An implementation of zippers over s-expressions represented as nested javascript arrays

zipper.js

class Node {
  constructor(value, children) {
    this.value = value;
    this.children = children;
  }
  
  setValue(newValue) {
    return new Node(newValue, this.children);
  }
  
  appendChild() {
    return new Node(this.value, this.children.concat(new Node(null, [])));
  }
  
  removeChild(index) {
    return new Node(this.value, this.children.slice(0, index).concat(this.children.slice(index+1)));
  }
  
  print(nest) {
    nest = nest || "";
    console.log(nest+this.value);
    (this.children || []).forEach(child => child.print("+"+nest));
  }
  
  zipper() {
    return new Zipper(this, []);
  }
}

class Crumb {
  constructor(parent, left, right) {
    this.parent = parent;
    this.left = left || [];
    this.right = right || [];
  }
}

class Zipper {
  constructor(item, crumbs) {
    this.item = item;
    this.crumbs = crumbs || [];
  }
  
  mutate(lambda) {
    return new Zipper(lambda(this.item), this.crumbs);
  }
  
  childAt(index) {
    return new Zipper(this.item.children[index], [this.crumbFrom(index)].concat(this.crumbs));
  }
  
  crumbFrom(index) {
    return new Crumb(this.item.value, this.item.children.slice(0, index), this.item.children.slice(index+1));
  }
  
  isRoot() {
    return this.crumbs.length == 0;
  }
  
  up() {
    let crumb = this.crumbs[0];
    return new Zipper(new Node(crumb.parent, crumb.left.concat(this.item, crumb.right)), this.crumbs.slice(1))
  }
  
  print() {
    console.log("ITEM");
    this.item.print();
    console.log("CRUMBS");
    console.log(this.crumbs);
  }
  
  printItem() {
    this.item.print();
  }
}

const sampleTree = new Node(
  "root", [
  new Node("A", [
    new Node("E", null)
  ]), 
  new Node("B", [
    new Node("C", [
      new Node("F", null)
    ]), 
    new Node("D", null)
  ])
]);

example:

sampleTree.zipper().
  childAt(1).mutate(n => n.setValue('Q')).up().
  childAt(0).mutate(n => n.appendChild()).
    childAt(1).mutate(n => n.setValue('new node')).
  up().
up().print();