.map() a Javascript ES6 Map?

Question

How would you do this? Instinctively, I want to do:

var myMap = new Map([["thing1", 1], ["thing2", 2], ["thing3", 3]]);

// wishful, ignorant thinking
var newMap = myMap.map((key, value) => value + 1); // Map { 'thing1' => 2, 'thing2' => 3, 'thing3' => 4 }

I've haven't gleaned much from the documentation on the new iteration protocol.

I am aware of wu.js, but I'm running a Babel project and don't want to include Traceur, which it seems like it currently depends on.

I also am a bit clueless as to how to extract how fitzgen/wu.js did it into my own project.

Would love a clear, concise explanation of what I'm missing here. Thanks!

---

Docs for ES6 Map, FYI

Answer 1

So .map itself only offers one value you care about... That said, there are a few ways of tackling this:

// instantiation
const myMap = new Map([
  [ "A", 1 ],
  [ "B", 2 ]
]);

// what's built into Map for you
myMap.forEach( (val, key) => console.log(key, val) ); // "A 1", "B 2"

// what Array can do for you
Array.from( myMap ).map(([key, value]) => ({ key, value })); // [{key:"A", value: 1}, ... ]

// less awesome iteration
let entries = myMap.entries( );
for (let entry of entries) {
  console.log(entry);
}

Note, I'm using a lot of new stuff in that second example... ...Array.from takes any iterable (any time you'd use [].slice.call( ), plus Sets and Maps) and turns it into an array... ...Maps, when coerced into an array, turn into an array of arrays, where el[0] === key && el[1] === value; (basically, in the same format that I prefilled my example Map with, above).

I'm using destructuring of the array in the argument position of the lambda, to assign those array spots to values, before returning an object for each el.

If you're using Babel, in production, you're going to need to use Babel's browser polyfill (which includes "core-js" and Facebook's "regenerator").
I'm quite certain it contains Array.from.

Answer 2

Just use Array.from(iterable, [mapFn]).

var myMap = new Map([["thing1", 1], ["thing2", 2], ["thing3", 3]]);

var newEntries = Array.from(myMap, ([key, value]) => [key, value + 1]);
var newMap = new Map(newEntries);

Answer 3

You should just use Spread operator:

var myMap = new Map([["thing1", 1], ["thing2", 2], ["thing3", 3]]);

var newArr = [...myMap].map(value => value[1] + 1);
console.log(newArr); //[2, 3, 4]

var newArr2 = [for(value of myMap) value = value[1] + 1];
console.log(newArr2); //[2, 3, 4]

Answer 4

You can use this function:

function mapMap(map, fn) {
  return new Map(Array.from(map, ([key, value]) => [key, fn(value, key, map)]));
}

usage:

var map1 = new Map([["A", 2], ["B", 3], ["C", 4]]);

var map2 = mapMap(map1, v => v * v);

console.log(map1, map2);
/*
Map { A → 2, B → 3, C → 4 }
Map { A → 4, B → 9, C → 16 }
*/

Answer 5

In typescript, in case somebody would need it :

export {}

declare global {
    interface Map<K, V> {
        map<T>(predicate: (key: K, value: V) => T): Map<V, T>
    }
}

Map.prototype.map = function<K, V, T>(predicate: (value: V, key: K) => T): Map<K, T> {
    let map: Map<K, T> = new Map()

    this.forEach((value: V, key: K) => {
        map.set(key, predicate(value, key))
    })
    return map
}

Answer 6

Using Array.from I wrote a Typescript function that maps the values:

function mapKeys<T, V, U>(m: Map<T, V>, fn: (this: void, v: V) => U): Map<T, U> {
  function transformPair([k, v]: [T, V]): [T, U] {
    return [k, fn(v)]
  }
  return new Map(Array.from(m.entries(), transformPair));
}

const m = new Map([[1, 2], [3, 4]]);
console.log(mapKeys(m, i => i + 1));
// Map { 1 => 3, 3 => 5 }

Answer 7

Actually you can still have a Map with the original keys after converting to array with Array.from. That's possible by returning an array, where the first item is the key, and the second is the transformed value.

const originalMap = new Map([
  ["thing1", 1], ["thing2", 2], ["thing3", 3]
]);

const arrayMap = Array.from(originalMap, ([key, value]) => {
    return [key, value + 1]; // return an array
});

const alteredMap = new Map(arrayMap);

console.log(originalMap); // Map { 'thing1' => 1, 'thing2' => 2, 'thing3' => 3 }
console.log(alteredMap);  // Map { 'thing1' => 2, 'thing2' => 3, 'thing3' => 4 }

If you don't return that key as the first array item, you loose your Map keys.

Answer 8

You can map() arrays, but there is no such operation for Maps. The solution from Dr. Axel Rauschmayer:

• Convert the map into an array of [key,value] pairs.
• Map or filter the array.
• Convert the result back to a map.

Example:

let map0 = new Map([
  [1, "a"],
  [2, "b"],
  [3, "c"]
]);

const map1 = new Map(
  [...map0]
  .map(([k, v]) => [k * 2, '_' + v])
);

resulted in

{2 => '_a', 4 => '_b', 6 => '_c'}

Answer 9

I prefer to extend the map

export class UtilMap extends Map {  
  constructor(...args) { super(args); }  
  public map(supplier) {
      const mapped = new UtilMap();
      this.forEach(((value, key) => mapped.set(key, supplier(value, key)) ));
      return mapped;
  };
}

Answer 10

You can use myMap.forEach, and in each loop, using map.set to change value.

myMap = new Map([
  ["a", 1],
  ["b", 2],
  ["c", 3]
]);

for (var [key, value] of myMap.entries()) {
  console.log(key + ' = ' + value);
}


myMap.forEach((value, key, map) => {
  map.set(key, value+1)
})

for (var [key, value] of myMap.entries()) {
  console.log(key + ' = ' + value);
}

Answer 11

const mapMap = (callback, map) => new Map(Array.from(map).map(callback))

var myMap = new Map([["thing1", 1], ["thing2", 2], ["thing3", 3]]);

var newMap = mapMap((pair) => [pair[0], pair[1] + 1], myMap); // Map { 'thing1' => 2, 'thing2' => 3, 'thing3' => 4 }

Answer 12

If you don't want to convert the entire Map into an array beforehand, and/or destructure key-value arrays, you can use this silly function:

/**
 * Map over an ES6 Map.
 *
 * @param {Map} map
 * @param {Function} cb Callback. Receives two arguments: key, value.
 * @returns {Array}
 */
function mapMap(map, cb) {
  let out = new Array(map.size);
  let i = 0;
  map.forEach((val, key) => {
    out[i++] = cb(key, val);
  });
  return out;
}

let map = new Map([
  ["a", 1],
  ["b", 2],
  ["c", 3]
]);

console.log(
  mapMap(map, (k, v) => `${k}-${v}`).join(', ')
); // a-1, b-2, c-3

Answer 13

Map.prototype.map = function(callback) {
  const output = new Map()
  this.forEach((element, key)=>{
    output.set(key, callback(element, key))
  })
  return output
}

const myMap = new Map([["thing1", 1], ["thing2", 2], ["thing3", 3]])
// no longer wishful thinking
const newMap = myMap.map((value, key) => value + 1)
console.info(myMap, newMap)

Depends on your religious fervor in avoiding editing prototypes, but, I find this lets me keep it intuitive.

Answer 14

Here is a Typescript variant which builds upon some of the existing ideas and attempts to be flexible in the following ways:

• Both key and value types of output map can differ from input
• The mapper function can access the input map itself, if it likes

function mapMap<TKI, TVI, TKO, TVO>(map: Map<TKI, TVI>,
        f: (k: TKI, v: TVI, m: Map<TKI, TVI>) => [TKO, TVO]) : Map<TKO, TVO>{
    return new Map([...map].map(p => f(p[0], p[1], map)))
}

Note: this code uses the spread operator, like some of the existing ideas, and so needs a target of of 'es2015' or higher according to my VS Code Intellisense.

Answer 15

Maybe this way:

const m = new Map([["a", 1], ["b", 2], ["c", 3]]);
m.map((k, v) => [k, v * 2]); // Map { 'a' => 2, 'b' => 4, 'c' => 6 }

You would only need to monkey patch Map before:

Map.prototype.map = function(func){
    return new Map(Array.from(this, ([k, v]) => func(k, v)));
}

We could have wrote a simpler form of this patch:

Map.prototype.map = function(func){
    return new Map(Array.from(this, func));
}

But we would have forced us to then write m.map(([k, v]) => [k, v * 2]); which seems a bit more painful and ugly to me.

Mapping values only

We could also map values only, but I wouldn't advice going for that solution as it is too specific. Nevertheless it can be done and we would have the following API:

const m = new Map([["a", 1], ["b", 2], ["c", 3]]);
m.map(v => v * 2); // Map { 'a' => 2, 'b' => 4, 'c' => 6 }

Just like before patching this way:

Map.prototype.map = function(func){
    return new Map(Array.from(this, ([k, v]) => [k, func(v)]));
}

Maybe you can have both, naming the second mapValues to make it clear that you are not actually mapping the object as it would probably be expected.