Sort elements in array by their frequencies

sortByFrequencies.js

function sortByFrequencies(coll) {
    'use strict';

    const frequencies = coll.reduce((freq, elem) => {
        if (freq.has(elem)) {
            return freq.set(elem, freq.get(elem) + 1);
        } else {
            return freq.set(elem, 1);
        }
    }, new Map());

    const sortedFreq = [];
    for (let item of frequencies.entries()) {
        sortedFreq.push(item);
    }

    sortedFreq.sort((prev, curr) => {
        const compared = curr[1] - prev[1];
        if (!compared) {
            return coll.indexOf(prev[0]) - coll.indexOf(curr[0]);
        } else {
            return compared;
        }
    });

    return sortedFreq.reduce((sorted, [item, frequency]) => {
        const repeatedItem = new Array(frequency).fill(item);
        return sorted.concat(repeatedItem);
    }, []);
}