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Map2

This is a tiny library which exposes a javascript Map2 class. ES6 Map maps from k -> v. Map2 maps from (k1, k2) -> v.

Why do you need this?

var m = new Map;
m.set([x, y], "dance");
// ...
m.get([x, y]); // undefined! The new list isn't in the map :(

With Map2:

var m = new Map2;
m.set(x, y, "dance");
// ...
m.get(x, y); // "dance" Woo!

The API is heavily based on the API for ES6 maps. Use map.set(k1, k2, v) to set values and map.get(k1, k2) to get them back again.

The map API supports both map.forEach and the new for (var e of map) { ... } syntax to iterate over maps. Be careful with forEach - your function is called with arguments (v, k1, k2) to match the es6 maps. Its kind of weird, and its tripped me up a few times.

This library depends on the javascript Map class, so its avaliable in NodeJS 0.12, IoJS and any modern browser. Any polyfill that adds Map to the global object should work fine too.

Install it with:

% npm install map2

A quick tour:

var Map2 = require('map2');

var m = new Map2;
m.set(1, 2, 3); // Chainable. Returns m.

m.get(1, 2); // Returns 3.

var obj1 = {x:5};
var obj2 = ['an','array'];
m.set(obj1, obj2, "oh hi");

m.get(obj1, obj2); // Returns "oh hi".

m.size; // 2

m.has(obj1, obj2); // true

// To delete things, you can either delete items individually:
m.delete(1, 2); // returns true if something was deleted.

// Or delete everything all at once:
m.clear();

// To iterate, either use .forEach():
m.forEach(function(v, k1, v2) {
  // The callback takes the value first - which is weird, but matches the
  // semantics of Map#forEach.
  console.log(k1, k2, v); // Prints 1,2,3 then {x:5},['an','array'] 
});

// Or use iterator syntax.
for (var entry of m) {
  console.log(entry); // An array of [k1, k2, v]
}

API

Constructor

The Map2 constructor makes a new Map2 and optionally initializes it with data.

var Map2 = require('map2');
var map = new Map2;

Or with data:

var Map2 = require('map2');

// Equalivant of calling map.set(1,2,3); map.set(4,5,6);
var map = new Map2([[1,2,3], [4,5,6]]);

map.set(key1, key2, value)

Set the tuple (key1, key2) to (value). Set returns the map, so you can chain it.

map.set(1, 2, 3).set(4, 5, 6);

map.get(1, 2); // 3.

Map.get(key1, key2)

Get the value stored at the tuple (key1, key2). Note that maps use reference equality, so identical object literals aren't treated the same.

Returns the value stored at (key1, key2) or undefined.

map.set(1, 2, 3);
map.get(1, 2); // 3.

var o1 = {}, o2 = {};
map.set(o1, o2, 10);
map.get(o1, o2); // 10

// Note:
map.set({}, [], 10);
map.get({}, []); // undefined because {} and [] make new objects.

Map.delete(key1, key2)

Deletes the map entry (key1, key2). Returns true if an item was removed from the map, false otherwise.

Map.clear()

Remove all objects from the map. After clear() is called, the object will have a size of 0.

Map.size

The number of entries in the map.

map.size; // 0
map.set(1, 2, 3); // 1
map.set(1, 2, 5); // still 1
map.delete(1, 2); // 0

Iterating

There are two ways to iterate through all the items in a Map2:

  • Use map.forEach
  • Use ES6 Iterators

I haven't done extensive performance testing, but until javascript iterators are better optimized, forEach is probably going to be faster. (Using a closure allocates a function, but using an iterator allocates an object for each iteration of the loop).

Its currently undefined what happens if you add new items to a map while iterating through it. (You may or may not see the new object). That said, removing the item you're currently iterating through is ok.

The order of iteration is not guaranteed. Do not depend on it in any way. (If you care about the order of your items, consider using an array instead).

Map.forEach(function(value, key1, key2) {...})

Call map.forEach to iterate through the map with a custom iterator function. Note the order of arguments here is (value, key1, key2).

map.set(1, 2, 3);
map.set(4, 5, 6);

map.forEach(function(value, key1, key2) {
  // Prints 1, 2, 3 and 4, 5, 6.
  console.log(key1, key2, value);
});

Map.entries()

Get an iterator for the entries in the map. Each entry in the result is an array of [key1, key2, value]. Map.entries is also exposed via map[Symbol.iterator] for usage in for..of loops.

map.set(1, 2, 3);
map.set(4, 5, 6);

for (var entry of map) {
  // Prints 1, 2, 3 and 4, 5, 6.
  console.log(entry[0], entry[1], entry[2]);
}

Or awkwardly from Coffeescript:

map.set 1, 2, 3
map.set 4, 5, 6

iter = map.entries()
while !(v = iter.next()).done
  [key1, key2, value] = v.value
  console.log key1, key2, value

From Coffeescript I recommend just sticking with map.forEach (v, k1, k2) -> ... until better syntax lands in coffeescript.

Map.values()

Returns an iterator which iterates over the values in the map. Note that this iterator does not dedup values. The same value may appear multiple times in the result.

map.set(1, 2, 3);
map.set(4, 5, 6);

for (var v of map.values()) {
  // Prints 3 and 6.
  console.log(v);
}

Map.keys()

Returns an iterator over key pairs in the map. Keys are returned in an array of length 2. Map.keys is included for completeness and API similarity with the raw Map class, but you should probably just use map.entries instead.


Changelog

1.1.2

  • Removed empty inner maps to fix assert.deepEquals comparison behaviour

1.1.1

  • Added the ability to initalize a map from an iterator (or another map).

1.1.0

  • Converted code to typescript. Added typing information and ES module support. Tests are still in coffeescript but they work fine, so eh.
  • Made library always expose (and depend on) javascript iterators. They're supported everywhere, and have been for years at this point.
  • Rewrote inspect method to use newer util.inspect.custom

License

Copyright (c) 2015, Joseph Gentle <me@josephg.com>

Permission to use, copy, modify, and/or distribute this software for any
purpose with or without fee is hereby granted, provided that the above
copyright notice and this permission notice appear in all copies.

THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES WITH
REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY AND
FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT,
INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM
LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR
OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
PERFORMANCE OF THIS SOFTWARE.