2024-09-09 web, development, javascript
Hash Tables in JavaScript
By O. Wolfson
Hash Tables in JavaScript
Hash tables are a type of data structure that let you store information using a "key" and a "value" pair. They're kind of like a phone book, where you can look up a name (the key) and find the corresponding phone number (the value).
Compared to a regular list, where you have to search through every item to find what you're looking for, hash tables make it much faster to find a specific item. This is because the hash function can quickly jump to the spot in memory where the data is stored, based on the key you're searching for.
When we have a large amount of data and we want to look up specific values, it can be very inefficient to search through all the data. Hash tables provide a more efficient way to store and look up data because they use a mathematical function called a hash function to map keys to specific locations in memory.
By using a hash function, we can avoid the need to search through the entire dataset to find the value we are looking for. Instead, we can use the key to compute the memory address where the value is stored and retrieve it directly. This means that we can look up values much more quickly than we could using a traditional list or array.
Hash tables are commonly used in programming because they provide a fast and efficient way to store and retrieve data. They are used in a wide range of applications, from databases and search engines to web servers and operating systems. In particular, hash tables are often used in cases where we need to quickly look up data based on a specific key, such as in a dictionary or phone book.
Here's a simple implementation of a hash table in JavaScript:
This implementation uses an array of arrays to store key-value pairs. The _hash method is used to compute an index in the array for a given key. The set method stores a key-value pair in the array, and the get method looks up a value by its key.
A Use-Case: Counting Word Frequencies
Here's an example of how hash tables can be used to count word frequencies in a text:
This code creates a hash table to store word frequencies. It uses the split method to break up the text into words, and then loops over the words, incrementing their frequency counts in the hash table. Finally, it looks up the frequencies of the words "the" and "fox" using the get method.
Alternative Implementations
JavaScript does not have a built-in implementation of a hash table, but there are several libraries and modules that provide similar functionality. One such example is the "Map" object in JavaScript.
The Map object in JavaScript is similar to a hash table in that it allows you to store key-value pairs and retrieve them by key. Here is an example of using a Map object to count the frequency of words in a string: