Skip to content

Immutable data structures with history for top-to-bottom properties in component based libraries like React. Based on Immutable.js

Notifications You must be signed in to change notification settings


Folders and files

Last commit message
Last commit date

Latest commit


Repository files navigation

Immstruct NPM version Build Status Dependency Status Gitter

A wrapper for Immutable.js to easily create cursors that notify when they are updated. Handy for use with immutable pure components for views, like with Omniscient or React.js.

See the API References for more documentation and usage.



// Require a default instance of immstruct (singleton instance)
var immstruct = require('immstruct');

// Create structure under the later retrievable ID `myKey`
var structure = immstruct('myKey', { a: { b: { c: 1 } } });

// Use event `swap` or `next-animation-frame`
structure.on('swap', function (newStructure, oldStructure, keyPath) {
  console.log('Subpart of structure swapped.');
  console.log('New structure:', newStructure.toJSON());

  // e.g. with usage with React
  // React.render(App({ cursor: structure.cursor() }), document.body);

var cursor = structure.cursor(['a', 'b', 'c']);

// Update the value at the cursor. As cursors are immutable,
// this returns a new cursor that points to the new data
var newCursor = cursor.update(function (x) {
  return x + 1;

// We unwrap the cursor, by getting the data it is pointing at using deref
// and see that the value of the old `cursor` to is still `1`
console.log(cursor.deref()); //=> 1

// `newCursor` points to the new data
console.log(newCursor.deref()); //=> 2

Note: The cursors you see here are cursors from Facebooks Immutable.js library. Read the complete API in their repo.


// Require a default instance of immstruct (singleton instance)
var immstruct = require('immstruct');

// Get the structure we previously defined under the ID `myKey`
var structure = immstruct('myKey');

var cursor = structure.cursor(['a', 'b', 'c']);

var updatedCursor = cursor.update(function (x) { // triggers `swap` in somefile.js
  return x + 1;

// Unwrap the value
console.log(updatedCursor.deref()); //=> 3


While Immutable.js cursors are immutable, Immstruct lets you create references to a piece of data from where cursors will always be fresh. The cursors you create are still immutable, but you have the ability to retrieve the newest and latest cursor pointing to a specific part of your immutable structure.

var structure = immstruct({ 'foo': 'bar' });
var ref = structure.reference('foo');

console.log(ref.cursor().deref()) //=> 'bar'

var oldCursor = structure.cursor('foo');
console.log(oldCursor.deref()) //=> 'bar'

var newCursor = structure.cursor('foo').update(function () { return 'updated'; });
console.log(newCursor.deref()) //=> 'updated'

assert(oldCursor !== newCursor);

// You don't need to manage and track fresh/stale cursors.
// A reference cursor will do it for you.
console.log(ref.cursor().deref()) //=> 'updated'

Updating a cursor created from a reference will also update the underlying structure.

This offers benefits similar to that of Om's reference cursors, where React.js or Omniscient components can observe pieces of application state without it being passed as cursors in props from their parent components.

References also allow for listeners that fire when their path or the path of sub-cursors change:

var structure = immstruct({
  someBox: { message: 'Hello World!' }
var ref = structure.reference(['someBox']);

var unobserve = ref.observe(function () {
  // Called when data the path 'someBox' is changed.
  // Also called when the data at ['someBox', 'message'] is changed.

// Update the data using the ref
ref.cursor().update(function () { return 'updated'; });

// Update the data using the initial structure
structure.cursor(['someBox', 'message']).update(function () { return 'updated again'; });

// Remove the listener


Parents' change listeners are also called when sub-cursors are changed.

Cursors created from references are still immutable. If you keep a cursor from a var cursor = reference.cursor() around, the cursor will still point to the data at time of cursor creation. Updating it may rewrite newer information.

Usage Undo/Redo

// optionalKey and/or optionalLimit can be omitted from the call
var optionalLimit = 10; // only keep last 10 of history, default Infinity
var structure = immstruct.withHistory('optionalKey', optionalLimit, { 'foo': 'bar' });
console.log(structure.cursor('foo').deref()); //=> 'bar'

structure.cursor('foo').update(function () { return 'hello'; });
console.log(structure.cursor('foo').deref()); //=> 'hello'

console.log(structure.cursor('foo').deref()); //=> 'bar'

console.log(structure.cursor('foo').deref()); //=> 'hello'


Creates or retrieves structures.

See examples:

var structure = immstruct('someKey', { some: 'jsObject' })
// Creates new structure with someKey
var structure = immstruct('someKey')
// Get's the structure named `someKey`.

Note: if someKey doesn't exist, an empty structure is created

var structure = immstruct({ some: 'jsObject' })
var randomGeneratedKey = structure.key;
// Creates a new structure with random key
// Used if key is not necessary
var structure = immstruct()
var randomGeneratedKey = structure.key;
// Create new empty structure with random key

You can also create your own instance of Immstruct, isolating the different instances of structures:

var localImmstruct = new immstruct.Immstruct()
var structure = localImmstruct.get('someKey', { my: 'object' });

API Reference

See API Reference.

Structure Events

A Structure object is an event emitter and emits the following events:

  • swap: Emitted when cursor is updated (new information is set). Is emitted on all types of changes, additions and deletions. The passed structures are always the root structure. One use case for this is to re-render design components. Callback is passed arguments: newStructure, oldStructure, keyPath.
  • next-animation-frame: Same as swap, but only emitted on animation frame. Could use with many render updates and better performance. Callback is passed arguments: newStructure, oldStructure, keyPath.
  • change: Emitted when data/value is updated and it existed before. Emits values: newValue, oldValue and path.
  • delete: Emitted when data/value is removed. Emits value: removedValue and path.
  • add: Emitted when new data/value is added. Emits value: newValue and path.
  • any: With the same semantics as add, change or delete, any is triggered for all types of changes. Differs from swap in the arguments that it is passed. Is passed newValue (or undefined), oldValue (or undefined) and full keyPath. New and old value are the changed value, not relative/scoped to the reference path as with swap.

NOTE: If you update cursors via Cursor.update or Cursor.set, and if the underlying Immutable collection is not inherently changed, swap and changed events will not be emitted, neither will the history (if any) be applied.

See tests for event examples


MIT License


Immutable data structures with history for top-to-bottom properties in component based libraries like React. Based on Immutable.js