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es6-array-map

This is an Map like class for JavaScript to allow Arrays to be used as map keys by array values

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ArrayMap

Description

ArrayMap is a ES6 Map-like object designed for use where arrays are needed to act as map keys by array-values rather than array identity for example

import ArrayMap from "es6-array-map"
arrayMap = new ArrayMap();
arrayMap.set([1,2,3], 10);
console.log(arrayMap.get([1,2,3])); // prints 10 even though [1,2,3] has
                                    // different identity to the list set with

Install

npm install es6-array-map

Documentation

ArrayMap has an identical set of available methods as EcmaScript 2016 Maps provide, the only major differences are the key equality and order of iteration.

Key Equality

The main difference from ES6 maps is that keys are equal if the values of the arrays are equal. For example with keys a=[1,2,3]; and b=[1,2,3]; then these are considered equal. Sub-arrays and objects in the array however are not considered by values so a=[1,2,{}]; and b=[1,2,{}] are not equal as each {} has a different identity.

Order of Iteration

Due to how the ArrayMap is stored (as a tree of values) we don't iterate over the values in insertion order, instead they're iterated using tree-search.

Properties

ArrayMap.length

For sake of consistency with ES6 Maps ArrayMap.length has length 0.

ArrayMap.prototype

Its the prototype for ArrayMap objects, feel free to extend it.

Methods

new ArrayMap(iterable, serialize)

When creating an ArrayMap you can pass in an iterable argument just like es6 Map however you can also pass in an additional argument: serialize, this argument must be a function and will be called on every arrayKey passed to any of the methods, this function must return an Array (or array-like)

function toSet(arr) {
    return Array.from(new Set(arr)).sort();
}

var map = new ArrayMap(null, toSet);
map.set([1,2,2], 'foo')
map.get([2,1]) // 'foo'

map.set([2,1], 'bar')
map.get([1,2,2]) // 'bar'

map.delete([1,1,1,1,1,2,2,2,2,2,2])
map.has([1,2,2]) // false

ArrayMap.prototype.clear()

Removes all [arrayKey, value] pairs from the ArrayMap object.

var map = new ArrayMap();
map.set([1,2,3], 'cats');
map.set([1,2], 12);

map.get([1,2,3]); // 'cats'
map.has([1,2,3]); // true

map.size; // 2
map.clear();

map.has([1,2,3]); // false
map.get([1,2,3]); // undefined
map.size; // 0

ArrayMap.prototype.delete(arrayKey)

Removes a value associated with arrayKey. ArrayMap.prototype.has(arrayKey) will return false afterwards.

var map = new ArrayMap();
map.set([1,2,3], 10);

map.get([1,2,3]); // 10
map.has([1,2,3]); // true

map.delete([1,2,3]); // true
map.delete([2,3]); // false as there wasn't any value associated with [2,3]

map.has([1,2,3]); // false
map.get([1,2,3]); // undefined

Be wary of using array elements by value though:

var map = new ArrayMap();
map.set([1,2,{}], 10);
map.size; // 1

map.delete([1,2,{}]); // false as {} isn't the same object as originally used
                      // in .set
map.size; // 1

ArrayMap.prototype.entries()

Returns a new Iterator that gives [arrayKey, value] pairs for each element of the ArrayMap. Unlike ES6 Maps this is not in insertion order but rather using depth-first-search (yielding as they're seen).

var map = new ArrayMap();
map.set([1,2], 10);
map.set([], 12);
map.set([1,2,3], 30);
map.set([2,3], 40);
map.set(['cats', 'hats'], 40);

Array.from(map.entries);
    // [
    //   [ [], 12 ], // Shortest comes first
    //   [ [ 1, 2 ], 10 ], // Depth-first search on subkeys happens next
    //   [ [ 1, 2, 3 ], 30 ], // So [1,2,3] comes next as it follows from [1,2]
    //   [ [ 2, 3 ], 40 ],
    //   [ ['cats', 'hats'], 40] // Any objects make valid keys in any map
    // ]

ArrayMap.prototype.forEach(callback[, thisArg])

Calls callback once for each key-value pair present in the Map object, in insertion order. If a thisArg parameter is provided to forEach, it will be used as the this value for each callback.

var map = new ArrayMap();
map.set([1,2,3], 12);
map.set([], 10);
map.set([11], 5);

var total = 0;
map.forEach(function(value, key, arrayMap) {
    total = total + value;
});
total; // 27

ArrayMap.prototype.get(arrayKey)

Returns the value associated with arrayKey or undefined if there is none.

var map = new ArrayMap();
map.set([1,2,3], 12);
map.set([], 10);

map.get([1,2,3]); // 12
map.get([]); // 10
map.get([11,12]); // undefined

// undefined is still a valid value though
map.set([1,2], undefined);
map.get([1,2]); // undefined
// so check with .has(arrayKey) first if undefined is a valid value
map.has([1,2]); // true
map.has([110,12]); // false

map.set(['cats', 12], {hats: /sats/}); // Keys and values can be types you want
map.get(['cats', 12]); // {hats: /sats/}, primitives are recommended for keys though
                       // as arrayKey elements are compared using ES6 Map key equality

Note that only the arrayKey itself is considered exempt from ES6 Map equality, the internal entries aren't so the following doesn't work:

var map = new ArrayMap();
var objectA = {};
var objectB = {};

map.set([1, objectA], 12);
map.get([1, objectB]); // undefined as [1, objectA] is a different key to
                       // [1, objectB]

map.set([1, NaN], 10);
map.get([1, NaN]); // 10, as while NaN !== NaN, ES6 map equality is used
                   // for entries within the arrayKey

ArrayMap.prototype.has(arrayKey)

Returns true if there is a value associated with arrayKey, false otherwise.

var map = new ArrayMap();
map.set([1,2,3], 12);

map.has([1,2,3]); // true
map.get([1,2,3]); // 12

map.set([2,3], undefined);
map.has([2,3]); // true, undefined is still a valid value for a key to
                // be associated with
map.has([999, 999]); // false
map.get([999,999]) // undefined, get will still return undefined so you
                   // you may need to explicitly check .has(arrayKey) before
                   // using .get(arrayKey) if undefined is a valid value

ArrayMap.prototype.keys()

Returns an iterator of all keys of the ArrayMap object, the keys are iterated in depth-first-search order (yielding as seen so smaller keys come before the corresponding larger subkeys).

var map = new ArrayMap();
map.set([1,2,3], 12);
map.set([1,2], 10);
map.set([], 'cats');
map.set([4,5], 1001);

Array.from(map.keys()); // [ [], [ 1, 2 ], [ 1, 2, 3 ], [ 4, 5 ] ]

ArrayMap.prototype.set(arrayKey, value)

Sets the value for the given arrayKey in the map, if two arrayKeys have identical elements then they will be set to the same value:

var map = new ArrayMap();
map.set([1,2,3], 12);
map.get([1,2,3]); // 12

map.set([1,2,3], -12); // Setting to an equal array will override original values
map.get([1,2,3]); // -12

var objectA = {};
var objectB = {};
map.set([10, objectA], 20);
map.get([10, objectB]); // undefined as [10, objectA] isn't the same key
                        // as [10, objectB]
map.get([10, objectA]); // 20, if you still can make the arrayKey you can still
                        // access the element

map.set(['cats', 10, {}, new ArrayMap()], 24); // Any objects can be used as parts
                                               // of a key, but its recommended to use primitives
                                               // as primitives are easy to recreate

map.set([], 42); // The empty array is also a valid key
map.get([]); // 42

map.set([NaN], 12);
map.get([NaN]); // 12, although NaN !== NaN elements of the arrayKey are considered
                // equal by ES6 Map key equality

ArrayMap.prototype.values()

Returns an Iterator of values of the map.

var map = new ArrayMap();
map.set([1,2,3], 12);
map.set([3,4], 16);
map.set([], 200);
map.set([12,13], 'llamas');

Array.from(map.keys()); // [200, 12, 16, 'llamas']

ArrayMap.prototype[@@iterator]()

Returns an Iterator of [key, value] pairs, this is equivalent to ArrayMap.prototype.entries()

Credits

Documentation based off Mozilla Developer Network's Map documentation https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Map

Original idea based on http://stackoverflow.com/questions/21838436/map-using-tuples-or-objects

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