Understanding Objects in JavaScript

You already know how to store a single value in a variable — const name = "Alice". But what if you need to store everything about Alice in one place — her name, age, and city? That's exactly what objects are for.

An object is a collection of related data, grouped together under one name using key-value pairs.

What Is an Object?

Think of an object like a contact card in your phone. One card holds multiple pieces of information about one person:

┌─────────────────────────────┐
│  name  →  "Alice"           │
│  age   →  25                │
│  city  →  "Mumbai"          │
└─────────────────────────────┘

Each piece of information has a key (like name) and a value (like "Alice"). Together, they form a key-value pair.

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Creating an Object

Use curly braces {} with key: value pairs separated by commas:

const person = {
  name: "Alice",
  age: 25,
  city: "Mumbai"
};

• name, age, city are the keys (also called properties)
• "Alice", 25, "Mumbai" are the values
• Values can be any data type — string, number, boolean, array, even another object

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Accessing Properties

Dot notation — the most common way

console.log(person.name);  // "Alice"
console.log(person.age);   // 25
console.log(person.city);  // "Mumbai"

Bracket notation — useful when the key is dynamic

console.log(person["name"]);  // "Alice"

const key = "age";
console.log(person[key]);     // 25 — key stored in a variable

Use dot notation by default. Switch to bracket notation when the key comes from a variable or contains special characters.

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Updating Object Properties

You can change the value of any property directly:

const person = {
  name: "Alice",
  age: 25,
  city: "Mumbai"
};

person.age = 26;
person.city = "Pune";

console.log(person.age);  // 26
console.log(person.city); // "Pune"

Even though person is declared with const, you can still modify its properties. const only prevents you from replacing the entire object — not from changing what's inside it.

person = { name: "Bob" }; //  Error — cannot reassign a const
person.name = "Bob";      //  Fine — updating a property

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Adding and Deleting Properties

Add a new property

Just assign to a key that doesn't exist yet:

person.email = "[e-mail]";

console.log(person.email); // "[e-mail]"

Delete a property

delete person.city;

console.log(person.city); // undefined — property is gone

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Objects Can Have Methods

A method is a function stored as a property of an object:

const person = {
  name: "Alice",
  age: 25,
  greet() {
    return "Hi, I'm " + this.name + "!";
  }
};

console.log(person.greet()); // "Hi, I'm Alice!"

this inside a method refers to the object itself — so this.name gives you "Alice".

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Looping Through Object Keys

Use for...in to iterate over every key in an object:

const person = {
  name: "Alice",
  age: 25,
  city: "Mumbai"
};

for (const key in person) {
  console.log(key + ": " + person[key]);
}

// name: Alice
// age: 25
// city: Mumbai

Useful object utility methods

console.log(Object.keys(person));
// ["name", "age", "city"]

console.log(Object.values(person));
// ["Alice", 25, "Mumbai"]

console.log(Object.entries(person));
// [["name", "Alice"], ["age", 25], ["city", "Mumbai"]]

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Nested Objects

Objects can contain other objects:

const student = {
  name: "Rahul",
  age: 20,
  address: {
    city: "Delhi",
    pincode: "110001"
  }
};

console.log(student.address.city);    // "Delhi"
console.log(student.address.pincode); // "110001"

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Destructuring — Extract Values Cleanly

Instead of writing person.name, person.age separately, pull them out in one line:

const { name, age, city } = person;

console.log(name); // "Alice"
console.log(age);  // 25
console.log(city); // "Mumbai"

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Array vs Object — Which to Use?

	Array []	Object {}
Structure	Ordered list	Named key-value pairs
Access	By index: arr[0]	By key: obj.name
Best for	List of similar items	One thing with multiple properties
Example	List of 5 scores	One student's full profile

// Array — a list of students
const students = ["Alice", "Bob", "Charlie"];

// Object — one student's details
const student = {
  name: "Alice",
  age: 20,
  grade: "A"
};

// Real-world combo — array of objects (most common pattern!)
const allStudents = [
  { name: "Alice", grade: "A" },
  { name: "Bob",   grade: "B" },
];

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