Hash
- during breakfast, hash is a delicious processed meat product
- during coding, a hash is an abstract data type
(image credit: Jeffrey W, CC-BY )
What makes a hash a hash?
a hash is an
- unordered
- indexed (by strings, not numbers)
- collection
- of key/value pairs
In other words, a hash defines a mapping from one group of things to another, like a phone book or a dictionary.
a hash is also known as a...
- map or mapping
- dictionary
- associative array
- lookup table
- key/value store
hash or object?
In JavaScript, a hash is officially called an object.
This is confusing since in every other computer language, "hash" and "object" are quite different things.
Object != Object-Oriented
- Just using an object as a key-value store does not make your code object-oriented.
- For OOP you need a few extra features, especially the keywords
new and this and class
- We discuss OOP in a separate OO_JS track
- In the rest of this lesson we use "object", but elsewhere, you may see "hash" and "object" used interchangeably
Sidebar: Why is it called a hash?
The most common and useful implementation of this data structure uses something called a hashing function to make the lookup efficient. A hashing function allows you to have huge amounts of data and still access a single item very quickly.
Think of the Dewey Decimal System: when you look up a book in the card index, it tells you what aisle and shelf to visit to find that book. You don't need to search through the entire library; you just need to search a single shelf.
Because programmers are humans, and humans can be very literal-minded, people named it based on how it works, rather than what it does or why it does it.
Map and Dictionary are much better metaphors (and in fact there is a recently-introduced JavaScript type called Map which behaves better than the built-in "object" hash; for instance, its keys are not limited to being strings).
Also, hash is a funny word, and programmers love jokes.