Selection List in Javascript using Immutable

selection-list.js

import Immutable from 'immutable';

/* Selection List Data Structure */

class SelectionList {
  constructor(list) {
    if (list.length > 0) {
      this._previous = new Immutable.Stack();
      this._selected = list[0];
      this._next = new Immutable.Stack(list.splice(1));
    } else {
      throw new Error('Selection list must include at least one element');
    }
  }

  toArray() {
    return this._previous.toSeq()
      .reverse()
      .toArray()
      .concat(this._selected)
      .concat(this._next.toSeq().toArray());
  }

  map(f) {
    const previousSize = this._previous.size;

    const previous = new Immutable.Stack(this._previous.toSeq().reverse().map(f).reverse());
    const selected = f(this._selected, previousSize);
    const next = new Immutable.Stack(this._next.toArray().map((x, index) => f(x, index + previousSize + 1)));

    return makeSelectionList(previous, selected, next);
  }

  next() {
    if (this._next.size > 0) {
      const previous = this._previous.push(this._selected);
      const selected = this._next.peek();
      const next = this._next.pop();
      return makeSelectionList(previous, selected, next);
    } else {
      return this;
    }
  }

  previous() {
    if (this._previous.size > 0) {
      const selected = this._previous.peek();
      const previous = this._previous.pop();
      const next = this._next.push(this._selected);
      return makeSelectionList(previous, selected, next);
    } else {
      return this;
    }
  }

  get selectedIndex() {
    return this._previous.size;
  }

  goto(index) {
    const currentIndex = this.selectedIndex;

    if (index === currentIndex) {
      return this;
    } else if (currentIndex < index && this._next.size > 0) {
      return this.next().goto(index);
    } else if (currentIndex > Math.max(0, index)) {
      return this.previous().goto(index);
    } else {
      return this;
    }
  }

  toObject() {
    return {
      previous: this._previous.toArray(),
      selected: this._selected,
      next: this._next.toArray()
    };
  }

  updateSelected(f) {
    return makeSelectionList(
      this._previous,
      f(this._selected),
      this._next
    );
  }
}

const SelectionListPrototype = SelectionList.prototype;

const makeSelectionList = (previous, selected, next) => {
  let selectionList = Object.create(SelectionListPrototype);
  selectionList._previous = previous;
  selectionList._selected = selected;
  selectionList._next = next;
  return selectionList;
}