## DEV Community

Oğuzhan Olguncu

Posted on • Updated on • Originally published at ogzhanolguncu.com

# Hash tables explained in Javascript

Hash tables are incredibly powerful and versatile beasts. They let you store your data in a key-value format similar to the plain javascript object. But for a hash table to be effective it should have unique keys. You might think this as a phone book, each name corresponds to a phone number, so you know exactly where to look.

``````   Adam    --> +3435232323
Ben     --> +2323231313
Cambell --> +4566464534
``````

If you type Adam in your phone it immediately returns +3435232323 with zero down-time. Because the hash table search has `O(1)` complexity just like delete and insertion.

Another example of this might be storing movies in alphabetical order. As you can imagine two different movie names might start with the same letter. So, you also need to run through these movies until finding the right one. Let's suppose we are looking for The Abyss.

``````A
B
C
.
.
.

T ---> The Lord of the Rings
The Godfather
The Abyss  // Found it.
``````

It first ran through alphabetically then ran through movies to find the right one.

But If you have two Adam on your phone with different phone numbers or two The Abyss on your movie list, now you got collision problem, meaning two keys are identical. This should be avoided in all cases.

The reason behind unique keys are to decrease time of our `get()`, `set()`, `delete()` and avoid collision. Let's implement unique table using Map.

``````const hashTable = new Map([
['Ben', +12346],
['Cambell ', +123457],
])
hashTable.get("Cambell") // +123457

hashTable.delete("Cambell") // Deletes Cambell from list

hashTable.set("Cambell", +123457) // Adds Cambell to list
[['Adam', +12345], ['Ben', +12346], ['Cambell ', +123457]]
``````

To prevent collision `Map` has useful method called `has()` which retuns true if `Map` has the searched element. I advise you to use `has()` before setting up new values to avoid collision.

``````const hashTable = new Map([
['Ben', +12346],
['Cambell ', +123457],
])
hashTable.has("Cambell") // true
``````

To override one of the values you can use `set()` again.

``````const hashTable = new Map([
['Ben', +12346],
['Cambell ', +123457],
])
hashTable.set("Cambell", +12345678)
[['Adam', +12345], ['Ben', +12346], ['Cambell ', +12345678]]
``````

Now let's suppose you've saved Adam-1 and Adam-2 under name Adam. Usually this operation takes `O(n)` because you need to iterate over Adam array, but in our case we are using `Map` this means `get()` operation has constant time with `O(1)` complexity. It's little tricky to work with it, but let's see.

``````const hashTable = new Map([
First, we use `get()` as usual then create new `Map` from returned value, now we can again use `get()` for nested children.