`Map`

The Map object holds key-value pairs and follows insertion order.

Any value may be used as either the key or value.

Maps are similar to Sets but have differences.

Both however, use insertion order: the first element inserted takes the first index, and so on.

The most recent is the last index.

A key in a Map is unique and can't be used more than once.

Also note that Map objects are iterated over by key-value pairs in insertion order.

Maps are required to be implemented such that access times are on average better than linear time.

This is very efficient because they can internally be represented as hash-tables with \(\Omega(1)\) time complexity.

A search tree can be \(\Omega(\log(N))\), and any other data structure can be used - if its better than \(\Omega(N)\)

Here are some basic Map operations:

Objects and Maps share the notion that they are formed via key-value pairs.

Within both one can set keys to values, retrieve values, delete pairs, detect if a key is stored, and more.

Historically, Object in languages like JavaScript have been used as Map actually.

Here are some important differences that can make Maps more efficient sometimes:

Accidental keys: Map has no keys by default. It only contains what one puts in it.
- Object has default keys that can collide with explicit keys.
Key types: Map keys can pair with any value. Object keys must be a String.
Key order: Maps are in insertion order. Object orders can be more complex.
Size: Map size is easily found via the size property. Objects are not directly iterable.
- Hence, one has to create methods to find things such as size.
Performance: Map performs better with frequent actions. Object is not optimized for frequent changes.

Regarding serialization/parsing, a downside of Map is that there is no native support for either.

One has to make their own, typically by using JSON.stringify() and JSON.parse().

Another key difference is setting properties:

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

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

Remember to not use Object methods

The corrected example:

Map has many methods. We will list the most important ones:

Constructor: Map()
get() - Retrieve the value associated to the passed key
clear() - remove all key-value pairs!
keys(), values() and entries() - returns an iterator object that goes over: keys, values and [key, value] arrays
forEach() - effectively iterates over the key-value pairs in insertion order

NaN can actually be used as a key in a Map!

A practical use is transforming an Array into a Map:

Map