sockMerchant.js

function stockAndCount(n, arr) {
	let pairs = 0; // pairs number
	const colors = arr.reduce((acc, val) => {
		acc[val] ? (acc[val] += 1) : (acc[val] = 1);
		return acc;
	}, {}); // return an object with colors (numbers) as keys and quantity as value
    
	Object.keys(colors).forEach((n) => { // for each key in colors
    let _pair = parseInt(colors[n] / 2); // looks for even numbers
    if (_pair >= 1) pairs += _pair; // total
	});

	return pairs;
}

const n = 9;
const socks = [10, 20, 20, 10, 10, 10, 10, 10, 20];

console.group('Stocked and counted');
console.log(`There is a total of ${stockAndCount(n, socks)} pairs`);
console.groupEnd();