Object in JS

JavaScriptMiddle~1 min read

In short: An object (Object) in JavaScript is a "key -> value" data structure, where keys are strings or symbols, and values can be any data type (including other objects and functions).

Detailed explanation

Objects are the main way to store and organize data in JS. They let you group related values into a single entity and access them by name (keys).

javascript

const user = {
  name: 'Alice',
  age: 25,
  isAdmin: true
};

Here user is an object with three properties: name, age, isAdmin.

1. Accessing properties

javascript

console.log(user.name);    // "Alice"
console.log(user['age']);  // 25

You can access them via:

dot notation (obj.key), when the name is known;
square brackets (obj['key']), when the name is stored in a variable.

2. Adding and removing properties

javascript

user.city = 'Kyiv';       // add a new one
delete user.isAdmin;      // remove

3. Iterating over an object's properties

javascript

for (let key in user) {
  console.log(key, user[key]);
}

Prints:

javascript

name Alice
age 25
city Kyiv

4. Nested objects

javascript

const user = {
  name: 'Alice',
  address: {
    city: 'Kyiv',
    zip: '123456'
  }
};

console.log(user.address.city); // "Kyiv"

A property can contain another object, which lets you build complex structures.

5. Object methods (functions as properties)

javascript

const user = {
  name: 'Alice',
  sayHi() {
    console.log(`Hi, ${this.name}!`);
  }
};

user.sayHi(); // "Hi, Alice!"

A function inside an object is called a method. this inside a method refers to the object itself (user).

6. Checking if a property exists

javascript

console.log('name' in user); // true
console.log(user.hasOwnProperty('age')); // true

7. Copying objects

javascript

const clone = { ...user };               // shallow copy (spread)
const deepClone = structuredClone(user); // deep copy

8. Objects can be created in different ways

javascript

const obj1 = {};                  // object literal
const obj2 = new Object();        // via constructor
const obj3 = Object.create(null); // without a prototype

Summary

Property	Description
Type	Composite (reference type)
Stores	"Key -> value" pairs
Keys	Strings or symbols
Values	Any data type
Main operations	Adding, reading, removing, iterating
Often used for	Representing entities, settings, data

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