Chalarangelo/30-seconds-of-code

---
title: JavaScript Data Structures - Tree
shortTitle: Tree
type: story
language: javascript
tags: [class]
cover: purple-flower-macro-2
excerpt: A tree is a data structure consisting of a set of linked nodes representing a hierarchical tree structure.
listed: true
dateModified: 2021-08-22
---

## Definition

A tree is a data structure consisting of a set of linked nodes that represent a hierarchical tree structure. Each node is linked to others via parent-children relationship. The first node in the tree is the root, whereas nodes without any children are the leaves.

![JavaScript Tree visualization](./illustrations/ds-tree.svg)

Each node in a tree data structure must have the following properties:

- `key`: The key of the node
- `value`: The value of the node
- `parent`: The parent of the node (`null` if there is none)
- `children`: An array of pointers to the node's children

The main operations of a tree data structure are:

- `insert`: Inserts a node as a child of the given parent node
- `remove`: Removes a node and its children from the tree
- `find`: Retrieves a given node
- `preOrderTraversal`: Traverses the tree by recursively traversing each node followed by its children
- `postOrderTraversal`: Traverses the tree by recursively traversing each node's children followed by the node

## Implementation

```js
class TreeNode {
  constructor(key, value = key, parent = null) {
    this.key = key;
    this.value = value;
    this.parent = parent;
    this.children = [];
  }

  get isLeaf() {
    return this.children.length === 0;
  }

  get hasChildren() {
    return !this.isLeaf;
  }
}

class Tree {
  constructor(key, value = key) {
    this.root = new TreeNode(key, value);
  }

  *preOrderTraversal(node = this.root) {
    yield node;
    if (node.children.length) {
      for (let child of node.children) {
        yield* this.preOrderTraversal(child);
      }
    }
  }

  *postOrderTraversal(node = this.root) {
    if (node.children.length) {
      for (let child of node.children) {
        yield* this.postOrderTraversal(child);
      }
    }
    yield node;
  }

  insert(parentNodeKey, key, value = key) {
    for (let node of this.preOrderTraversal()) {
      if (node.key === parentNodeKey) {
        node.children.push(new TreeNode(key, value, node));
        return true;
      }
    }
    return false;
  }

  remove(key) {
    for (let node of this.preOrderTraversal()) {
      const filtered = node.children.filter(c => c.key !== key);
      if (filtered.length !== node.children.length) {
        node.children = filtered;
        return true;
      }
    }
    return false;
  }

  find(key) {
    for (let node of this.preOrderTraversal()) {
      if (node.key === key) return node;
    }
    return undefined;
  }
}
```

- Create a `class` for the `TreeNode` with a `constructor` that initializes the appropriate `key`, `value`, `parent` and `children` properties.
- Define an `isLeaf` getter, that uses `Array.prototype.length` to check if `children` is empty.
- Define a `hasChildren` getter, that is the reverse of the `isLeaf` getter.
- Create a `class` for the `Tree` with a `constructor` that initializes the `root` of the tree.
- Define a `preOrderTraversal()` generator method that traverses the tree in pre-order, using the `yield*` syntax to recursively delegate traversal to itself.
- Define a `postOrderTraversal()` generator method that traverses the tree in post-order, using the `yield*` syntax to recursively delegate traversal to itself.
- Define an `insert()` method, that uses the `preOrderTraversal()` method and `Array.prototype.push()` to add a new `TreeNode` to the tree.
- Define a `remove()` method, that uses the `preOrderTraversal()` method and `Array.prototype.filter()` to remove a `TreeNode` from the tree.
- Define a `find()` method, that uses the `preOrderTraversal()` method to retrieve the given node in the tree.

```js
const tree = new Tree(1, 'AB');

tree.insert(1, 11, 'AC');
tree.insert(1, 12, 'BC');
tree.insert(12, 121, 'BG');

[...tree.preOrderTraversal()].map(x => x.value);
// ['AB', 'AC', 'BC', 'BCG']

tree.root.value;              // 'AB'
tree.root.hasChildren;        // true

tree.find(12).isLeaf;         // false
tree.find(121).isLeaf;        // true
tree.find(121).parent.value;  // 'BC'

tree.remove(12);

[...tree.postOrderTraversal()].map(x => x.value);
// ['AC', 'AB']
```