Hashmap values in javascript

How to Create a Hashmap in JavaScript (Part 1)

Hash maps are data structures that serve as efficient key-value stores. They are capable of assigning and retrieving data in the fastest way possible. This is because the underlying data structure that hash maps use is an array. A value is stored at an array index determined by plugging the key into a hash function. Because we always know exactly where to find values in a hash map, we have constant access to any of the values it contains. This quick access to values makes a hash map a good choice of data structure whenever we need to store a lot of values but need fast look-up time. In JavaScript, objects are often used to map keys to values as in a hash map, but in this lesson, you’ll create your own implementation of a hash map by building out a HashMap class. You’ll build methods to hash and compress a given key, assign an index at which to store a value, and retrieve that value. To implement a hash map, the HashMap constructor method will create an empty array that will hold values. A hashing function will return an index in the array where the value will be stored. While going through the following exercises, remember that the purpose of this lesson is to gain a deeper understanding of the data structure rather than creating a production-quality data structure.

class HashMap  constructor(size = 0)  this.hashmap = new Array(size) .fill(null); > > 

Hashing

The hashing function is the secret to efficiently storing and retrieving values in a hash map. A hashing function takes a key as input and returns an index within the hash map’s underlying array. This function is said to be deterministic. That means the hashing function must always return the same index when given the same key. This is important because we will need to hash the key again later to retrieve the stored value. We won’t be able to do this unless our hashing function behaves in a predictable and consistent way. Getting an integer representing an index can be done by summing the character codes, or integer representation, of all the characters in the key, a string. A variable can be used to keep track of the running total of the character codes until .hash() is finished looping over the key and the sum can be returned. The code for the hashing function should look something like this:

declare hashCode variable with value of 0 for each character in the key add the character code value to hashCode return hashCode 

class HashMap  constructor(size = 0)  this.hashmap = new Array(size) .fill(null); > hash(key)  let hashCode = 0; for (let i = 0; i  key.length; i++)  hashCode += hashCode + key.charCodeAt(i); > return hashCode > > 

Compression

The current hashing function will return a wide range of integers — some of which are not indices of the hash map array. To fix this, we need to use compression. Compression means taking some input and returning an output only within a specific range. In our hash map implementation, we’re going to have our hashing function handle compression in addition to hashing. This means we’ll add an additional line of code to compress the hashCode before we return it. The updated .hash() should follow these steps:

initialize hashCode variable to 0 for each character in the key add the character code and hashCode to hashCode return compressed hashCode 

class HashMap  constructor(size = 0)  this.hashmap = new Array(size) .fill(null); > hash(key)  let hashCode = 0; for (let i = 0; i  key.length; i++)  hashCode += hashCode + key.charCodeAt(i); > return hashCode % this.hashmap.length; > > 

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Hashmaps: Javascript Edition

A hashmap is a data structure containing an unordered collection of keys that are mapped to values using hashing. Because of their array-like format, hashmaps map key labels to corresponding array indices where values are stored. This removes the limitation of sequential numerical indices for ordering data, in turn allowing the use of flexible keys instead!

Properties

Key/Value

When using hashmaps, data is stored in the form of key/value pairs. The key, used to reference the data, can be any data type. Yes, even an object or an array can be a key when using a hashmap! Similarly, values in hashmaps are allowed to be null.

Hashing

Hashing is a term used to describe the manipulation of a string or input key, and representing it with a hash value. This hash value is typically determined by an algorithm or hash function. Hashing functions are used to return indexes in the array where the value will be stored. Hashing functions take keys as inputs and return an index with the hashmap’s array. Hashing functions are deterministic, meaning the hashing function always returns the same index when provided the same key. Hashing functions must be predictable and consistent to retrieve the stored value via the key. A good hash function should be efficient and assign unique keys. The three most common hash functions are Arithmetic Modular, Truncation, and Folding. Sometimes collisions occur when a hash function generates the same index for more than one key. Four common strategies to handle collisions include Chaining, Open Addressing or resizing the Array or List, Linear probing, and Double Hashing.

Collisions

A collision occurs when multiple keys hash to the same index. This is a situation in which two or more keys produce the same hash value, subsequently occupying the same array index. When this happens, you need to make sure that you can differentiate between conflicting keys. Chaining, specifically separate chaining is one way to resolve this. This happens by storing multiple key-value pairs at the index in question. In this situation, you store all the key-pairs that collide in a linked-list and parse through them. Open addressing is another approach to dealing with collisions. In this situation, all elements are stored in the hash table itself. This means that, at any given point, the hashmap’s size must be greater than or equal to the number of keys stored in it. Another solution, linear probing, involves linearly probing for the next open slot. To insert an element using a given key, compute to find the index at which there is an available slot and place the value in question there. If the slot is full, find the next available index to store the value. Otherwise, try for the next index. Repeat this process until an available slot is found in the hashmap. The last solution, double hashing, uses the idea of applying a second hash function to the key when a collision occurs. In this instance, hashString1() and hashString2() are hash functions and this.length represents the size of our hashmap. We repeat by increasing i when a collision occurs. This can be thought of as:

(hashString1(key) + i * hashString2(key)) % this.length 

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Hashmap values in javascript

HashMap Class for JavaScript

You can download the last stable version from the releases page.

If you like risk, you can download the latest master version, it’s usually stable.

This project provides a HashMap class that works both on Node.js and the browser. HashMap instances store key/value pairs allowing keys of any type.

Unlike regular objects, keys will not be stringified. For example numbers and strings won’t be mixed, you can pass Date ‘s, RegExp ‘s, DOM Elements, anything! (even null and undefined )

HashMap constructor overloads

• new HashMap() creates an empty hashmap
• new HashMap(map:HashMap) creates a hashmap with the key-value pairs of map
• new HashMap(arr:Array) creates a hashmap from the 2D key-value array arr , e.g. [[‘key1′,’val1’], [‘key2′,’val2’]]
• new HashMap(key:*, value:*, key2:*, value2:*, . ) creates a hashmap with several key-value pairs

• get(key:*) : * returns the value stored for that key.
• set(key:*, value:*) : HashMap stores a key-value pair
• multi(key:*, value:*, key2:*, value2:*, . ) : HashMap stores several key-value pairs
• copy(other:HashMap) : HashMap copies all key-value pairs from other to this instance
• has(key:*) : Boolean returns whether a key is set on the hashmap
• search(value:*) : * returns key under which given value is stored ( null if not found)
• delete(key:*) : HashMap deletes a key-value pair by key
• remove(key:*) : HashMap Alias for delete(key:*) (deprecated)
• type(key:*) : String returns the data type of the provided key (used internally)
• keys() : Array returns an array with all the registered keys
• values() : Array returns an array with all the values
• entries() : Array returns an array with Hashmap values in javascript pairs
• size : Number the amount of key-value pairs
• count() : Number returns the amount of key-value pairs (deprecated)
• clear() : HashMap deletes all the key-value pairs on the hashmap
• clone() : HashMap creates a new hashmap with all the key-value pairs of the original
• hash(key:*) : String returns the stringified version of a key (used internally)
• forEach(function(value, key)) : HashMap iterates the pairs and calls the function for each one

All methods that don’t return something, will return the HashMap instance to enable chaining.

Assume this for all examples below

If you’re using this within Node, you first need to import the class

var HashMap = require('hashmap');

map.set("some_key", "some value");
map.get("some_key"); // --> "some value"

Map size / number of elements

var map = new HashMap();
map.set("key1", "val1");
map.set("key2", "val2");
map.size; // -> 2

map.set("some_key", "some value");
map.delete("some_key");
map.get("some_key"); // --> undefined

map.set("1", "string one");
map.set(1, "number one");
map.get("1"); // --> "string one"

A regular Object used as a map would yield «number one»

var key = >;
var key2 = >;
map.set(key, 123);
map.set(key2, 321);
map.get(key); // --> 123

A regular Object used as a map would yield 321

map.set(1, "test 1");
map.set(2, "test 2");
map.set(3, "test 3");
 
map.forEach(function(value, key) 
 console.log(key + " : " + value);
>);
// ES6 Iterators version
for (const pair of map) 
 console.log(`$pair.key> : $pair.value>`)
>

map
 .set(1, "test 1")
 .set(2, "test 2")
 .set(3, "test 3")
 .forEach(function(value, key) 
 console.log(key + " : " + value);
 >);

Copyright (c) 2012 Ariel Flesler

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the «Software»), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED «AS IS», WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF

• (?) Allow extending the hashing function in a AOP way or by passing a service
• Make tests work on the browser

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