Map, Set, WeakMap and WeakSet

Now we’ve learned about the following complex data structures:

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, undefined if key doesn’t exist in map.
• 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 – returns the current element 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 convert 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 their 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
visitsCounts[john.id] = 123;

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

…But Map is much more elegant.

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) with key-value pairs, like this:

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

There is a built-in method Object.entries(obj) that returns an array of key/value pairs for an object exactly in that format.

So we can initialize a map from an object like this:

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 for keys,
• map.values() – returns an iterable for values,
• map.entries() – returns an iterable 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, tomatoes, 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,500 (and so on)
}

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

recipeMap.forEach( (value, key, map) => {
  alert(`${key}: ${value}`); // cucumber: 500 etc
});

Set

A Set is a collection of values, where each value may occur only once.

Its 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 for compatibility with Map where forEach has three arguments. Looks a bit strange, for sure. But may help to replace Map with Set in certain cases with ease, and vice versa.

The same methods 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 removing its items from memory. WeakMap is the same thing for Map.

As we know from the chapter زباله‌روبی, JavaScript engine stores a value in memory while it is reachable (and can potentially be 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

Usually, properties of an object or elements of an array or another data structure are considered reachable and kept in memory while that data structure is in memory.

For instance, if we put an object into an array, then while the array is alive, the object will be alive as well, even if there are no other references to it.

Like this:

let john = { name: "John" };

let array = [ john ];

john = null; // overwrite the reference

// john is stored inside the array, so it won't be garbage-collected
// we can get it as array[0]

Or, if we use an object as the key in a regular Map, then while the Map exists, that object exists as well. It occupies memory and may not be garbage collected.

For instance:

let john = { name: "John" };

let map = new Map();
map.set(john, "...");

john = null; // overwrite the reference

// john is stored inside the map,
// we can get it by using map.keys()

WeakMap/WeakSet are fundamentally different in this aspect. They do not prevent garbage-collection of key objects.

Let’s explain it starting with WeakMap.

The first difference from Map is that WeakMap keys must be objects, not primitive values:

let weakMap = new WeakMap();

let obj = {};

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

// can't use a string as the key
weakMap.set("test", "Whoops"); // Error, because "test" is not an object

Now, if we use an object as the key in it, and there are no other references to that object – it will be removed from memory (and from the map) automatically.

let john = { name: "John" };

let weakMap = new WeakMap();
weakMap.set(john, "...");

john = null; // overwrite the reference

// john is removed from memory!

Compare it with the regular Map example above. Now if john only exists as the key of WeakMap – it is to be automatically deleted.

WeakMap does not support iteration and methods keys(), values(), entries(), so there’s no way to get all keys or values from it.

WeakMap has only the following methods:

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

Why such a limitation? That’s for technical reasons. If an object has lost all other references (like john in the code above), then it is to be garbage-collected 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 a WeakMap is not known. The engine may have cleaned it up or not, or did it partially. For that reason, methods that access WeakMap as a whole are not supported.

Now where do we need such thing?

The idea of WeakMap is that we can store something for an object that should exist only while the object exists. But we do not force the object to live by the mere fact that we store something for it.

weakMap.set(john, "secret documents");
// if john dies, secret documents will be destroyed automatically

That’s useful 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 look at an example.

For instance, we have 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 their visit count anymore.

One way would be to keep track of users, and when they leave – clean up the map 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 anymore
john = null;

// but it's still in the map, we need to clean it!
alert( visitsCountMap.size ); // 1
// and john is 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 anymore
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, cleaning up after a user has left becomes a tedious task: we not only need to remove the user from its 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 in another place and is getting no information about removals.

WeakSet behaves similarly:

• It is analogous to Set, but we may only add objects to WeakSet (not primitives).
• An object exists in the set while it is reachable from somewhere else.
• Like Set, it supports add, has and delete, but not size, keys() and no iterations.

For instance, we can use it to keep track of whether a message is read:

let messages = [
    {text: "Hello", from: "John"},
    {text: "How goes?", from: "John"},
    {text: "See you soon", from: "Alice"}
];

// fill it with array elements (3 items)
let unreadSet = new WeakSet(messages);

// use unreadSet to see whether a message is unread
alert(unreadSet.has(messages[1])); // true

// remove it from the set after reading
unreadSet.delete(messages[1]); // true

// and when we shift our messages history, the set is cleaned up automatically
messages.shift();

// no need to clean unreadSet, it now has 2 items
// (though technically we don't know for sure when the JS engine clears it)

The most notable limitation of WeakMap and WeakSet is the absence of iterations, and inability to get all current content. That may appear inconvenient, but does 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

Regular collections:

• Map – is 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.

Collections that allow garbage-collection:

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

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

WeakMap and WeakSet are used as “secondary” data structures in addition to the “main” object storage. Once the object is removed from the main storage, if it is only found in the WeakMap/WeakSet, it will be cleaned up automatically.