a hash is an
In other words, a hash defines a mapping from one group of things to another, like a phone book or a dictionary.
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.
new and this and class
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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