Javascript Hash Array Mapped Trie
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Latest commit 207584b Sep 8, 2017 @mattbierner committed on GitHub Merge pull request #26 from sonicdoe/patch-1
Fix grammatical error


Javascript Hash Array Mapped Trie


The hash array mapped trie is a persistent map data structure with good lookup and update performance. This library provides an immutable map with an API that resembles ES6's Map.

var hamt = require('hamt');

// Keys can be any string and the map can store any value.
var h = hamt.empty
    .set('key', 'value')
    .set('object', { prop: 1 })
    .set('falsy', null);

h.size === 0
h.has('key') === true
h.has('falsy') === true
h.get('key') === 'value'

// Iteration
for (let [key, value] of h)
    console.log(key, value);

Array.from(h.values()) === [{ prop: 1 }, 'value'], null];

// The data structure is fully immutable
var h2 = h.delete('key');
h2.get('key') === undefined
h.get('key') === 'value'

Benchmarks show that this library performs well, even as the size of the map becomes very large. Check out Hamt+ if you want support for additional features such as custom key types and transient mutation.


Source code is in hamt.js and generated from lib/hamt.js. The library supports node, AMD, and use as a global.


$ npm install hamt
var hamt = require('hamt');

var h = hamt.empty.set('key', 'value');



    paths: {
        'hamt': 'path/to/hamt/lib/hamt'

require(['hamt'], function(hamt) {
    var h = hamt.empty.set('key', 'value');


Hamt provides a method chaining interface and free functions for updating and querying the map. Both APIs provide identical functionality, but the free functions are designed for binding and composition, while the method chaining API is more legible and more Javascripty.

HAMTs are is persistent, so operations always return a modified copy of the map instead of alterting the original.

Custom Hash Values

Most update and lookup methods have two versions: one that takes a key and uses an internal hash function to compute its hash, and a version that takes a custom computed hash value.

var h = hamt.empty.set('key', 'value');
var h2 = hamt.empty.setHash(5, 'key', 'value');

h.get('key') === 'value'
h2.getHash(5, 'key') === 'value'

If using a custom hash, you must only use the *Hash variant of functions to interact with the map.

// Because the internally computed hash of `key` is not `5`, a direct
// look will not work.
h2.get('key') === undefined

// You must use `getHash` with the same hash value originally passed in.
h2.getHash(5, 'key') === 'value'


  • Basic – Basic map operations.
  • Lookup – Looking up values in a map.
  • Modify – Updating a map.
  • Iteration – Looping and folding over a map.



An empty map.


Is value a map?



Is a map empty?

This is the correct method to check if a map is empty. Direct comparisons to hamt.empty will not work.

Returns false if map is not a map.




Get number of elements in map.

  • map - Hamt map.
hamt.empty.count() === 0;
hamt.empty.set('a', 3).count() === 1;
hamt.empty.set('a', 3).set('b', 3).size === 2;


hamt.get(key, map)


Lookup the value for key in map.

  • key - String key.
  • map - Hamt map.
var h = hamt.empty.set('key', 'value');

h.get('key') === 'value'
hamt.get('key', k) === 'value'

h.get('no such key') === undefined

hamt.getHash(hash, key, map)

map.getHash(hash, key)

Same as get but uses a custom hash value.

hamt.tryGet(alt, key, map)

map.tryGet(alt, key)

Same as get but returns alt if no value for key exists.

  • alt - Value returned if no such key exists in the map.
  • key - String key.
  • map - Hamt map.

hamt.has(key, map)


Does an entry for key exist in map?

  • key - String key.
  • map - Hamt map.
var h = hamt.empty.set('key', 'value');

h.has('key') === true
h.has('no such key') === false

hamt.tryGetHash(alt, hash, key, map)

map.tryGetHash(alt, hash, key)

Same as tryGet but uses a custom hash value.

hamt.set(key, value, map)

map.set(key, value)

Set the value for key in map.

  • value - Value to store. Hamt supports all value types, including: literals, objects, falsy values, null, and undefined. Keep in mind that only the map data structure itself is guaranteed to be immutable. Using immutable values is recommended but not required.
  • key - String key.
  • map - Hamt map.

Returns a new map with the value set. Does not alter the original.

var h = hamt.empty
    .set('key', 'value');
    .set('key2', 'value2');

var h2 = h.set('key3', 'value3');

h2.get('key') === 'value'
h2.get('key2') === 'value2'
h2.get('key3') === 'value3'

// original `h` was not modified
h.get('key') === 'value'
h.get('key2') === 'value2'
h.get('key3') === undefined

hamt.setHash(hash, key, value, map)

map.setHash(hash, key, value)

Same as set but uses a custom hash value.


hamt.modify(f, key, map)

map.modify(key, f)

Update the value stored for key in map.

  • f - Function mapping current value to new value. If no current value exists, f is invoked with no arguments.
  • key - String key.
  • map - Hamt map.

Returns a new map with the modified value. Does not alter the original.

var h = hamt.empty
    .set('i', 2);

var h2 = h.modify('i', x => x * x);

h2.get('i') === 4
h.get('i') === 2
h2.count() === 1
h.count() === 1

// Operate on value that does not exist
var h3 = h.modify('new', x => {
    if (x === undefined) {
        return 10;
    return -x;

h3.get('new') === 10
h3.count() === 2

hamt.modifyHash(f, hash, key, map)

map.modifyHash(hash, key, f)

Same as modify but uses a custom hash value.

hamt.modifyValue(f, defaultValue, key, map)

map.modifyValue(key, f, defaultValue)

Similar to modify, but invokes f with defaultValue if no value exists in map for key.

hamt.modifyValueHash(f, defaultValue, hash, key, map)

map.modifyValueHash(hash, key, f, defaultValue)

Same as modifyValue but uses a custom hash value.

hamt.remove(key, map)



Remove key from map.

  • key - String key.
  • map - Hamt map.

Returns a new map with the value removed. Does not alter the original.

var h = hamt.empty
    .set('a', 1)
    .set('b', 2)
    .set('c', 3);

var h2 = h.remove('b');

h2.count() === 2;
h2.get('a') === 1
h2.get('b') === undefined
h2.get('c') === 3

hamt.removeHash(hash, key, map)

map.removeHash(hash, key)

map.deleteHash(hash, key)

Same as remove but uses a custom hash value.


hamt.fold(f, z, map)

map.fold(f, z)

Fold over the map, accumulating result value.

  • f - Function invoked with accumulated value, current value, and current key.
  • z - Initial value.
  • map - Hamt map.

Order is not guaranteed.

var max = hamt.fold.bind(null,
    (acc, value, key) => Math.max(acc, value),

max(hamt.empty.set('key', 3).set('key', 4)) === 4;



Get a Javascript iterator to all key value pairs in map.

  • map - Hamt map.

Order is not guaranteed.

Array.from(hamt.empty.entries()) === [];
Array.from(hamt.empty.set('a', 3).entries()) === [['a', 3]];
Array.from(hamt.empty.set('a', 3).set('b', 3).entries()) === [['a', 3], ['b', 3]];

You can also iterated directly over a map with ES6:

const h = hamt.empty.set('a', 3).set('b', 3);

for (let [key, value] of h)

Array.from(h) === [['a', 3], ['b', 3]];



Get a Javascript iterator to all keys in map.

  • map - Hamt map.

Order is not guaranteed.

Array.from(hamt.empty.keys()) === [];
Array.from(hamt.empty.set('a', 3).keys()) === ['a'];
Array.from(hamt.empty.set('a', 3).set('b', 3).keys()) === ['a', 'b'];



Get a Javascript iterator to all values in map.

  • map - Hamt map.

Order is not guaranteed. Duplicate entries may exist.

Array.from(hamt.empty.values()) === [];
Array.from(hamt.empty.set('a', 3).values()) === [3];
Array.from(hamt.empty.set('a', 3).values('b', 3).values()) === [3, 3];

hamt.forEach(f, map)


Invoke function f for each value in the map.

  • f - Function invoked with (value, key, map).
  • map - Hamt map.

Order is not guaranteed.


Any contributions to Hamt are welcome. Feel free to open an issue if you run into problems or have a suggested improvement.

To develop Hamt, fork the repo and install the development node packages:

cd hamt
$ npm install

The source is written in ES6 and lives in lib/hamt.js. Gulp and Bable are used to translate the ES6 code to an ES5 distribution found in hamt.js. To start the compiler:

$ gulp default

Tests are written in Mocha and found in tests\*. To run the tests:

$ mocha tests


Code originally based on exclipy's Haskell port.