Hash Table

Overview

A hash table is a data structure that stores data as key value pairs. It uses a hashing function to map each key to a position in memory which gives fast access when storing or retrieving values by that key.

JavaScript provides objects and Maps as hash table based collections.

• Using a plain object:

  const user = {};
  
  user['age'] = 25;
  user['name'] = 'John';
  
  console.log(user); // {age: 25, name: 'John'}
  console.log(user['age']); // 25

• Using Map:

  const user = new Map();
  
  user.set('age', 25);
  user.set('name', 'John');
  
  console.log(user); // Map(2) {'age' => 25, 'name' => 'John'}
  console.log(user.get('age')); // 25

Background

A hash table is built on three core components:

1. Key: The identifier used to access stored data.
2. Hash Function: A process that turns the key into a numeric hash code.
3. Buckets: An array in memory that holds the entries.

How it works:

1. Input:

   You insert the value John using the key name.

2. Hashing:

   The hash function converts name into a number such as 417. This example uses character code sums. Real systems may use stronger functions for better distribution.

3. Indexing:

   The table reduces that number to a valid index with a modulo step such as 417 % 10 which becomes 7. Again, real systems may use different indexing methods.

4. Storage:

   The pair is stored in bucket 7.

When you request name, the table repeats the hash and index calculations and retrieves the entry from bucket 7 immediately O(1).

Handling collisions:

A collision happens when two different keys map to the same bucket.

Solution:

• Chaining:

  Each bucket holds a small list of entries. If two keys map to bucket 7, both are stored in that list.

  Lookups check bucket 7 in O(1) time then scan the small list in O(n) time to find the matching key.

  Keeping entries well distributed across buckets is important for performance because it keeps these lists short which reduces how often lookups fall back to O(n) work.