Object vs array

JavaScriptJunior~2 min read

Both objects and arrays in JavaScript are data structures, but they have a different purpose and way of storing information.

In short

Property	Object	Array
What it stores	Key -> value pairs	An ordered list of elements by index
Keys	Strings or `Symbol`	Automatic numeric indexes
Order of elements	Not guaranteed	Guaranteed
Structure type	Associative	Sequential
Suited for	Describing entities (a user, a product, etc.)	Lists, collections, data sets
Methods	`Object.keys()`, `Object.values()`, etc.	`map()`, `filter()`, `push()`, `pop()`, etc.
Type check	`typeof obj === 'object'`	`Array.isArray(arr)`

Example of an object

javascript

const user = {
  name: 'Bohdan',
  age: 25,
  city: 'Kyiv'
};

console.log(user.name); // "Bohdan"

Here the keys are strings ("name", "age", "city") and the order of properties does not matter.

Example of an array

javascript

const numbers = [10, 20, 30];
console.log(numbers[0]); // 10

Here the elements are ordered: 0 → 10, 1 → 20, 2 → 30.

Key differences

1. Indexing

In an array, indexes are numeric and go in order.
In an object, keys are arbitrary strings.

javascript

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

console.log(obj['a']); // 1
console.log(arr[0]);   // 1

2. Order

An array guarantees the order of elements, while in an object the order of properties is not always important (although in modern JS it is preserved by insertion order).

3. Methods

Arrays have special methods for working with collections:

javascript

const arr = [1, 2, 3];
console.log(arr.map(x => x * 2)); // [2, 4, 6]

Objects do not, but they can be converted into an array:

javascript

const obj = { a: 1, b: 2, c: 3 };
console.log(Object.values(obj)); // [1, 2, 3]

4. Purpose

Type	Used for
Object	Structured data (a user, an order, settings)
Array	Sequences (product lists, messages, numbers)

5. Type check

javascript

const arr = [1, 2, 3];
const obj = { a: 1 };

console.log(typeof arr); // "object" (!) - technically an array is also an object
console.log(Array.isArray(arr)); // true

In JavaScript an array is a special case of an object, just with additional properties and behavior.

SUMMARY

Criterion	Object	Array
Storage	"key -> value"	indexed elements
Keys	strings / symbols	numbers
Order	not guaranteed	preserved
Methods	`Object.*`	`map`, `filter`, `push`, `forEach`
Ideal for	describing entities	working with lists
Type check	`typeof obj === 'object'`	`Array.isArray(arr)`

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