en.javascript.info/1-js/5-data-structures/9-map-set-weakmap-weakset/article.md

Map, Set, WeakMap and WeakSet

Now we know the following complex data structures:

• Objects for storing keyed collections
• Arrays for storing ordered collections

But that's not enough for real life. That's why there also exist Map and Set.

Map

Map is a collection of keyed data items. Just like an Object. But the main difference is that Map allows keys of any type.

The main methods are:

• new Map() -- creates the map.
• map.set(key, value) -- stores the value by the key.
• map.get(key) -- returns the value by the key.
• map.has(key) -- returns true if the key exists, false otherwise.
• map.delete(key) -- removes the value by the key.
• map.clear() -- clears the map
• map.size -- is the current elements count.

For instance:

let map = new Map();

map.set('1', 'str1');   // a string key
map.set(1, 'num1');     // a numeric key
map.set(true, 'bool1'); // a boolean key

// remember the regular Object? it would keys to string
// Map keeps the type, so these two are different:
alert( map.get(1)   ); // 'num1'
alert( map.get('1') ); // 'str1'

alert( map.size ); // 3

As we can see, unlike objects, keys are not converted to strings. Any type of key is possible.

Map can also use objects as keys.

For instance:

let john = { name: "John" };

// for every user, let's store his visits count
let visitsCountMap = new Map();

// john is the key for the map
visitsCountMap.set(john, 123);

alert( visitsCountMap.get(john) ); // 123

Using objects as keys is one of most notable and important Map features. For string keys, Object can be fine, but it would be difficult to replace the Map with a regular Object in the example above.

In the old times, before Map existed, people added unique identifiers to objects for that:

// we add the id field
let john = { name: "John", *!*id: 1*/!* };

let visitsCounts = {};

// now store the value by id
visitCounts[john.id] = 123;

alert( visitsCounts[john.id] ); // 123

...But Map is much more elegant.

```smart header="How Map compares keys" To test values for equivalence, Map uses the algorithm SameValueZero. It is roughly the same as the strict equality ===, but the difference is that NaN is considered equal to NaN. So NaN can be used as the key as well.

This algorithm can't be changed or customized.

````smart header="`Map` allows *any* strings as keys, `Object` does not"
In objects, there is a built-in property named `__proto__`. We'll see the details about it later. Here the important thing is that it should not be overwritten. If it is set to a primitive then the write operation is ignored, so we can say that JavaScript protects it to a certain extent.

But if key/value pairs come from the user (like a user naming himself `__proto__`), then we can meet unexpected problems with it. That's actually an endless source of bugs in many well-known JavaScript libraries. `Map`, from the other hand, is totally safe:

```js run
let map = new Map();
map.set('__proto__', 123);
alert( map.get('__proto__') ); // 123

````smart header="Chaining"

Every `map.set` call returns the map itself, so we can "chain" the calls:

```js
map.set('1', 'str1')
  .set(1, 'num1')
  .set(true, 'bool1');
```

Map from Object

When a Map is created, we can pass an array (or another iterable object) with key-value pairs, like this:

// array of [key, value] pairs
let map = new Map([
  ['1',  'str1'],
  [1,    'num1'],
  [true, 'bool1']
]);

Note that this is exactly the format of Object.entries, so we can initialize a map from an object:

let map = new Map(Object.entries({
  name: "John",
  age: 30
}));

Here, Object.entries returns the array of key/value pairs: [ ["name","John"], ["age", 30] ]. That's what Map needs.

Iteration over Map

For looping over a map, there are 3 methods:

• map.keys() -- returns an iterable object for keys,
• map.values() -- returns an iterable object for values,
• map.entries() -- returns an iterable object for entries [key, value], it's used by default in for..of.

For instance:

let recipeMap = new Map([
  ['cucumber', 500],
  ['tomatoes', 350],
  ['onion',    50]
]);

// iterate over keys (vegetables)
for(let vegetable of recipeMap.keys()) {
  alert(vegetable); // cucumber, tomateos, onion
}

// iterate over values (amounts)
for(let amount of recipeMap.values()) {
  alert(amount); // 500, 350, 50
}

// iterate over [key, value] entries
for(let entry of recipeMap) { // the same as of recipeMap.entries()
  alert(entry); // cucumber,50 (and so on)
}

The iteration goes in the same order as the values were inserted. `Map` guarantees that unlike a regular `Object`.

Besides that, Map has a built-in forEach method, similar to Array:

recipeMap.forEach( (value, key, map) => {
  alert(`${key}: ${value}`); // cucumber: 50 etc
});

Set

Set -- is a collection of values, where each value may occur.

The main methods are:

• new Set(iterable) -- creates the set, optionally from an array of values (any iterable will do).
• set.add(value) -- adds a value, returns the set itself.
• set.delete(value) -- removes the value, returns true if value existed at the moment of the call, otherwise false.
• set.has(value) -- returns true if the value exists in the set, otherwise false.
• set.clear() -- removes everything from the set.
• set.size -- is the elements count.

For example, we have visitors coming, and we'd like to remember everyone. But repeated visits should not lead to duplicates. A visitor must be "counted" only once.

Set is just the right thing for that:

let set = new Set();

let john = { name: "John" };
let pete = { name: "Pete" };
let mary = { name: "Mary" };

// visits, some users come multiple times
set.add(john);
set.add(pete);
set.add(mary);
set.add(john);
set.add(mary);

// set keeps only unique values
alert( set.size ); // 3

for(let user of set) {
  alert(user.name); // John (then Pete and Mary)
}

The alternative to Set could be an array of users, and the code to check for duplicates on every insertion using arr.find. But the performance would be much worse, because this method walks through the whole array checking every element. Set is much better optimized internally for uniqueness checks.

Iteration over Set

We can loop over a set either with for..of or using forEach:

let set = new Set(["oranges", "apples", "bananas"]);

for(let value of set) alert(value);

// the same with forEach:
set.forEach((value, valueAgain, set) => {
  alert(value); 
});

Note the funny thing. The forEach function in the Set has 3 arguments: a value, then again a value, and then the target object. Indeed, the same value appears in the arguments twice.

That's made for compatibility with Map where forEach has three arguments.

The same methods as Map has for iterators are also supported:

• set.keys() -- returns an iterable object for values,
• set.values() -- same as set.keys, for compatibility with Map,
• set.entries() -- returns an iterable object for entries [value, value], exists for compatibility with Map.

WeakMap and WeakSet

WeakSet is a special kind of Set that does not prevent JavaScript from memory cleaning. WeakMap is the same thing for Map.

That is: usually the JavaScript engine stores a value in memory while it can potentially be accessed/used.

For instance:

let john = { name: "John" };

// the object can be accessed, john is the reference to it

// overwrite the reference
john = null;

// the object will be removed from memory 

We'll go into more details later, but the example here is somewhat obvious, right? If nothing references the object, it can be safely removed.

Usually, if an object is in a set or an array or another data structure, and the data structure is in memory, then the object remains in memory too. With the exception of WeakMap/WeakSet.

WeakMap/WeakSet does not prevent the object removal from the memory.

For instance, let's start with WeakMap. The first difference from Map is that its keys can only be objects, not primitive values:

let weakMap = new WeakMap();

let obj = {};

weakMap.set(obj, "ok"); // works fine (object key)

*!*
weakMap.set("test", "wops"); // Error, because "test" is a primitive
*/!*

So it stores data for objects. And the data only exists while the object exists. That's handy for situations when we have a main storage for the objects somewhere and need to keep additional information that is only relevant while the object lives.

Let's see an example.

For instance, we have a code that keeps a visit count for each user. The information is stored in a map: a user is the key and the visit count is the value. When a user leaves, we don't want to store his visit count any more.

One way would be to keep track of leaving users and clean up the storage manually:

let john = { name: "John" };

// map: user => visits count
let visitsCountMap = new Map();

// john is the key for the map
visitsCountMap.set(john, 123);

// now john leaves us, we don't need him any more
john = null;

*!*
// but it's still in the map, we need to clean it!
*/!*
alert( visitsCountMap.size ); // 1
// it's also in the memory, because Map uses it as the key

Another way would be to use WeakMap:

let john = { name: "John" };

let visitsCountMap = new WeakMap();

visitsCountMap.set(john, 123);

// now john leaves us, we don't need him any more
john = null;

// there are no references except WeakMap, 
// so the object is removed both from the memory and from visitsCountMap automatically

With a regular Map, the user deletion becomes a tedious task: we not only need to remove the user from it's main storage (be it a variable or an array), but also need to clean up the additional stores like visitsCountMap. And it can become cumbersome in more complex cases when users are managed in one place of the code and the additional structure is at another place and is getting no information about removals.

WeakMap can make things simpler, because it is cleaned up automatically.

WeakMap has only the following methods:

• weakMap.get(key)
• weakMap.set(key, value)
• weakMap.delete(key, value)
• weakMap.has(key)

Please note WeakMap does not support methods clear() and has no size property. Also we can not iterate over it.

That's for technical reasons. If the object is to be removed (like john in the code above), then it WeakMap will be cleaned up automatically. But technically it's not exactly specified when the cleanup happens.

The JavaScript engine decides that. It may choose to perform the memory cleanup immediately or to wait and do the cleaning later when more deletions happen. So, technically the current element count of the WeakMap is not known. The engine may have cleaned it up or not, or did it partially. For that reason, the methods that access the WeakMap as a whole are not supported.

The same refers to WeakSet.

• It keeps a set of objects, an object exists while it is referenced from anywhere else.
• Like Set, it supports add, has and delete, but not size and no iterations.

The limitations may appear inconvenient, but they actually do not prevent WeakMap/WeakSet from doing their main job -- be an "additional" storage of data for objects which are stored/managed at another place.

Summary

• Map -- is a a collection of keyed values.

  The differences from a regular Object:

  • Any keys, objects can be keys.
  • Iterates in the insertion order.
  • Additional convenient methods, the size property.

• Set -- is a collection of unique values.

  • Unlike an array, does not allow to reorder elements.
  • Keeps the insertion order.

• WeakMap -- a variant of Map that allows only objects as keys and removes them once they become unaccessible by other means.

  • It does not support operations on the structure as a whole: no size, no clear(), no iterations.

• WeakSet -- is a variant of Set that only stores objects and removes them once they become unaccessible by other means.

  • Also does not support size/clear() and iterations.

WeakMap and WeakSet are used as "secondary" data structures in additional to the "main" object storage. Once the object is removed from the main storage, so it only stays in WeakMap/WeakSet, they clean up aumatically.