Higher-level tree operations

One extremely useful aspect of tree-based development is that we can define higher-level operations on trees that solve general problems.

In the previous step, we combined several trees backed by different data representations into a single tree with several branches.

In some cases, it’s useful to merge two or more trees together to create a single tree. Let’s solve that general problem by creating a higher-level tree that merges multiple trees together.

MergeTree class

We’ll start creating a MergeTree class whose constructor accepts multiple trees.

/* src/merge/MergeTree.js */

export default class MergeTree {
  constructor(...trees) {
    this.trees = trees;

  async keys() {
    /* TODO */

  async get(key) {
    /* TODO */

The keys will return all the keys in all of the supplied trees. We can use a Set to de-duplicate the keys.

  async keys() {
    const keys = new Set();
    for (const tree of this.trees) {
      for await (const key of tree) {
    return keys;

The get method looks in each of the trees in turn, returning the first defined value from any of the trees.

  async get(key) {
    const subtrees = [];
    for (const tree of this.trees) {
      const value = await tree.get(key);

      const isAsyncDictionary =
        typeof value?.get === "function" &&
        typeof value?.keys === "function";

      if (value !== undefined) {
        if (isAsyncDictionary) {
        } else {
          return value;

    return subtrees.length === 0
      ? undefined
      : subtrees.length === 1
      ? subtrees[0]
      : new this.constructor(...subtrees);

We also take care to handle the case where multiple trees define async subtrees for the same key. In that case, we collect the subtrees and wrap them in a new MergeTree instance. This implements a deep merge operation.

Use MergeTree to define the site

We can update our site tree to use this new higher-level MergeTree operation.

/* src/merge/siteTree.js */

import htmlFiles from "./htmlFiles.js";
import htmlFn from "./htmlFn.js";
import htmlObject from "./htmlObject.js";
import indexPages from "./indexPages.js";
import MergeTree from "./MergeTree.js";

const merged = new MergeTree(htmlObject, htmlFiles, htmlFn);

export default indexPages(merged);

We apply our indexPages transform to give the merged tree index pages.

The site is now the deep merge of all three trees:

g Alice.html <p>Hello, <strong>Alice</strong>.</p> ->Alice.html Alice.html Bob.html <p>Hello, <strong>Bob</strong>.</p> ->Bob.html Bob.html Carol.html <p>Hello, <strong>Carol</strong>.</p> ->Carol.html Carol.html more ->more more Frank.html <p>Hello, <strong>Frank</strong>.</p> ->Frank.html Frank.html Grace.html <p>Hello, <strong>Grace</strong>.</p> ->Grace.html Grace.html Henry.html <p>Hello, <strong>Henry</strong>.</p> ->Henry.html Henry.html Kelly.html <p>Hello, <strong>Kelly</strong>.</p> ->Kelly.html Kelly.html Larry.html <p>Hello, <strong>Larry</strong>.</p> ->Larry.html Larry.html Michelle.html <p>Hello, <strong>Michelle</strong>.</p> ->Michelle.html Michelle.html index.html <!DOCTYPE html> <html> <body> <ul>… ->index.html index.html more/David.html <p>Hello, <strong>David</strong>.</p> more->more/David.html David.html more/Eve.html <p>Hello, <strong>Eve</strong>.</p> more->more/Eve.html Eve.html more/Isabella.html <p>Hello, <strong>Isabella</strong>.</p> more->more/Isabella.html Isabella.html more/James.html <p>Hello, <strong>James</strong>.</p> more->more/James.html James.html more/index.html <!DOCTYPE html> <html> <body> <ul>… more->more/index.html index.html

If you compare this to the previous step, where we treated each of the component trees as separate branches, you can see that the merged tree is flatter.

From inside the src/merge directory, run the updated server.

$ cd ../merge
$ node serve
Server running at http://localhost:5000. Press Ctrl+C to stop.

Browse the merged site.

Some of the pages are defined by an object, some by files, and some by a function. In fact, that function will handle any requests for .html pages that aren’t otherwise defined — if you browse to a page not defined by the object- or files-based trees, the function tree will provide a default value. For example, browsing to Sara.html returns a default page for Sara.

You can also build the site.

$ node build
$ ls dist
Alice.html    Carol.html    Grace.html    Kelly.html    Michelle.html more
Bob.html      Frank.html    Henry.html    Larry.html    index.html

Other higher-level operations

You can define a range of useful higher-level tree operations to handle situations commonly encountered in website development and other contexts, such as:

  • A cache operation that accepts two trees: a source tree that produces values, and a read-write tree to cache values which have already been produced. The latter is consulted first. If no value for a given key has been produced yet, the source tree is consulted, and the resulting value cached for later use.
  • A filter operation that accepts two trees: a source tree that produces values, and a tree that defines a filter which should be used to decide whether or not to return a value from the source tree.
  • A sort operation that wraps a source tree, with an keys method that returns the source tree’s keys in a particular sort order.
  • A shuffle operation that wraps a source tree, with an keys method that returns the source tree’s keys in a random order.

Such higher-level operations are common enough that they would be of value in many projects. Rather than having many people implement such operations, it’d be nice to share solid implementations of them in a common library.


Next: Sharing a common library and tools