a hash is an abstract data type

corned beef hash

Hash Example

let hashMenu = {
    'corned beef hash': 3.69,
    'roast beef hash': 3.89,
    'homestyle hash': 4.47,

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?

and 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.

Setting and Getting Hash Values with square brackets

let states = {}

states['CA'] = 'California'
states['MA'] = 'Massachusetts'
states['NY'] = 'New York'

states['MA']                // 'Massachusetts'
states['MA'].toUpperCase()  // 'MASSACHUSETTS'

Setting and Getting Hash Values with dots

let states = {}

states.CA = 'California'
states.MA = 'Massachusetts'
states.NY = 'New York'

states.MA                // 'Massachusetts'
states.MA.toUpperCase()  // 'MASSACHUSETTS'

Dots vs. Brackets

Dots are prettier than square brackets, but less versatile:

> capitals = {}

> capitals.New York = 'Albany'
capitals.New York = 'Albany'
SyntaxError: Unexpected identifier

> capitals.'New York' = 'Albany'
capitals.'New York' = 'Albany'
SyntaxError: Unexpected string

> capitals['New York'] = 'Albany'
> capitals
{ 'New York': 'Albany' }

Dots vs. Brackets vs. Variables

Given this code:

var items = {
    brick: 'red'
var item = 'brick'

Two of these expressions look for a key named item but only one looks for a key named the contents of the variable named item

items.item      // undefined
items['item']   // undefined
items[item]     // "red"

This is confusing!

Hash literals

a Hash can be defined literally (inline) with curly braces, like this:

states = {
           CA: 'California',
           MA: 'Massachusetts',
           NY: 'New York'

states['MA']  // 'Massachusetts'
states.MA     // also 'Massachusetts'

Looping through a hash with

for (let state in states) {

Note: use "for...of" for arrays, use "" for hashes -- see this article for more detail.

WARNING: remember the let or you will be defining a global variable named state

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 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 names (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.

JS Object Hash Rules

  • All keys are strings

  • Beware of using these as keys, since they get converted to strings in unexpected ways:

    • null
    • undefined
    • '' (empty string)
    • false or true
    • 0 (or any number)


To remove a key-value pair from a hash, use the keyword delete.

states = {
            CA: 'California',
            MA: 'Massachusetts',
            NY: 'New York'
{ CA: 'California', MA: 'Massachusetts', NY: 'New York' }
> delete states.MA
> states
{ CA: 'California', NY: 'New York' }

fake delete

You can get a similar effect by setting the value to null or undefined, but beware: the key remains!

> states.CA = null
> states.NY = undefined
> states
{ CA: null, NY: undefined }
> for (state of states) { console.log(state) }

You probably shouldn't do this.

Hash methods

let rectangle = {
    height: 10,
    width: 8,
    area: function() {
        return this.height * this.width;

rectangle.height   //=> 10
rectangle.area()   //=> 80

this is a magic word that means "this object I'm in right now"