = "Bob";
const age = 25;
const bob = { name, age };
// { name: "Bob", age: 25 }

const user = {

name:"Alice"

age:28

isAdmin:false

}

Accessing Properties

Dot notation or bracket notation.

const user = { name: "Alice", age: 28 };

user.name;    // "Alice"
user.age;     // 28
user.email;   // undefined (doesn't exist)

// Clean and readable — use when:
// • Key is a valid identifier
// • Key is known at write time

Modifying Objects

Add, update, and delete properties.

const user = { name: "Alice", age: 28 };

// Update existing property:
user.age = 29;

// Add new property:
user.email = "alice@example.com";

// Delete a property:
delete user.email;

console.log(user);
// { name: "Alice", age: 29 }

⚠️ constprevents reassigning the variable, but you can still modify the object's properties. Use Object.freeze() to make an object truly immutable.

Nested Objects

Objects inside objects — common for structured data.

const user = {
  name: "Alice",
  address: {
    street: "123 Main St",
    city: "Springfield",
    country: "US",
  },
  hobbies: ["reading", "coding"],
};

// Access nested properties:
user.address.city;    // "Springfield"
user.hobbies[0];     // "reading"

// Optional chaining (safe access):
user.address?.zip;        // undefined (no error)
user.contact?.email;      // undefined (no error)
// Without ?. → TypeError if contact is undefined

Checking Properties

Iterate and inspect object contents.

const user = { name: "Alice", age: 28, role: "dev" };

// Check if property exists:
"name" in user;              // true
"email" in user;             // false
user.hasOwnProperty("age");  // true

// Get all keys, values, or entries:
Object.keys(user);    // ["name", "age", "role"]
Object.values(user);  // ["Alice", 28, "dev"]
Object.entries(user); // [["name","Alice"], ["age",28], ...]

// Loop through properties:
for (const [key, value] of Object.entries(user)) {
  console.log(`${key}: ${value}`);
}

References

Objects are stored by reference, like arrays.

const a = { x: 1 };
const b = a;

b.x = 99;
console.log(a.x); // 99 ← same object!

Assigning an object to another variable creates a reference — both point to the same data.

💡 b = a does NOT copy the object

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FAQ

Common questions about objects.