pairs.js

'use strict';

function findPairs(inputs) {
    const unpairedNumbers = new Set();
    const pairs = [];
    for (let element of inputs) {
        if (unpairedNumbers.has(element)) {
            pairs.push(element);
            unpairedNumbers.delete(element);
        } else {
            unpairedNumbers.add(element);
        }
    }
    return pairs;
}

console.log(findPairs([ 1, 2, 3, 5, 5, 5, 5, 5, 1, 8 ]));