Map

## Description

`Map` objects are collections of key-value pairs. A key in the `Map` **may only occur once**; it is unique in the `Map`'s collection. A `Map` object is iterated by key-value pairs — a {{jsxref("Statements/for...of", "for...of")}} loop returns a 2-member array of `[key, value]` for each iteration. Iteration happens in _insertion order_, which corresponds to the order in which each key-value pair was first inserted into the map by the [`set()`](/en-US/docs/Web/JavaScript/Reference/Global_Objects/Map/set) method (that is, there wasn't a key with the same value already in the map when `set()` was called).

The specification requires maps to be implemented "that, on average, provide access times that are sublinear on the number of elements in the collection". Therefore, it could be represented internally as a hash table (with O(1) lookup), a search tree (with O(log(N)) lookup), or any other data structure, as long as the complexity is better than O(N).

### Key equality

Value equality is based on the [SameValueZero](/en-US/docs/Web/JavaScript/Guide/Equality_comparisons_and_sameness#same-value-zero_equality) algorithm. (It used to use [SameValue](/en-US/docs/Web/JavaScript/Guide/Equality_comparisons_and_sameness#same-value_equality_using_object.is), which treated `0` and `-0` as different. Check [browser compatibility](#browser_compatibility).) This means {{jsxref("NaN")}} is considered the same as `NaN` (even though `NaN !== NaN`) and all other values are considered equal according to the semantics of the `===` operator. Also, for object keys, equality is based on object identity. They are compared by reference, not by value. See [Using the Map object](#using_the_map_object) for examples.

### Objects vs. Maps

{{jsxref("Object")}} is similar to `Map`—both let you set keys to
values, retrieve those values, delete keys, and detect whether something is
stored at a key. For this reason (and because there were no built-in
alternatives), `Object` has been used as `Map` historically.

However, there are important differences that make `Map` preferable in some
cases:

<table class="standard-table">
  <thead>
    <tr>
      <th scope="row"></th>
      <th scope="col">Map</th>
      <th scope="col">Object</th>
    </tr>
  </thead>
  <tbody>
    <tr>
      <th scope="row">Accidental Keys</th>
      <td>
        A <code>Map</code> does not contain any keys by default. It only
        contains what is explicitly put into it.
      </td>
      <td>
        <p>
          An <code>Object</code> has a prototype, so it contains default keys
          that could collide with your own keys if you're not careful.
        </p>
        <div class="notecard note">
          <p>
            <strong>Note:</strong> This can be bypassed by using
            {{jsxref("Object.create", "Object.create(null)")}},
            but this is seldom done.
          </p>
        </div>
      </td>
    </tr>
    <tr>
      <th scope="row">Security</th>
      <td>
        A <code>Map</code> is safe to use with user-provided keys and values.
      </td>
      <td>
        <p>
          Setting user-provided key-value pairs on an <code>Object</code> may allow
          an attacker to override the object's prototype, which can lead to
          <a href="https://github.com/eslint-community/eslint-plugin-security/blob/main/docs/the-dangers-of-square-bracket-notation.md">
            object injection attacks
          </a> or <a href="/en-US/docs/Web/Security/Attacks/Prototype_pollution">prototype pollution attacks</a>. Like the accidental keys issue, this can also be mitigated by using
          a <code>null</code>-prototype object.
        </p>
      </td>
    </tr>
    <tr>
      <th scope="row">Key Types</th>
      <td>
        A <code>Map</code>'s keys can be any value (including functions,
        objects, or any primitive).
      </td>
      <td>
        The keys of an <code>Object</code> must be either a
        {{jsxref("String")}} or a {{jsxref("Symbol")}}.
      </td>
    </tr>
    <tr>
      <th scope="row">Key Order</th>
      <td>
        <p>
          The keys in <code>Map</code> are ordered in a straightforward
          way: A <code>Map</code> object iterates entries, keys, and values in
          the order of entry insertion.
        </p>
      </td>
      <td>
        <p>
          Although the keys of an ordinary <code>Object</code> are ordered now,
          this was not always the case, and the order is complex. As a result,
          it's best not to rely on property order.
        </p>
        <p>
          The order was first defined for own properties only in ECMAScript
          2015; ECMAScript 2020 defines order for inherited properties as well.
          But note that no single mechanism
          iterates
          <strong>all</strong> of an object's properties; the various mechanisms
          each include different subsets of properties.
          ({{jsxref("Statements/for...in", "for-in")}}
          includes only enumerable string-keyed properties;
          {{jsxref("Object.keys")}} includes only own, enumerable,
          string-keyed properties;
          {{jsxref("Object.getOwnPropertyNames")}} includes own,
          string-keyed properties even if non-enumerable;
          {{jsxref("Object.getOwnPropertySymbols")}} does the same
          for just <code>Symbol</code>-keyed properties, etc.)
        </p>
      </td>
    </tr>
    <tr>
      <th scope="row"><p>Size</p></th>
      <td>
        The number of items in a <code>Map</code> is easily retrieved from its
        {{jsxref("Map.prototype.size", "size")}} property.
      </td>
      <td>
        Determining the number of items in an <code>Object</code> is more roundabout and less efficient. A common way to do it is through the {{jsxref("Array/length", "length")}} of the array returned from {{jsxref("Object.keys()")}}.
      </td>
    </tr>
    <tr>
      <th scope="row">Iteration</th>
      <td>
        A <code>Map</code> is an
        <a href="/en-US/docs/Web/JavaScript/Reference/Iteration_protocols"
          >iterable</a
        >, so it can be directly iterated.
      </td>
      <td>
        <p>
          <code>Object</code> does not implement an <a
            href="/en-US/docs/Web/JavaScript/Reference/Iteration_protocols#the_iterable_protocol"
            >iteration protocol</a
          >, and so objects are not directly iterable using the JavaScript
          <a href="/en-US/docs/Web/JavaScript/Reference/Statements/for...of"
            >for...of</a
          >
          statement (by default).
        </p>
        <div class="notecard note">
          <p><strong>Note:</strong></p>
          <ul>
            <li>
              An object can implement the iteration protocol, or you can get an
              iterable for an object using <a
                href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/keys"
                ><code>Object.keys</code></a
              > or <a
                href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/entries"
                ><code>Object.entries</code></a
              >.
            </li>
            <li>
              The
              <a href="/en-US/docs/Web/JavaScript/Reference/Statements/for...in"
                >for...in</a
              >
              statement allows you to iterate over the
              <em>enumerable</em> properties of an object.
            </li>
          </ul>
        </div>
      </td>
    </tr>
    <tr>
      <th scope="row">Performance</th>
      <td>
        <p>
          Performs better in scenarios involving frequent additions and removals
          of key-value pairs.
        </p>
      </td>
      <td>
        <p>
          Not optimized for frequent additions and removals of key-value pairs.
        </p>
      </td>
    </tr>
    <tr>
      <th scope="row">Serialization and parsing</th>
      <td>
        <p>No native support for serialization or parsing.</p>
        <p>
          (But you can build your own serialization and parsing support for
          <code>Map</code> by using {{jsxref("JSON.stringify()")}}
          with its <em>replacer</em> argument, and by using
          {{jsxref("JSON.parse()")}} with its
          <em>reviver</em> argument. See the Stack Overflow question
          <a href="https://stackoverflow.com/q/29085197/"
            >How do you JSON.stringify an ES6 Map?</a
          >).
        </p>
      </td>
      <td>
        <p>
          Native support for serialization from {{jsxref("Object")}} to
          JSON, using {{jsxref("JSON.stringify()")}}.
        </p>
        <p>
          Native support for parsing from JSON to {{jsxref("Object")}},
          using {{jsxref("JSON.parse()")}}.
        </p>
      </td>
    </tr>
  </tbody>
</table>

### Setting object properties

Setting Object properties works for Map objects as well, and can cause
considerable confusion.

Therefore, this appears to work in a way:

```js example-bad
const wrongMap = new Map();
wrongMap["bla"] = "blaa";
wrongMap["bla2"] = "blaaa2";

console.log(wrongMap); // Map { bla: 'blaa', bla2: 'blaaa2' }

The correct usage for storing data in the Map is through the `set(key, value)`
method.

```js example-good
const contacts = new Map();
contacts.set("Jessie", { phone: "213-555-1234", address: "123 N 1st Ave" });
contacts.has("Jessie"); // true
contacts.get("Hilary"); // undefined
contacts.set("Hilary", { phone: "617-555-4321", address: "321 S 2nd St" });
contacts.get("Jessie"); // {phone: "213-555-1234", address: "123 N 1st Ave"}
contacts.delete("Raymond"); // false
contacts.delete("Jessie"); // true
console.log(contacts.size); // 1

interface RTCStatsReport {
  readonly maplike<DOMString, object>;
};

const myMap = new Map();

const keyString = "a string";
const keyObj = {};
const keyFunc = () => {};

// setting the values
myMap.set(keyString, "value associated with 'a string'");
myMap.set(keyObj, "value associated with keyObj");
myMap.set(keyFunc, "value associated with keyFunc");

console.log(myMap.size); // 3

// getting the values
console.log(myMap.get(keyString)); // "value associated with 'a string'"
console.log(myMap.get(keyObj)); // "value associated with keyObj"
console.log(myMap.get(keyFunc)); // "value associated with keyFunc"

console.log(myMap.get("a string")); // "value associated with 'a string'", because keyString === 'a string'
console.log(myMap.get({})); // undefined, because keyObj !== {}
console.log(myMap.get(() => {})); // undefined, because keyFunc !== () => {}

const myMap = new Map();
myMap.set(NaN, "not a number");

myMap.get(NaN);
// "not a number"

const otherNaN = Number("foo");
myMap.get(otherNaN);
// "not a number"

const myMap = new Map();
myMap.set(0, "zero");
myMap.set(1, "one");

for (const [key, value] of myMap) {
  console.log(`${key} = ${value}`);
}
// 0 = zero
// 1 = one

for (const key of myMap.keys()) {
  console.log(key);
}
// 0
// 1

for (const value of myMap.values()) {
  console.log(value);
}
// zero
// one

for (const [key, value] of myMap.entries()) {
  console.log(`${key} = ${value}`);
}
// 0 = zero
// 1 = one

myMap.forEach((value, key) => {
  console.log(`${key} = ${value}`);
});
// 0 = zero
// 1 = one

const kvArray = [
  ["key1", "value1"],
  ["key2", "value2"],
];

// Use the regular Map constructor to transform a 2D key-value Array into a map
const myMap = new Map(kvArray);

console.log(myMap.get("key1")); // "value1"

// Use Array.from() to transform a map into a 2D key-value Array
console.log(Array.from(myMap)); // Will show you exactly the same Array as kvArray

// A succinct way to do the same, using the spread syntax
console.log([...myMap]);

// Or use the keys() or values() iterators, and convert them to an array
console.log(Array.from(myMap.keys())); // ["key1", "key2"]

const original = new Map([[1, "one"]]);

const clone = new Map(original);

console.log(clone.get(1)); // one
console.log(original === clone); // false (useful for shallow comparison)

const first = new Map([
  [1, "one"],
  [2, "two"],
  [3, "three"],
]);

const second = new Map([
  [1, "uno"],
  [2, "dos"],
]);

// Merge two maps. The last repeated key wins.
// Spread syntax essentially converts a Map to an Array
const merged = new Map([...first, ...second]);

console.log(merged.get(1)); // uno
console.log(merged.get(2)); // dos
console.log(merged.get(3)); // three

const first = new Map([
  [1, "one"],
  [2, "two"],
  [3, "three"],
]);

const second = new Map([
  [1, "uno"],
  [2, "dos"],
]);

// Merge maps with an array. The last repeated key wins.
const merged = new Map([...first, ...second, [1, "un"]]);

console.log(merged.get(1)); // un
console.log(merged.get(2)); // dos
console.log(merged.get(3)); // three