reverse-iterable-map

The ReverseIterableMap object is a reverse-iterable map implementation based on the built-in Map object.

It implements a linked list meaning that each element in the internal data structure (a Map object) knows about its previous and next element; thus, allowing iteration in both directions at the same time. This implies added memory usage because in addition to its key and value, an element also needs to store the two references for the previous and next elements.

Links:

• npmjs.com/package/reverse-iterable-map
  • on BundlePhobia
• github.com/kleinfreund/reverse-iterable-map

See also:

• ReverseIterableArray: reverse-iterable-array
• ReverseIterableSet: reverse-iterable-set

Table of contents

• Installation
• Usage
• Examples
• Tests
• Documentation
  • Constructor
  • size
  • [Symbol.toStringTag]
  • clear()
  • delete()
  • entries()
  • forEach()
  • forEachReverse()
  • get()
  • has()
  • iteratorFor()
  • keys()
  • reverseIterator()
  • set()
  • setFirst()
  • [Symbol.iterator]()
  • values()
• Why this was implemented
• How to update this package

Installation

npm install reverse-iterable-map

Usage

import ReverseIterableMap from 'reverse-iterable-map';

const map = new ReverseIterableMap();

Examples

For some live usage examples, clone the repository and run the following:

npm install
npm run build
npm start

Then, open localhost:8080/examples in a browser.

Tests

In order to run the tests, clone the repository and run the following:

npm install
npm test

Documentation

Disclaimer: The documentation section copies a lot of content from the Map documentation on the Mozilla Developer Network.

A ReverseIterableMap object iterates its elements in insertion or reverse-insertion order — a for...of loop returns an array of [key, value] for each iteration.

Constructor

Syntax

new ReverseIterableMap([iterable])

Parameters:

• iterable: An Array or other iterable object whose elements are key-value pairs.

Usage

• Without arguments

  const map = new ReverseIterableMap();

• Array

  const map = new ReverseIterableMap([1, 2, 3].entries());

• Array of Arrays

  const map = new ReverseIterableMap([[0, 1], [1, 2], [2, 3]]);

• Map

  const builtInMap = new Map([['key1', 1], ['key2', 2], ['key3', 3]]);
  const map = new ReverseIterableMap(builtInMap);

• NodeList

  const nodeList = document.querySelectorAll('a');
  const map = new ReverseIterableMap(nodeList.entries());

size

The size accessor property returns the number of elements in a ReverseIterableMap object.

Syntax

map.size

Usage

const map = new ReverseIterableMap()
  .set('one', 'I')
  .set('two', 'lack')
  .set('three', 'creativity');

map.size
//> 3

[Symbol.toStringTag]

The ReverseIterableMap[@@toStringTag] property has an initial value of “ReverseIterableMap”.

clear()

Syntax

map.clear();

Return value:

undefined.

Usage

// Clears the underlying Map object
// Sets the first and last node references to `null`
map.clear();

delete()

Syntax

map.delete(key);

Parameters:

• key: Required. The key of the element to remove from the ReverseIterableMap object.

Return value:

• Boolean: Returns true if an element in the ReverseIterableMap object existed and has been removed, or false if the element does not exist.

Usage

const map = new ReverseIterableMap(['hey', 'beauty'].entries());

map.delete(0);
//> true (deletes the key value pair [0, 'hey'])

map.delete(1);
//> true (deletes the key value pair [1, 'beauty'])

map.delete(2);
//> false (key 2 does not exist in map)

entries()

Returns an iterator containing the [key, value] pairs for each element in the ReverseIterableMap object in insertion order.

An iterator containing the same pairs in reverse-insertion order can be obtained with entries().reverseIterator().

Syntax

map.entries();

Return value:

A new ReverseIterableMap iterator object.

Usage

const map = new ReverseIterableMap([1, 2, 4].entries());

const iterator = map.entries();

iterator.next().value;
//> [0, 1]

iterator.next().value;
//> [1, 2]

iterator.next().value;
//> [2, 4]

iterator.next().value;
//> undefined

forEach()

The forEach() method executes a provided function once for each [key, value] pair in the ReverseIterableMap object, in insertion order.

Syntax

map.forEach(callback[, thisArg]);

Parameters:

• callbackfn: Function to execute for each element. The callbackfn gets passed references to the current value, key, and a reference to the ReverseIterableMap object itself.

• thisArg: Value to use as this when executing callback.

Return value:

undefined.

Usage

const map = new ReverseIterableMap([
  [0, 'a'],
  [1, 'b'],
  [2, 'c']
]);

map.forEach(value => {
  console.log(value);
});
//> a
//> b
//> c

map.forEach(function (value, key, mapReference) {
  console.log(key, value, mapReference.size);
});
//> 0 a 3
//> 1 b 3
//> 2 c 3

forEachReverse()

The forEachReverse() method executes a provided function once per each [key, value] pair in the ReverseIterableMap object, in reverse-insertion order.

Syntax

map.forEachReverse(callback[, thisArg]);

Parameters:

• callback: Function to execute for each element. The callbackfn gets passed references to the current value, key, and a reference to the ReverseIterableMap object itself.
• thisArg: Value to use as this when executing callback.

Return value:

undefined.

Usage

const map = new ReverseIterableMap([
  [0, 'a'],
  [1, 'b'],
  [2, 'c']
]);

map.forEachReverse(value => {
  console.log(value);
});
//> c
//> b
//> a

map.forEachReverse(function (value, key, mapReference) {
  console.log(key, value, mapReference.size);
});
//> 2 c 3
//> 1 b 3
//> 0 a 3

get()

Syntax

map.get(key);

Parameters:

• key: Required. The key of the element to return from the ReverseIterableMap object.

Return value:

• Returns the element associated with the specified key or undefined if the key can't be found in the ReverseIterableMap object.

Usage

const map = new ReverseIterableMap(['hey', 'beauty'].entries());

map.get(0);
//> 'hey'

map.get(1);
//> 'beauty'

map.get(2);
//> undefined

has()

Syntax

map.has(key);

Parameters:

• key: Required. The key of the element to test for presence in the ReverseIterableMap object.

Return value:

• Boolean: Returns true if an element with the specified key exists in the ReverseIterableMap object; otherwise false.

Usage

const map = new ReverseIterableMap(['hey', 'beauty'].entries());

map.has(0);
//> true

map.has(1);
//> true

map.has(2);
//> false

iteratorFor()

Returns an iterator containing the [key, value] pairs for each element in the ReverseIterableMap object in insertion order starting with the pair specified by the key parameter.

This allows starting iteration at a specific element in the map.

An iterator containing the same pairs in reverse-insertion order can be obtained with iteratorFor().reverseIterator().

Syntax

map.iteratorFor(key);

Parameters:

• key: Required. The key of the element to start iterating from.

Return value:

A new ReverseIterableMap iterator object.

Usage

const map = new ReverseIterableMap([1, 2, 4].entries());

// Iterator, starting at the element with key 1.
const iterator = map.iteratorFor(1);

iterator.next().value;
//> [1, 2]

iterator.next().value;
//> [2, 4]

iterator.next().value;
//> undefined

// Reverse-iterator, starting at the element with key 1.
const reverseIterator = map.iteratorFor(1).reverseIterator();

reverseIterator.next().value;
//> [1, 2]

reverseIterator.next().value;
//> [0, 1]

reverseIterator.next().value;
//> undefined

keys()

Returns an iterator containing the keys for each element in the ReverseIterableMap object in insertion order.

An iterator containing the same keys in reverse-insertion order can be obtained with keys().reverseIterator().

Syntax

map.keys();

Return value:

A new ReverseIterableMap iterator object.

Usage

const map = new ReverseIterableMap([1, 2, 4].entries());

const iterator = map.keys();

iterator.next().value;
//> 2

iterator.next().value;
//> 1

iterator.next().value;
//> 0

iterator.next().value;
//> undefined

reverseIterator()

In theory, following the semantics of [Symbol.iterator](), this should be [Symbol.reverseIterator](). However, as a developer, I cannot define a well-known symbol myself and make use of it. In the future, the a proposal like The ReverseIterable Interface, by Lee Byron might make it’s way into the specification. For the time being, the reverseIterator() function serves the same purpose.

Syntax

map.reverseIterator();

Return value:

The map reverse-iterator function, which is the entries().reverseIterator() function by default.

Usage

const map = new ReverseIterableMap([1, 2, 4].entries());

const reverseIterator = map.reverseIterator();

reverseIterator.next().value;
//> [2, 4]

reverseIterator.next().value;
//> [1, 2]

reverseIterator.next().value;
//> [0, 1]

reverseIterator.next().value;
//> undefined

set()

Syntax

map.set(key, value);

Parameters:

• key: Required. The key of the element to add to the ReverseIterableMap object.
• value: Required. The value of the element to add to the ReverseIterableMap object.

Return value:

• The ReverseIterableMap object.

Usage

const map = new ReverseIterableMap();

map.set('you', 'beauty');
//> map

map.set('the-magic-key', 'hey');
//> map

The set() method returns a reference to the map object. This makes the set operation chainable.

const map = new ReverseIterableMap()
  .set('key', '… is spelled like tea')
  .set('hey', '… somehow ney');

setFirst()

The setFirst() method functions like set() but uses reverse-insertion order.

Syntax

map.set(key, value);

Parameters:

• key: Required. The key of the element to add to the ReverseIterableMap object.
• value: Required. The value of the element to add to the ReverseIterableMap object.

Return value:

• The ReverseIterableMap object.

Usage

const map = new ReverseIterableMap()
  .setFirst('key1', 'was inserted first')
  .setFirst('key2', 'was inserted last');

map.values().next().value;
//> 'was inserted last'

map.values().reverseIterator().next().value;
//> 'was inserted first'

[Symbol.iterator]()

Returns the map iterator function. By default, this is the entries() function.

Syntax

map[Symbol.iterator]();

Return value:

The map iterator function, which is the entries() function by default.

Usage

const map = new ReverseIterableMap([1, 2, 4].entries());

const iterator = map[Symbol.iterator]();

iterator.next().value;
//> [0, 1]

iterator.next().value;
//> [1, 2]

iterator.next().value;
//> [2, 4]

iterator.next().value;
//> undefined

values()

Returns an iterator containing the values for each element in the ReverseIterableMap object in insertion order.

An iterator containing the same values in reverse-insertion order can be obtained with values().reverseIterator().

Syntax

map.values();

Return value:

A new ReverseIterableMap iterator object.

Usage

const map = new ReverseIterableMap([1, 2, 4].entries());

const iterator = map.values();

iterator.next().value;
//> 1

iterator.next().value;
//> 2

iterator.next().value;
//> 4

iterator.next().value;
//> undefined

Why this was implemented

Part of the additions to ECMAScript 2015 are the iteration protocols: Iterable and iterator. The former allows arbitrary objects to become iterable. Following the rules of the protocol gives one iteration capabilities via the following techniques:

• for...of statement
• Array.from() method
• Spread syntax

However, only the iteration in one direction is considered by the specification at the time. This means that we only get forward-iteration by default. There is a draft for a proposal to add a ReverseIterable interface to the specification: “The ReverseIterable Interface” by Lee Byron.

Now, with the iteration protocols, we could redefine the iteration behavior for our purpose and make an object backwards-iterable. At the same time, this means losing the ability to iterate forwards.

If you need both a forwards- and backwards-iterable object, this implementation might be for you.

But why a map?

That’s what I needed. To be precise, I needed to access an iterator at a specific location in my data structure and be able to iterate in both directions.

I tried to stick to the Map interface as close as possible.

Implementing a reverse-iterable array, for example, can be accomplished by using the same techniques of this implementation.

How to update this package

… because I keep forgetting that.

Let’s assume a minor update was made. First of all, the working directory needs to be cleaned up; all changes need to be committed. It’s important to run the build script to make sure new CommonJS and ES modules are compiled from the TypeScript source module.

npm run build
git commit -am "Implemented extremely nice feature"

Next, make sure you have a valid NPM authentication token set up:

npm whoami

If not, do that with npm login and continue. We now create a new commit with the next minor version tag and update the package.json. Actually, the following command will do that:

npm version minor # See `npm version --help` for more options

This creates a new git tag that we need to publish as well. With that, we can now publish the new version.

git push && git push --tags
npm publish

That’s it.