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.

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