function indexOf(set, item, idx, prop) {
        for (/*idx = ~~idx*/; idx < set.length; idx++) {// start idx is always given as this is internal
            if ((prop ? set[idx][prop] : set[idx]) === item) return idx;
        }
        return -1;
    }
**indexOf** find index of item in collection.
@param {Array|NodeList} set
@param {Object} item
@param {number} idx
@param {string} [prop] Property on set item to compare to item