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JavaScript Objects

What are Objects?​

"Key-value pairs"

Objects are collections of properties (key-value pairs) in JavaScript.

"JavaScript objects are collections of key-value pairs, used to represent structured data and enable object-oriented programming."

Object Creation​

Literal · Constructor · Object.create()

// Object literal (most common)
const obj = { name: "John", age: 30 };

// Constructor
const obj = new Object();
obj.name = "John";

// Object.create()
const obj = Object.create(null);

"Objects can be created with literals (most common), constructors, or Object.create()."

Property Access​

Dot notation · Bracket notation

const obj = { name: "John", "key with spaces": "value" };

// Dot notation
obj.name; // 'John'

// Bracket notation
obj["name"]; // 'John'
obj["key with spaces"]; // 'value' (required for special keys)

"Properties accessed with dot notation or bracket notation - bracket required for special characters or dynamic keys."

Property Types​

Data properties · Accessor properties

// Data property
const obj = {
  name: "John", // value, writable, enumerable, configurable
};

// Accessor property
const obj = {
  get name() {
    return this._name;
  },
  set name(value) {
    this._name = value;
  },
};

"Properties can be data properties (value) or accessor properties (getter/setter)."

Property Descriptors​

value · writable · enumerable · configurable

Object.defineProperty(obj, "name", {
  value: "John",
  writable: false, // Can't change
  enumerable: true, // Shows in for...in
  configurable: false, // Can't delete or reconfigure
});

"Property descriptors control value, writability, enumerability, and configurability."

Object Methods​

Object.keys · values · entries · assign

const obj = { a: 1, b: 2 };

Object.keys(obj); // ['a', 'b']
Object.values(obj); // [1, 2]
Object.entries(obj); // [['a', 1], ['b', 2]]
Object.assign({}, obj); // Shallow copy

"Object methods include keys, values, entries for iteration, and assign for copying."

Object Destructuring​

Extract properties easily

const obj = { name: "John", age: 30 };

// Destructuring
const { name, age } = obj;

// Rename
const { name: personName } = obj;

// Default values
const { name, city = "Unknown" } = obj;

"Destructuring extracts object properties into variables, with support for renaming and default values."

Spread Operator​

Copy and merge objects

const obj1 = { a: 1, b: 2 };
const obj2 = { c: 3 };

// Copy
const copy = { ...obj1 };

// Merge
const merged = { ...obj1, ...obj2 }; // { a: 1, b: 2, c: 3 }

// Override
const updated = { ...obj1, b: 5 }; // { a: 1, b: 5 }

"Spread operator copies and merges objects, with later properties overriding earlier ones."

Reference Types​

Objects are passed by reference

const obj1 = { name: "John" };
const obj2 = obj1; // Reference, not copy

obj2.name = "Jane";
console.log(obj1.name); // 'Jane' (both reference same object)

"Objects are reference types - assigning doesn't copy, both variables reference the same object."

Object Comparison​

Reference equality, not value equality

const obj1 = { name: "John" };
const obj2 = { name: "John" };

obj1 === obj2; // false (different references)
obj1 === obj1; // true (same reference)

"Object comparison uses reference equality - two objects with same properties are not equal unless same reference."

🧠 Ultra-Short Cheat Sheet​

Key-value pairs
Literal, constructor, Object.create()
Dot/bracket notation
Property descriptors
Reference types
Object methods (keys/values/entries)
Destructuring
Spread operator