Map.map() should be a thing in ES6. It just makes sense.

Map.map.js

/**
How to use:

let m = new Map();
m.set(1, 1);
m.set(2, 2);
// Map(2) {1 => 1, 2 => 2}

let n = m.map((key, value) => value * 2);
// Map(2) {1 => 2, 2 => 4}

**/

Map.prototype.map = function(mapfn) {
  //mapfn((key, value) => ...)
  let newMap = new Map(); 
  for(let key of this.keys()) {
    let value = this.get(key); 
    newMap.set(key, mapfn(key, value));
  } 
  return newMap;
};