Building a React-Like Library: Functional Components

This is the second of four issues where we build a React-like library to learn how React works under the hood.

May 10, 2023

Building a React-Like Library

The series contains the following articles:

Video

If you prefer, I recorded a live-coding video explaining this article. Feel free to complement this article with it, or go and watch that instead.

Introduction

In the previous article, we built a site from a static virtual DOM. You can find the code in this gist.

const render = (vnode, parent) => {
  // manage string type
  if (typeof vnode === "string") {
    return parent.appendChild(document.createTextNode(vnode));
  }

  // create HTML element
  const element = document.createElement(vnode.tag);

  // set attributes
  if (vnode.props) {
    Object.keys(vnode.props).forEach(key => {
      const value = vnode.props[key];
      element.setAttribute(key, value);
    });
  }

  // iterate over children and render them
  if (vnode.children) {
    vnode.children.forEach(child => render(child, element));
  }

  // append the element created
  return parent.appendChild(element);
};

const vDom = {
  tag: "div",
  props: {},
  children: [
    {
      tag: "h1",
      props: { id: "title" },
      children: ["Hello, World!"]
    },
    {
      tag: "p",
      props: {},
      children: ["I'm learning about virtual DOM"]
    }
  ]
};

render(vDom, document.getElementById("root"));

Build a Node With a Function

A node is an object with tag, props, and children:

{
  tag: “h1”,
  props: { id: “title” },
  children: ["Hello, World!"]
}
// or
{
  tag: "div",
  props: {},
  children: [...]
}

Let’s build the node with a function.

const createElement = (tag, props, children) => ({ tag, props, children });

Let’s use this function:

const node = createElement(“h1”, { id: “title” }, [“Hello from app!”]);

Building the Virtual DOM With a Function

To create all the virtual DOM with createElement, all the children nodes need to be created also with createElement:

const vDom = createElement(
  "div",
  {},
  [
    createElement("h1", {}, ["Hello from app!"]),
    createElement("p", {}, ["This was built with createElement"])
  ]
);

That’s it; now our vDom is the same as before, but we built it with the function createElement. Which is similar to what React does with React.createElement.

Functional Components

One of the main features of React is the functional components. Let’s refactor the h1 to be a functional component:

const Title = () => createElement("h1", {}, ["Hello from dynamic app!"]);

Our virtual DOM now has a functional component:

Let’s build our vDom again:

const vDom = createElement(
  "div",
  {},
  [
    // Functional component
    createElement(Title, {}, []),
    // createElement("h1", {}, ["Hello from app!"])
    createElement("p", {}, ["This was built with createElement"])
  ]
);

Render Function With Functional Components

Notice how now one node uses the functional component;

createElement(Title, {}, []),

The render function doesn’t know what to do with a “tag” that is a function. It can only handle tags as strings:

const render = (vnode, parent) => {
  //…
  // create HTML element
  const element = document.createElement(vnode.tag);
  //…
}

This “tag” function is our functional component, which returns a node when called:

const Title = () => createElement("h1", {}, ["Hello from dynamic app!"]);

We call the “tag” function and pass the returned value (another node) to the render function:

const render = (vnode, parent) => {
  // ...
  // manage function type
  if (typeof vnode.tag === "function") {
    // call function to get the virtual DOM node
    const nextVnode = vnode.tag();
    return render(nextVnode, parent);
  }
  // ...
}

And now it works! Our React-like “library” can now render functional components.

Put It Together

I recorded a live-coding video explaining this article. It might be helpful as a recap also.

Let’s take a look at it all together. First, our new virtual DOM:

const Title = () => createElement("h1", {}, ["Hello from dynamic app!"]);

const vDom = createElement(
  "div",
  {},
  [
    createElement(Title, {}, []),
    createElement("p", {}, ["This was built with createElement"])
  ]
);

Title is a functional component, but now our render function knows how to deal with functional components:

const render = (vnode, parent) => {
  // manage string type
  if (typeof vnode === "string") {
    return parent.appendChild(document.createTextNode(vnode));
  }

  // manage functional component
  if (typeof vnode.tag === "function") {
    const nextVnode = vnode.tag();
    render(nextVnode, parent);
    return;
  }

  // create HTML element
  const element = document.createElement(vnode.tag);

  // set attributes
  if (vnode.props) {
    Object.keys(vnode.props).forEach(key => {
      const value = vnode.props[key];
      element.setAttribute(key, value);
    });
  }

  // iterate over children and render them
  if (vnode.children) {
    vnode.children.forEach(child => render(child, element));
  }

  // append the element created
  return parent.appendChild(element);
};

Finally, we render the vDom into the website:

render(vDom, document.getElementById("root"));

IMHO, the next issue is the most impressive. We’ll create interactive applications by adding state to the functional components (useState)! Stay tuned!

Thanks to Bernat and Sebastià for reviewing this article 🙏