How React Works Under the Hood: A Deep Dive for Full-Stack Developers

React, the popular JavaScript library for building user interfaces, has revolutionized web development since its introduction by Facebook in 2013. Its declarative, component-based approach has made it easier than ever to build complex, interactive UIs. But what‘s happening under the hood that makes React so efficient and powerful? In this deep dive, we‘ll explore the key concepts and mechanisms that drive React‘s inner workings, providing insights valuable to any full-stack developer working with React.

The Virtual DOM: React‘s Secret Weapon

At the core of React‘s efficiency is the virtual DOM, an in-memory representation of the actual DOM (Document Object Model). When you render a React component, you‘re creating a lightweight JavaScript object that describes what the actual DOM should look like, rather than manipulating the DOM directly.

What is the Virtual DOM?

The virtual DOM is a programming concept where an ideal, or "virtual" representation of a UI is kept in memory and synced with the "real" DOM. This process is called reconciliation.

Why is the Virtual DOM Fast?

Manipulating the actual DOM is slow. Manipulating the virtual DOM is much faster, because nothing gets drawn onscreen. Think of manipulating the virtual DOM as editing a blueprint, as opposed to moving rooms in an actual house.

Virtual DOM Statistics

According to benchmarks, React‘s virtual DOM can handle:

• 200,000 nodes in ~400ms
• 100,000 nodes in ~200ms
• 50,000 nodes in ~100ms

This is significantly faster than vanilla JavaScript‘s direct DOM manipulation. (Source)

JSX: Syntactic Sugar for Creating Elements

If you‘ve written React, you‘re familiar with JSX, a syntax extension that allows you to write HTML-like code in your JavaScript. But JSX doesn‘t actually run in the browser – it gets compiled into JavaScript first.

What is JSX?

JSX stands for JavaScript XML. It allows us to write HTML in React, making it easier to write and add HTML in React.

How JSX Gets Compiled

Under the hood, JSX is converted into JavaScript. Specifically, JSX gets compiled into calls to React.createElement().

For example, this JSX:

<div className="greeting">
  Hello, {name}!
</div>

gets compiled to this JavaScript:

React.createElement(
  "div",
  { className: "greeting" },
  "Hello, ",
  name,
  "!"
);

Creating Elements with React.createElement

React.createElement creates a React element, which is a lightweight description of what to render. These elements aren‘t actual DOM nodes – they‘re more like instructions for how React should create the nodes.

Here‘s the signature for React.createElement:

React.createElement(
  type,
  [props],
  [...children]
)

• type: the type of the HTML element or the custom React component
• props: an object containing the properties of the element
• children: the children of the element, which can be more React elements or strings

Example of Compiled JSX

Let‘s look at a more complex JSX example and its compiled output:

<div>
  <img src="avatar.png" className="profile" />
  <Hello />
</div>

This would be compiled to:

React.createElement(
  "div",
  null,
  React.createElement("img", {
    src: "avatar.png",
    className: "profile"
  }),
  React.createElement(Hello, null)
);

As you can see, JSX provides a more readable and familiar syntax for defining tree structures with attributes than raw JavaScript function calls.

The Reconciliation Process: Diffing the Virtual and Real DOM

When a component‘s props or state change, React decides whether an actual DOM update is necessary by comparing the newly returned element with the previously rendered one. This process is called "reconciliation".

What is Reconciliation?

Reconciliation is the process through which React updates the DOM. At a high level, here‘s what happens when you attempt to update a component:

1. A new element is created from the component.
2. The new element is diffed against the previously rendered element.
3. The DOM is updated to reflect changes, if necessary.

The Diffing Algorithm

When diffing two trees, React first compares the two root elements. The behavior is different depending on the types of the root elements.

• If the root elements are of different types, React will tear down the old tree and build the new tree from scratch.
• If the root elements are of the same type, React will keep the same underlying DOM node and only update the changed attributes.

When updating the DOM, React only changes what‘s necessary. This is a key reason why React is so fast.

Reconciliation and Keys

When children have keys, React uses the key to match children in the original tree with children in the subsequent tree. Keys should be stable, predictable, and unique.

Here‘s an example of how keys impact reconciliation:

// Without keys
<ul>
  <li>Duke</li>
  <li>Villanova</li>
</ul>

<ul>
  <li>Connecticut</li>
  <li>Duke</li>
  <li>Villanova</li>
</ul>

// With keys
<ul>
  <li key="2015">Duke</li>
  <li key="2016">Villanova</li>
</ul>

<ul>
  <li key="2014">Connecticut</li>
  <li key="2015">Duke</li>
  <li key="2016">Villanova</li>
</ul>

In the first case, React will mutate every child instead of realizing it can keep the <li>Duke</li> and <li>Villanova</li> subtrees intact. This inefficiency can be a problem.

In the second case, by supplying unique keys, React can reorder the elements without needing to mutate them.

Code Example of Reconciliation

Let‘s see reconciliation in action with a code example:

class CounterButton extends React.Component {
  constructor(props) {
    super(props);
    this.state = {count: 1};
  }

  shouldComponentUpdate(nextProps, nextState) {
    if (this.props.color !== nextProps.color) {
      return true;
    }
    if (this.state.count !== nextState.count) {
      return true;
    }
    return false;
  }

  render() {
    return (
      <button
        color={this.props.color}
        onClick={() => this.setState(state => ({count: state.count + 1}))}>
        Count: {this.state.count}
      </button>
    );
  }
}

In this example, the shouldComponentUpdate method is checking if the color prop or the count state have changed. If either have changed, the component will re-render. If neither have changed, the component will not re-render.

This is a form of optimization in React. By default, when a component‘s state or props change, it will re-render. However, if we know that a certain prop or state change doesn‘t necessitate a re-render, we can save resources by skipping the re-render.

React Fiber: A Reimplemented Core Algorithm

React Fiber is a reimplementation of React‘s core algorithm. It was introduced in React 16.

Motivation Behind Fiber

The main goals of Fiber are:

• Ability to split interruptible work in chunks.
• Ability to prioritize, rebase and reuse work in progress.
• Ability to yield back and forth between parents and children to support layout in React.
• Ability to return multiple elements from render().

How Fiber Works

In essence, Fiber is a virtual stack frame. A fiber can be thought of as a data structure that represents some work to do or, in other words, a unit of work.

React Fiber‘s reconciliation algorithm works in two phases:

1. The render phase
2. The commit phase

In the render phase, React applies updates to components and figures out what needs to change in the DOM. This phase is interruptible.

In the commit phase, React applies the changes to the DOM. This phase is not interruptible.

Advantages of Fiber

• It makes apps more fluid and responsive.
• It improves perceived performance for complex applications.
• It reduces the risk of dropping frames for animations and interactions.

React Render Process: Initial Render and Re-renders

Understanding React‘s render process is crucial for optimizing the performance of your React applications.

Initial Render

When a React application first loads in the browser, it goes through an initial render. Here‘s what happens during the initial render:

1. Your application‘s root component gets mounted onto the DOM.
2. The component‘s JSX is transformed into React elements.
3. A virtual DOM tree is constructed from these elements.
4. The virtual DOM is used to create an actual DOM tree, which is inserted into the DOM.

Re-renders

After the initial render, re-renders can be triggered by changes to a component‘s props or state.

Here‘s what happens during a re-render:

1. A new virtual DOM tree is created.
2. The new virtual DOM tree is compared to the previous one.
3. The differences found in step 2 are applied to the actual DOM.

It‘s important to note that React only updates what‘s necessary during a re-render. It doesn‘t recreate the entire DOM tree from scratch.

Render Phase and Commit Phase

As we mentioned in the Fiber section, React‘s render process can be divided into two phases:

1. The render phase: This is where React updates the virtual DOM. This phase can be interrupted, as it doesn‘t make any changes to the actual DOM.

2. The commit phase: This is where React applies the changes to the actual DOM. This phase is uninterruptible.

Concurrent Mode: The Future of React (Experimental)

Concurrent Mode is an experimental feature that lets React apps be more responsive by rendering component trees without blocking the main thread.

What is Concurrent Mode?

In Concurrent Mode, rendering is not a blocking operation. This means that the browser can interrupt rendering to handle user input, network responses, or anything else.

Motivation Behind Concurrent Mode

The main motivation is to improve the user experience of React applications. With Concurrent Mode, high priority tasks like user input can interrupt low priority tasks like rendering, making the application feel more responsive.

How Concurrent Mode Works

Concurrent Mode is opt-in. You can enable it for part or all of your application. In Concurrent Mode, updates are divided into two categories:

1. Urgent updates, like user input.
2. Transition updates, like UI updates.

Urgent updates are rendered immediately. Transition updates are rendered when resources are available.

Writing Better React Code: Insights for Full-Stack Developers

As a full-stack developer, understanding how React works under the hood can help you write better, more performant React code.

Importance of Understanding React Internals

Understanding React‘s internals empowers you to:

• Write more efficient React components
• Optimize your application‘s performance
• Debug complex issues more effectively
• Make informed design decisions

Performance Optimization Techniques

Here are a few techniques you can use to optimize your React application‘s performance:

• Use shouldComponentUpdate to avoid unnecessary re-renders.
• Use React.memo for function components to memoize them.
• Virtualize long lists using libraries like react-window or react-virtualized.
• Lazy load components that aren‘t immediately needed.
• Use the production build of React for deployment.

Avoiding Common Pitfalls

Here are a few common pitfalls to avoid:

• Avoid using indexes as keys for list items. Use unique, stable identifiers instead.
• Don‘t mutate state directly. Always use setState.
• Be cautious when using refs to manipulate the DOM directly. Let React handle the DOM manipulation when possible.
• Avoid over-optimizing. Measure performance before optimizing.

Conclusion

React‘s innovative approach to building user interfaces has revolutionized web development. By understanding how React works under the hood – the virtual DOM, JSX compilation, the reconciliation process, and the Fiber architecture – you can write more performant and maintainable React applications.

As a full-stack developer, this deep understanding of React‘s internals is invaluable. It allows you to optimize your code, troubleshoot complex issues, and make informed design decisions.

Remember, React‘s goal is to provide a simple, declarative way to build complex user interfaces. By abstracting away many of the complexities of manual DOM manipulation and providing a component-based model for UI development, React allows developers to focus on the core logic of their applications.

As you continue your journey with React, keep exploring, keep learning, and keep coding. The better you understand the tools you work with, the better developer you‘ll be. Happy coding!