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36 Maps (Map) ES6

Before ES6, JavaScript didn’t have a data structure for dictionaries and (ab)used objects as dictionaries from strings to arbitrary values. ES6 brought Maps, which are dictionaries from arbitrary values to arbitrary values.

36.1 Using Maps

An instance of Map maps keys to values. A single key-value mapping is called an entry.

36.1.1 Creating Maps

There are three common ways of creating Maps.

First, we can use the constructor without any parameters to create an empty Map:

const emptyMap = new Map();
assert.equal(emptyMap.size, 0);

Second, we can pass an iterable (e.g., an Array) over key-value “pairs” (Arrays with two elements) to the constructor:

const map = new Map([
  [1, 'one'],
  [2, 'two'],
  [3, 'three'], // trailing comma is ignored
]);

Third, the .set() method adds entries to a Map and is chainable:

const map = new Map()
  .set(1, 'one')
  .set(2, 'two')
  .set(3, 'three')
;

36.1.2 Working with single entries

map.set() and map.get() are for writing and reading values (given keys).

const map = new Map();

map.set('foo', 123);

assert.equal(map.get('foo'), 123);
// Unknown key:
assert.equal(map.get('bar'), undefined);
// Use the default value '' if an entry is missing:
assert.equal(map.get('bar') ?? '', '');

map.has() checks if a Map has an entry with a given key. map.delete() removes entries.

const map = new Map([['foo', 123]]);

assert.equal(map.has('foo'), true);
assert.equal(map.delete('foo'), true)
assert.equal(map.has('foo'), false)

36.1.3 Determining the size of a Map and clearing it

map.size contains the number of entries in a Map. map.clear() removes all entries of a Map.

const map = new Map()
  .set('foo', true)
  .set('bar', false)
;
assert.equal(map.size, 2)
map.clear();
assert.equal(map.size, 0)

36.1.4 Example: Counting characters

countChars() returns a Map that maps characters to numbers of occurrences.

function countChars(chars) {
  const charCounts = new Map();
  for (let ch of chars) {
    ch = ch.toLowerCase();
    const prevCount = charCounts.get(ch) ?? 0;
    charCounts.set(ch, prevCount+1);
  }
  return charCounts;
}

const result = countChars('AaBccc');
assert.deepEqual(
  countChars('AaBccc'),
  new Map([
    ['a', 2],
    ['b', 1],
    ['c', 3],
  ])
);

36.2 Iterating over Maps

This is how Maps support iteration:

36.2.1 Looping over Maps

Maps are iterables over key-value pairs. This is a common way of iterating over them:

const map = new Map()
  .set(false, 'no')
  .set(true, 'yes')
;
for (const [key, value] of map) { // (A)
  console.log(JSON.stringify(key) + ' = ' + JSON.stringify(value));
}

In line A, we use destructuring to access the components of the key-value pairs returned by the iterator.

Output:

false = "no"
true = "yes"

The following example loops over the iterable iterator returned by method .keys():

for (const key of map.keys()) {
  console.log(JSON.stringify(key));
}

Output:

false
true

36.2.2 Listed in insertion order: entries, keys, values

Maps record in which order entries were created and honor that order when listing keys, values or entries:

const map1 = new Map([
  ['a', 1],
  ['b', 2],
]);
for (const key of map1.keys()) {
  console.log(JSON.stringify(key));
}

Output:

"a"
"b"

const map2 = new Map([
  ['b', 2],
  ['a', 1],
]);
for (const key of map2.keys()) {
  console.log(JSON.stringify(key));
}

Output:

"b"
"a"

Maps listing their contents in insertion order has two benefits:

36.3 Converting from and to Maps

36.3.1 Converting keys, values, entries to Arrays

On one hand, the values returned by .keys(), .values() and .entries() are iterables – which enables us to use Array.from():

const map = new Map()
  .set(false, 'no')
  .set(true, 'yes')
;

assert.deepEqual(
  Array.from(map.keys()),
  [false, true]
);
assert.deepEqual(
  Array.from(map.values()),
  ['no', 'yes']
);
assert.deepEqual(
  Array.from(map.entries()),
  [
    [false, 'no'],
    [true, 'yes'],
  ]
);

On the other hand, the values returned by .keys(), .values() and .entries() are also iterators – which enables us to use the iterator method .toArray():

const map = new Map()
  .set(false, 'no')
  .set(true, 'yes')
;

assert.deepEqual(
  map.keys().toArray(),
  [false, true]
);
assert.deepEqual(
  map.values().toArray(),
  ['no', 'yes']
);
assert.deepEqual(
  map.entries().toArray(),
  [
    [false, 'no'],
    [true, 'yes'],
  ]
);

36.3.2 Converting between Maps and Objects

As long as a Map only uses strings and symbols as keys, we can convert it to an object (via Object.fromEntries()):

const map = new Map([
  ['a', 1],
  ['b', 2],
]);
const obj = Object.fromEntries(map);
assert.deepEqual(
  obj, {a: 1, b: 2}
);

We can also convert an object to a Map with string or symbol keys (via Object.entries()):

const obj = {
  a: 1,
  b: 2,
};
const map = new Map(Object.entries(obj));
assert.deepEqual(
  map, new Map([['a', 1], ['b', 2]])
);

36.4 Processing Maps

36.4.1 Copying Maps

As we have seen, Maps are iterables over key-value pairs. Therefore, we can use the constructor to create a copy of a Map. That copy is shallow: keys and values are the same; they are not copied/cloned themselves.

const original = new Map()
  .set(false, 'no')
  .set(true, 'yes')
;
const copy = new Map(original);
assert.deepEqual(original, copy);

36.4.2 Combining multiple Maps into a single Map

There are no methods for combining Maps, which is why we must use a workaround. Let’s combine the following two Maps:

const map1 = new Map()
  .set(1, '1a')
  .set(2, '1b')
  .set(3, '1c')
;

const map2 = new Map()
  .set(2, '2b')
  .set(3, '2c')
  .set(4, '2d')
;

To combine map1 and map2, we create a new Array and spread (...) the entries (key-value pairs) of map1 and map2 into it (via iteration). Then we convert the Array back into a Map. All of that is done in line A:

const combinedMap = new Map([...map1, ...map2]); // (A)
assert.deepEqual(
  Array.from(combinedMap), // convert to Array for comparison
  [
    [ 1, '1a' ],
    [ 2, '2b' ],
    [ 3, '2c' ],
    [ 4, '2d' ],
  ]
);

Icon “exercise” Exercise: Combining Maps

exercises/maps/combine_maps_test.mjs

36.4.3 Mapping and filtering Maps via iterator methods ES2025

We can .map() and .filter() an Array, but there are no such operations for a Map. The solution is:

  1. Convert the Map to an iterator over [key, value] pairs.
  2. Map or filter the iterator.
  3. Convert the result back to a Map.

We’ll use the following Map to explore how that works.

const originalMap = new Map()
  .set(1, 'a')
  .set(2, 'b')
  .set(3, 'c')
;

Mapping originalMap:

const mappedMap = new Map( // step 3
  originalMap.entries() // step 1
  .map(([k, v]) => [k * 2, '_' + v]) // step 2
);
assert.deepEqual(
  mappedMap,
  new Map([[2,'_a'], [4,'_b'], [6,'_c']])
);

Filtering originalMap:

const filteredMap = new Map( // step 3
  originalMap.entries() // step 1
  .filter(([k, v]) => k < 3) // step 2
);
assert.deepEqual(
  filteredMap,
  new Map([[1,'a'], [2,'b']])
);

What if we can’t use iterator methods? Then we can switch to Array methods, by

36.5 A few more details about the keys of Maps (advanced)

Any value can be a key, even an object:

const map = new Map();

const KEY1 = {};
const KEY2 = {};

map.set(KEY1, 'hello');
map.set(KEY2, 'world');

assert.equal(map.get(KEY1), 'hello');
assert.equal(map.get(KEY2), 'world');

36.5.1 What keys are considered equal?

Most Map operations need to check whether a value is equal to one of the keys. They do so via the internal operation SameValueZero, which works like === but considers NaN to be equal to itself.

As a consequence, we can use NaN as a key in Maps, just like any other value:

> const map = new Map();

> map.set(NaN, 123);
> map.get(NaN)
123

Different objects are always considered to be different. That is something that can’t be changed (yet – configuring key equality is on TC39’s long-term roadmap).

> new Map().set({}, 1).set({}, 2).size
2

36.6 Quick reference: Map

Note: For the sake of conciseness, I’m pretending that all keys have the same type K and that all values have the same type V.

36.6.1 new Map()

36.6.2 Map.*

36.6.3 Map.prototype.*: handling single entries

36.6.4 Map.prototype: handling all entries

36.6.5 Map.prototype: iterating and looping

Both iterating and looping happen in the order in which entries were added to a Map.

36.7 FAQ: Maps

36.7.1 When should I use a Map, and when should I use an object?

If we need a dictionary-like data structure with keys that are neither strings nor symbols, we have no choice: we must use a Map.

If, however, our keys are either strings or symbols, we must decide whether or not to use an object. A rough general guideline is:

36.7.2 When would I use an object as a key in a Map?

We normally want Map keys to be compared by value (two keys are considered equal if they have the same content). That excludes objects. However, there is one use case for objects as keys: externally attaching data to objects. But that use case is served better by WeakMaps, where entries don’t prevent keys from being garbage-collected (for details, see the next chapter).

36.7.3 Why do Maps preserve the insertion order of entries?

In principle, Maps entries are unordered. The main reason for ordering entries is so that operations that list entries, keys, or values are deterministic. That helps, for example, with testing.

36.7.4 Why do Maps have a .size, while Arrays have a .length?

In JavaScript, indexable sequences (such as Arrays and strings) have a .length, while unindexed collections (such as Maps and Sets) have a .size:

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