Keyed collections
This chapter introduces collections of data which are indexed by a key; Map
and Set
objects contain elements which are iterable in the order of insertion.
Maps
Map object
A Map
object is a simple key/value map and can iterate its elements in insertion order.
The following code shows some basic operations with a Map
. See also the Map
reference page for more examples and the complete API. You can use a for...of
loop to return an array of [key, value]
for each iteration.
js
const sayings = new Map();
sayings.set("dog", "woof");
sayings.set("cat", "meow");
sayings.set("elephant", "toot");
sayings.size; // 3
sayings.get("dog"); // woof
sayings.get("fox"); // undefined
sayings.has("bird"); // false
sayings.delete("dog");
sayings.has("dog"); // false
for (const [key, value] of sayings) {
console.log(`${key} goes ${value}`);
}
// "cat goes meow"
// "elephant goes toot"
sayings.clear();
sayings.size; // 0
Object and Map compared
Traditionally, objects have been used to map strings to values. Objects allow you to set keys to values, retrieve those values, delete keys, and detect whether something is stored at a key. Map
objects, however, have a few more advantages that make them better maps.
- The keys of an
Object
areStrings
orSymbols
, where they can be of any value for aMap
. - You can get the
size
of aMap
easily, while you have to manually keep track of size for anObject
. - The iteration of maps is in insertion order of the elements.
- An
Object
has a prototype, so there are default keys in the map. (This can be bypassed usingmap = Object.create(null)
.)
These three tips can help you to decide whether to use a Map
or an Object
:
- Use maps over objects when keys are unknown until run time, and when all keys are the same type and all values are the same type.
- Use maps if there is a need to store primitive values as keys because object treats each key as a string whether it's a number value, boolean value or any other primitive value.
- Use objects when there is logic that operates on individual elements.
WeakMap object
A WeakMap
is a collection of key/value pairs whose keys must be objects or non-registered symbols, with values of any arbitrary JavaScript type, and which does not create strong references to its keys. That is, an object's presence as a key in a WeakMap
does not prevent the object from being garbage collected. Once an object used as a key has been collected, its corresponding values in any WeakMap
become candidates for garbage collection as well — as long as they aren't strongly referred to elsewhere. The only primitive type that can be used as a WeakMap
key is symbol — more specifically, non-registered symbols — because non-registered symbols are guaranteed to be unique and cannot be re-created.
The WeakMap
API is essentially the same as the Map
API. However, a WeakMap
doesn't allow observing the liveness of its keys, which is why it doesn't allow enumeration. So there is no method to obtain a list of the keys in a WeakMap
. If there were, the list would depend on the state of garbage collection, introducing non-determinism.
For more information and example code, see also "Why WeakMap?" on the WeakMap
reference page.
One use case of WeakMap
objects is to store private data for an object, or to hide implementation details. The following example is from Nick Fitzgerald's blog post "Hiding Implementation Details with ECMAScript 6 WeakMaps". The private data and methods belong inside the object and are stored in the privates
object, which is a WeakMap
. Everything exposed on the instance and prototype is public; everything else is inaccessible from the outside world because privates
is not exported from the module.
js
const privates = new WeakMap();
function Public() {
const me = {
// Private data goes here
};
privates.set(this, me);
}
Public.prototype.method = function () {
const me = privates.get(this);
// Do stuff with private data in `me`
// …
};
module.exports = Public;
Sets
Set object
Set
objects are collections of unique values. You can iterate its elements in insertion order. A value in a Set
may only occur once; it is unique in the Set
's collection.
The following code shows some basic operations with a Set
. See also the Set
reference page for more examples and the complete API.
js
const mySet = new Set();
mySet.add(1);
mySet.add("some text");
mySet.add("foo");
mySet.has(1); // true
mySet.delete("foo");
mySet.size; // 2
for (const item of mySet) {
console.log(item);
}
// 1
// "some text"
Converting between Array and Set
You can create an Array
from a Set using Array.from
or the spread syntax. Also, the Set
constructor accepts an Array
to convert in the other direction.
Note: Set
objects store unique values—so any duplicate elements from an Array are deleted when converting!
js
Array.from(mySet);
[...mySet2];
mySet2 = new Set([1, 2, 3, 4]);
Array and Set compared
Traditionally, a set of elements has been stored in arrays in JavaScript in a lot of situations. The Set
object, however, has some advantages:
- Deleting Array elements by value (
arr.splice(arr.indexOf(val), 1)
) is very slow. Set
objects let you delete elements by their value. With an array, you would have tosplice
based on an element's index.- The value
NaN
cannot be found withindexOf
in an array. Set
objects store unique values. You don't have to manually keep track of duplicates.
WeakSet object
WeakSet
objects are collections of garbage-collectable values, including objects and non-registered symbols. A value in the WeakSet
may only occur once. It is unique in the WeakSet
's collection.
The main differences to the Set
object are:
- In contrast to
Sets
,WeakSets
are collections of objects or symbols only, and not of arbitrary values of any type. - The
WeakSet
is weak: References to objects in the collection are held weakly. If there is no other reference to an object stored in theWeakSet
, they can be garbage collected. That also means that there is no list of current objects stored in the collection. WeakSets
are not enumerable.
The use cases of WeakSet
objects are limited. They will not leak memory, so it can be safe to use DOM elements as a key and mark them for tracking purposes, for example.
Key and value equality of Map and Set
Both the key equality of Map
objects and the value equality of Set
objects are based on the SameValueZero algorithm:
- Equality works like the identity comparison operator
===
. -0
and+0
are considered equal.NaN
is considered equal to itself (contrary to===
).