Method

Use Case Best Method Time Complexity Why?
Count occurrences of each element reduce() O(n) Efficient for counting
Get only elements that appear more than once filter() + Set O(n²) (can be slow for large arrays) Identifies duplicates
Remove duplicates (keep unique values) Set or filter() O(n) Set is fastest
Keep only elements appearing once filter() O(n²) Finds truly unique values
Group duplicates into arrays reduce() O(n) Organizes duplicates
Find first duplicate element Set + find() O(n) Fast and stops early
Remove duplicates without Set filter() + indexOf() O(n²)

const arr = ['2', '2', '2', '3', '3', '3', '4', '4'];

const grouped = arr.reduce((acc, item) => {
acc[item] = acc[item] ? [...acc[item], item] : [item];
return acc;
}, {});

console.log(grouped);
// Output: { '2': ['2', '2', '2'], '3': ['3', '3', '3'], '4': ['4', '4'] }

///inStcock. asd discount
const products = [
{ name: "Laptop", price: 800, inStock: true },
{ name: "Phone", price: 500, inStock: false },
{ name: "Tablet", price: 300, inStock: true }
]

const result = products.filter(product=> product.inStock).map(product=>({...product, price:product.price*0.9}))
console.log(result)