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Different type of Collections in JavaScript

Different type of Collections in JavaScript

When you are using a programming language, you need use something for keep your data information. Someting as a container.

For do it you need to use data structure, that defines a particular way to organizing the data.

Collection is a type of data structure that implement some kind of iterable interface and they internally use the same iteration method. This is easy to view in language like Java when exist a inheritance hierarchy defined by an interface.

In JavaScript is similar, but is less complete and, because is not possible define interfaces, we use the inheritance based in Object.

Object, Array and Functions

The first thing that you need to understand is that everything is a object in JavaScript unless primitives types, that are:

boolean, number, string, null, undefined  

About the collections, basically an object, array and function are instance of object, so you can use as data structures. The difference how to internally the iteration is defined.

If you check functional libraries as lodash you can see that exist method for collection and methods specific for arrays and objects. Exists little differences between them that determine how they should be iterated: For example, an array is a collection of element in order, while object is a collection of keys, and is not important the order.

But consider an Object (and Functions) as data structure at this moment with a ECMAScript6 is an error: Now we have a explicit class for do it.


A Map is the new ECMAScript6 data structure. You can use a Object as a Map, but this have some problems:

  • An Object has a prototype, so there are default keys in the map. However, this can be bypassed using map = Object.create(null).

  • The keys of an Object are Strings, where they can be any value for a Map. (At this point, Object more like Dictionary structure).

  • You can get the size of a Map easily while you have to manually keep track of size for an Object.


This is another new ECMAScript6 data structure. Is the same that Map, but only can store for unique values.

WeakMap & WeakSet

First we need to understand what is a Weak Reference and how works the reference in JavaScript.

All things in JavaScript unless primitives types (remember the first lines of this posts) works with references. If you create a new object that reference another object, basically you are linking the reference, not copy the object into another new:

> var var1 = {};
> var var2 = var1;

> var1.foo = 123;
> var2.foo

Basically a clone method that is very typical in other language doesn't exist natively.

When you use a Map or Set, internally the data structure is created using two arrays: One for store the key and one for store the value that is saved clone the value, not with linking method.

With WeakMap and WeakSet we use collections that store the key based in how works object natively: we are telling the garbage collector can remove the value if there is no reference.

Because of references being weak, WeakMap keys are not enumerable (i.e. there is no method giving you a list of the keys). If they were, the list would depend on the state of garbage collection, introducing non-determinism. If you want to have a list of keys, you should maintain it yourself.

A little example about the difference of behaivor between Map and WeakMap:

> map = new Map();
> var foo = 'bar'
> map.set('foo', foo)
> wmap = new WeakMap()
> var objt = {foo: 'bar'}
> map.get('foo')
> foo = 'it was changed'
'it was changed'  
> map.get('foo');
> wmap.set(objt, 'bar')
> map.get('foo');
> wmap.get(objt)
> objt = {foo: 'test'}
{ foo: 'test' }
> wmap.get(objt)

You need more?

Maybe you throw at fault other data structures. What's happens with Heap, List, MultiMap?

Of course you can create this data structures based in others, but are not available nativelly. I recommend use the library collections that provide you more data structures and the native .

But I miss more interesting structures such as Tree, B-Tree.