work.suroh.tk/node_modules/immutable/contrib/cursor/README.md

Cursors

Cursors allow you to hold a reference to a path in a nested immutable data structure, allowing you to pass smaller sections of a larger nested collection to portions of your application while maintaining a central point aware of changes to the entire data structure: an onChange function which is called whenever a cursor or sub-cursor calls update.

This is particularly useful when used in conjuction with component-based UI libraries like React or to simulate "state" throughout an application while maintaining a single flow of logic.

var Immutable = require('immutable');
var Cursor = require('immutable/contrib/cursor');

var data = Immutable.fromJS({ a: { b: { c: 1 } } });
var cursor = Cursor.from(data, ['a', 'b'], newData => {
  data = newData;
});

// ... elsewhere ...

cursor.get('c'); // 1
cursor = cursor.update('c', x => x + 1);
cursor.get('c'); // 2

// ... back to data ...

data.getIn(['a', 'b', 'c']); // 2