merge-descending-collections.js

function mergeDescendingCollections(target, source) {

    var nextTargetIndex = 0, i = 0,
        targetItem = target[nextTargetIndex++],
        result = [];

    while (i < source.length) {
        var sourceItem = source[i];

        if (!targetItem || sourceItem.counter > targetItem.counter) {
            result.push(sourceItem);
            i++;
        } else {
            if (sourceItem.counter === targetItem.counter) {
                i++; // not to add duplicates
            }

            result.push(targetItem);
            targetItem = target.length > nextTargetIndex ? target[nextTargetIndex++] : null;
        }
    }

    while (targetItem) {
        result.push(targetItem);
        targetItem = target.length > nextTargetIndex ? target[nextTargetIndex++] : null;
    }

    return result;
}