array-keyed-map

A map data structure (a.k.a. associative array, dictionary) which maps from arrays of arbitrary values ("paths") to arbitrary values. Like if the JS built-in Map took arrays as keys. Uses the key objects' identities; does not stringify anything, because that way lies madness.

const ArrayKeyedMap = require('array-keyed-map')
const m = new ArrayKeyedMap()

const obj = { x: true }
const objIdentical = { x: true }
const fun = function() {}
const reg = /regexp/

// Set values
m.set([obj],            1)
m.set([obj, fun],       2)
m.set([reg, reg, true], 3)
m.set([],               4)

// Get values
console.log( m.get([obj]) )            // => 1
console.log( m.get([objIdentical]) )   // => undefined
console.log( m.get([obj, fun]) )       // => 2
console.log( m.get([reg, reg, true]) ) // => 3
console.log( m.get([]) )               // => 4

Features:

• Implements all the same methods as Map, with the only API difference of not iterating in insertion order.
• Stores paths compactly as a tree. Shared prefixes are stored once only.
• Algorithms are iterative, because it's faster than recursive. (I checked.)
• Thoroughly unit-tested.
• No dependencies.

API

new ArrayKeyedMap([iterable])

Arguments:

• (optional) iterable: any iterable value of [key, value] entries from which to initialise contents

Returns ArrayKeyedMap akmap.

Array keyed maps are iterable, so you can use them in for-loops, pass them to Array.from, pass them into the constructor to create a copy (let copy = new ArrayKeyedMap(akmap)), etc. (See .entries.)

akmap.set(array, value)

Arguments:

• array: Array of values
• value: any value

Sets the value for the given array.

Objects in the array are treated by identity. The identity of the array object itself is irrelevant.

Returns ArrayKeyedMap akmap: a reference to the same map, handy for chaining multiple .set calls.

akmap.has(array)

Arguments:

• array: Array of values

Returns a Boolean: whether a previously set value exists for that key array.

akmap.get(array)

Arguments:

• array: Array of values

Returns the previously assigned value for this array, or undefined otherwise.

akmap.delete(array)

Arguments:

• array: Array of values

Deletes the value at this exact array. Does not affect other array, even if they are prefixes or extensions of this one. Remember to do this if you no longer need a array: the keys and values are not automatically garbage-collected, even if the objects used as keys go out of scope!

Returns a Boolean: true if an entry with that key existed and was deleted, or false if no such entry was found.

akmap.clear()

Deletes all entries from akmap.

Returns undefined.

akmap.hasPrefix(array)

Arguments:

• array: Array of values

Returns a Boolean: whether the map has some key starting with values matching the given array.

akmap.entries()

Returns an iterator that yields [key, value] for every entry in akmap.

:warning: Note that these are in arbitrary order; not insertion order! This differs from the basic Map!

akmap.keys()

Returns an iterator that yields the key part (type Array) of each entry in akmap.

:warning: Note that these are in arbitrary order; not insertion order! This differs from the basic Map!

akmap.values()

Returns an iterator that yields the value part of each entry in akmap.

:warning: Note that these are in arbitrary order; not insertion order! This differs from the basic Map!

akmap.forEach(callback[, thisArg])

Arguments:

• callback: Function that will be called for each entry in akmap, passing the value, key, and map as arguments.
• (optional) thisArg: Object passed to the callback as the value for this.

Returns undefined.

:warning: Note that these are in arbitrary order; not insertion order! This differs from the basic Map!

Performance characteristics

• The paths are stored as a tree. If multiple paths are stored that share a prefix, the prefix is not duplicated in storage, but shared between them. For example: ['a', 'b'] and ['a', 'c'] have a shared prefix ['a']. Only 1 instance of 'a' is stored, with 'b' and 'c' branching from it.

  This means any operation involving a path scales linearly with that path's length, as it is traversed.

• .size is cached, so it does not traverse the data structure.

• The algorithms are implemented iteratively, because the VM stack is faster than a JS stack.

FAQ

Why is this better than stringify → .join('/') → regular Map?

1. Because you might want your key array to contain objects (by identity) rather than strings, and objects cannot be stringified by identity, so identical objects would get mixed up. But this module can handle that:

   let akmap = new ArrayKeyedMap()
   // These are distinct paths!
   const path1 = [{}, {}, {}]
   const path2 = [{}, {}, {}]
   akmap.set(path1, 1)
   akmap.set(path2, 2)
   console.log(akmap.get(path1)) // → 1
   console.log(akmap.get(path2)) // → 2

2. Even if you only care about the object's content (and not identity), objects may contain cyclic references, which can't be stringified in isolation. But this module can handle that.

   const akmap = new ArrayKeyedMap()
   const cyclic = {}
   // Contains a reference to itself.  How would you stringify this?
   cyclic.x = cyclic
   akmap.set([ cyclic ], 1)
   console.log(akmap.get([ cyclic ])) // → 1

3. Even if you are only using string keys, the separator you choose (e.g. /) may appear as part of your path elements, so the arrays ['a/b'] and ['a', 'b'] would both resolve to the key a/b and overwrite each other.

   So use a separator other than /? Sure, but then you have the same problem with elements possibly containing that.

   So use a sufficiently long probabilistically unguessable separator like 03f2a8291a700b95904190583dba17c4ae1bf3bdfc2834391d60985ac6724940? That wastes RAM/disk. Also this is the code police speaking, you are under assert for crimes against humanity, go to BSD jail.

So please use this module instead of a hack.

What version of JS does this rely on?

ES2015 I think—it uses Maps and Symbols (← caniuse links). At time of writing, it works in any recent Node.js or browser. Except IE, of course.

Development

Pull requests with improvements of any size are appreciated. If anything about the code or documentation is unclear, do ask.

To install the testing dependencies, run npm install.

To run the automated tests and coding style check, run npm test.

License

ISC.