Building custom components

February 02, 2020

This is my second post in a series where I try to rebuild React from scratch in an effect to improve my understanding about it. As I mentioned before, has named my library recoil.

In the last post, we ended up with a simple function that could generate a string of HTML text for stock HTML elements like a div.

The next step is to support custom components, one of React’s main features. We need to go beyond the divs, spans, etc.

We last ended off with an API like this:

    children: [
      createElement("span", { children: "Hello" }, renderHtmlComponent),
      createElement("span", { children: "World" }, renderHtmlComponent),

and we defined createElement as:

function createElement(elementType, props, render) {
  let html = ""

  // Create the opening tag
  html += `<${elementType}>`

  // Call render, which we expect to return an array of 0 or more children.
  // By now, each child is just a string of HTML, since that's what createElement returns.
  // All we need to do is join the resulting strings together to build up the entire app.
  html += render(props).join("")

  // Create the closing tag
  // NOTE: We're assuming all elements have both opening and closing tags.
  // This isn't always the case
  html += `</${elementType}>`

  return html

To handle both custom components and regular HTML elements, I decided to go with the simple approach of treating regular HTML elements as custom components. In this way I can just create DivComponent, SpanComponent, etc, to handle the common elements. I believe that treating all elements as components will simplify the architecture, but only time will tell.

Let’s start with a component for the <div> element:

function DivComponent({ children, ...otherProps }) {
  // Turn each child into a string of HTML
  let processedChildren = (children || []).map(child =>
    createElement(child, otherProps)

  // Wrap the html string of the children in the div tags
  return `<div>${processedChildren.join("\n")}</div>`

Since DivComponent is creating the opening and closing tags, we can simplify our createElement call to just handle the string vs component case, and no longer worry about the elementType. We’ll consider the first argument (component) to be either a function or a string.

function createElement(component, props = {}) {
  // String "component", so just return the string. Like a text node in HTML
  if (typeof component === "string") {
    return component

  // 'render' the component
  return component(props)

So now we have a cleaner version of our code from last time, but we still don’t have custom components. Let’s build a custom component that takes a name prop and prints Hello {name}.

const GreetingComponent = ({ name }) =>
  createElement(DivComponent, {
    children: [createElement(`Hello ${name}`)],

// The JSX equivalent would be:
  hello {name}

This code produces both the div and our custom component!

<div id="recoil-root">
  <div>Hello Jeff</div>

And that’s it! Now recoil supports classic elements like <div>, and custom components lke our GreetingComponent.

As cool as that is, at this point we’re still only rendering once. There’s no data binding that causes elements to re-render. There’s also no internal component state.

In the next post, we’ll dive into the useState React hook and how we can implement it in recoil.

If you want to discuss this project or this post, reach out to me on twitter.