map-utils.js

const applyMapFn = (fn, {value}, thisArg) => fn.apply(null, value.reverse());

const generators = {
    *filter([map, iteratee, thisArg]) {
        const iterator = map.entries();
        let nextItem = iterator.next();
        while (!nextItem.done)  {
          const {value} = nextItem;
          const result = iteratee.call(thisArg, value[1], value[0], map);
           if (result) {
                yield nextItem.value;
            }
            nextItem = iterator.next();
        };
    },
    *map([map, iteratee, thisArg]) {
       const iterator = map.entries();
       let nextItem = iterator.next();
       while (!nextItem.done) {
           const {value} = nextItem;
          yield iteratee.call(thisArg, value[1], value[0], map);
          nextItem = iterator.next();
       }
   },
   *reduce([map, iteratee, initialValue, thisArg]) {
       const iterator = map.entries();
       let nextItem = iterator.next();
       let reducedValue = initialValue;
       while (!nextItem.done) {
           const {value} = nextItem;
            reducedValue = iteratee.call(thisArg, value, value[1], value[0], map);
            nextItem = iterator.next();
       }
       yield reducedValue;
   }
};

export default ['filter', 'map', 'reduce'].reduce((obj, fnName) => {
    obj[fnName] = function() {
        return new Map(generators[fnName](arguments));
    };
    return obj;
}, {});