Collections in JavaScript

Complete Guide to Collections in JavaScript

JavaScript provides powerful built-in collection types to store and manage data efficiently.

1️⃣ Array

Arrays store ordered elements indexed from 0.

Creating Array

let fruits = ["Apple", "Banana", "Mango"];

Add Elements

// Add at end
fruits.push("Orange");

// Add at beginning
fruits.unshift("Grapes");

Remove Elements

// Remove last
fruits.pop();

// Remove first
fruits.shift();

// Remove specific index
fruits.splice(1, 1);

Loop Through Array

// forEach
fruits.forEach(fruit => console.log(fruit));

// for loop
for(let i = 0; i < fruits.length; i++){
    console.log(fruits[i]);
}

Check if Element Exists

fruits.includes("Banana"); // true
fruits.indexOf("Apple") !== -1; // true

2️⃣ Object

Objects store key-value pairs.

let student = {
    name: "John",
    age: 22
};

Add Property

student.grade = "A";

Remove Property

delete student.age;

Loop Through Object

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

Check if Key Exists

"name" in student; // true
student.hasOwnProperty("grade"); // true

3️⃣ Set

Set stores unique values only.

let numbers = new Set([1,2,3]);

Add Value

numbers.add(4);

Remove Value

numbers.delete(2);

Loop Through Set

numbers.forEach(num => console.log(num));

Check if Value Exists

numbers.has(3); // true

4️⃣ Map

Map stores key-value pairs (keys can be any type).

let map = new Map();
map.set("name", "Alice");

Add Entry

map.set("age", 25);

Remove Entry

map.delete("age");

Loop Through Map

map.forEach((value, key) => {
    console.log(key, value);
});

Check if Key Exists

map.has("name"); // true

5️⃣ WeakSet

WeakSet stores only objects.

let weakSet = new WeakSet();
let obj = {};
weakSet.add(obj);

Check Existence

weakSet.has(obj); // true

6️⃣ WeakMap

WeakMap stores object keys only.

let weakMap = new WeakMap();
let user = {};
weakMap.set(user, "Private Data");

Check Existence

weakMap.has(user); // true

Summary

• Array → push(), pop(), forEach(), includes()
• Object → add/delete property, for...in, hasOwnProperty()
• Set → add(), delete(), has()
• Map → set(), delete(), has()
• WeakSet / WeakMap → object-only storage