React Components, Elements, and Instances

December 18, 2015 dodane przez Dan Abramov

The difference between components, their instances, and elements confuses many React beginners. Why are there three different terms to refer to something that is painted on screen?

Managing the Instances

If you’re new to React, you probably only worked with component classes and instances before. For example, you may declare a Button component by creating a class. When the app is running, you may have several instances of this component on screen, each with its own properties and local state. This is the traditional object-oriented UI programming. Why introduce elements?

In this traditional UI model, it is up to you to take care of creating and destroying child component instances. If a Form component wants to render a Button component, it needs to create its instance, and manually keep it up to date with any new information.

class Form extends TraditionalObjectOrientedView {
  render() {
    // Read some data passed to the view
    const { isSubmitted, buttonText } = this.attrs;

    if (!isSubmitted && !this.button) {
      // Form is not yet submitted. Create the button!
      this.button = new Button({
        children: buttonText,
        color: 'blue'
      });
      this.el.appendChild(this.button.el);
    }

    if (this.button) {
      // The button is visible. Update its text!
      this.button.attrs.children = buttonText;
      this.button.render();
    }

    if (isSubmitted && this.button) {
      // Form was submitted. Destroy the button!
      this.el.removeChild(this.button.el);
      this.button.destroy();
    }

    if (isSubmitted && !this.message) {
      // Form was submitted. Show the success message!
      this.message = new Message({ text: 'Success!' });
      this.el.appendChild(this.message.el);
    }
  }
}

This is pseudocode, but it is more or less what you end up with when you write composite UI code that behaves consistently in an object-oriented way using a library like Backbone.

Each component instance has to keep references to its DOM node and to the instances of the children components, and create, update, and destroy them when the time is right. The lines of code grow as the square of the number of possible states of the component, and the parents have direct access to their children component instances, making it hard to decouple them in the future.

So how is React different?

Elements Describe the Tree

In React, this is where the elements come to rescue. An element is a plain object describing a component instance or DOM node and its desired properties. It contains only information about the component type (for example, a Button), its properties (for example, its color), and any child elements inside it.

An element is not an actual instance. Rather, it is a way to tell React what you want to see on the screen. You can’t call any methods on the element. It’s just an immutable description object with two fields: type: (string | ReactClass) and props: Object¹.

DOM Elements

When an element’s type is a string, it represents a DOM node with that tag name, and props correspond to its attributes. This is what React will render. For example:

{
  type: 'button',
  props: {
    className: 'button button-blue',
    children: {
      type: 'b',
      props: {
        children: 'OK!'
      }
    }
  }
}

This element is just a way to represent the following HTML as a plain object:

<button class='button button-blue'>
  <b>
    OK!
  </b>
</button>

Note how elements can be nested. By convention, when we want to create an element tree, we specify one or more child elements as the children prop of their containing element.

What’s important is that both child and parent elements are just descriptions and not the actual instances. They don’t refer to anything on the screen when you create them. You can create them and throw them away, and it won’t matter much.

React elements are easy to traverse, don’t need to be parsed, and of course they are much lighter than the actual DOM elements—they’re just objects!

Component Elements

However, the type of an element can also be a function or a class corresponding to a React component:

{
  type: Button,
  props: {
    color: 'blue',
    children: 'OK!'
  }
}

This is the core idea of React.

An element describing a component is also an element, just like an element describing the DOM node. They can be nested and mixed with each other.

This feature lets you define a DangerButton component as a Button with a specific color property value without worrying about whether Button renders to a DOM <button>, a <div>, or something else entirely:

const DangerButton = ({ children }) => ({
  type: Button,
  props: {
    color: 'red',
    children: children
  }
});

You can mix and match DOM and component elements in a single element tree:

const DeleteAccount = () => ({
  type: 'div',
  props: {
    children: [{
      type: 'p',
      props: {
        children: 'Are you sure?'
      }
    }, {
      type: DangerButton,
      props: {
        children: 'Yep'
      }
    }, {
      type: Button,
      props: {
        color: 'blue',
        children: 'Cancel'
      }
   }]
});

Or, if you prefer JSX:

const DeleteAccount = () => (
  <div>
    <p>Are you sure?</p>
    <DangerButton>Yep</DangerButton>
    <Button color='blue'>Cancel</Button>
  </div>
);

This mix and matching helps keep components decoupled from each other, as they can express both is-a and has-a relationships exclusively through composition:

Button is a DOM <button> with specific properties.
DangerButton is a Button with specific properties.
DeleteAccount contains a Button and a DangerButton inside a <div>.

Components Encapsulate Element Trees

When React sees an element with a function or class type, it knows to ask that component what element it renders to, given the corresponding props.

When it sees this element:

{
  type: Button,
  props: {
    color: 'blue',
    children: 'OK!'
  }
}

React will ask Button what it renders to. The Button will return this element:

{
  type: 'button',
  props: {
    className: 'button button-blue',
    children: {
      type: 'b',
      props: {
        children: 'OK!'
      }
    }
  }
}

React will repeat this process until it knows the underlying DOM tag elements for every component on the page.

React is like a child asking “what is Y” for every “X is Y” you explain to them until they figure out every little thing in the world.

Remember the Form example above? It can be written in React as follows¹:

const Form = ({ isSubmitted, buttonText }) => {
  if (isSubmitted) {
    // Form submitted! Return a message element.
    return {
      type: Message,
      props: {
        text: 'Success!'
      }
    };
  }

  // Form is still visible! Return a button element.
  return {
    type: Button,
    props: {
      children: buttonText,
      color: 'blue'
    }
  };
};

That’s it! For a React component, props are the input, and an element tree is the output.

The returned element tree can contain both elements describing DOM nodes, and elements describing other components. This lets you compose independent parts of UI without relying on their internal DOM structure.

We let React create, update, and destroy instances. We describe them with elements we return from the components, and React takes care of managing the instances.

Components Can Be Classes or Functions

In the code above, Form, Message, and Button are React components. They can either be written as functions, like above, or as classes descending from React.Component. These three ways to declare a component are mostly equivalent:

// 1) As a function of props
const Button = ({ children, color }) => ({
  type: 'button',
  props: {
    className: 'button button-' + color,
    children: {
      type: 'b',
      props: {
        children: children
      }
    }
  }
});

// 2) Using the React.createClass() factory
const Button = React.createClass({
  render() {
    const { children, color } = this.props;
    return {
      type: 'button',
      props: {
        className: 'button button-' + color,
        children: {
          type: 'b',
          props: {
            children: children
          }
        }
      }
    };
  }
});

// 3) As an ES6 class descending from React.Component
class Button extends React.Component {
  render() {
    const { children, color } = this.props;
    return {
      type: 'button',
      props: {
        className: 'button button-' + color,
        children: {
          type: 'b',
          props: {
            children: children
          }
        }
      }
    };
  }
}

When a component is defined as a class, it is a little bit more powerful than a function component. It can store some local state and perform custom logic when the corresponding DOM node is created or destroyed.

A function component is less powerful but is simpler, and acts like a class component with just a single render() method. Unless you need features available only in a class, we encourage you to use function components instead.

However, whether functions or classes, fundamentally they are all components to React. They take the props as their input, and return the elements as their output.

Top-Down Reconciliation

When you call:

ReactDOM.render({
  type: Form,
  props: {
    isSubmitted: false,
    buttonText: 'OK!'
  }
}, document.getElementById('root'));

React will ask the Form component what element tree it returns, given those props. It will gradually “refine” its understanding of your component tree in terms of simpler primitives:

// React: You told me this...
{
  type: Form,
  props: {
    isSubmitted: false,
    buttonText: 'OK!'
  }
}

// React: ...And Form told me this...
{
  type: Button,
  props: {
    children: 'OK!',
    color: 'blue'
  }
}

// React: ...and Button told me this! I guess I'm done.
{
  type: 'button',
  props: {
    className: 'button button-blue',
    children: {
      type: 'b',
      props: {
        children: 'OK!'
      }
    }
  }
}

This is a part of the process that React calls reconciliation which starts when you call ReactDOM.render() or setState(). By the end of the reconciliation, React knows the resulting DOM tree, and a renderer like react-dom or react-native applies the minimal set of changes necessary to update the DOM nodes (or the platform-specific views in case of React Native).

This gradual refining process is also the reason React apps are easy to optimize. If some parts of your component tree become too large for React to visit efficiently, you can tell it to skip this “refining” and diffing certain parts of the tree if the relevant props have not changed. It is very fast to calculate whether the props have changed if they are immutable, so React and immutability work great together, and can provide great optimizations with the minimal effort.

You might have noticed that this blog entry talks a lot about components and elements, and not so much about the instances. The truth is, instances have much less importance in React than in most object-oriented UI frameworks.

Only components declared as classes have instances, and you never create them directly: React does that for you. While mechanisms for a parent component instance to access a child component instance exist, they are only used for imperative actions (such as setting focus on a field), and should generally be avoided.

React takes care of creating an instance for every class component, so you can write components in an object-oriented way with methods and local state, but other than that, instances are not very important in the React’s programming model and are managed by React itself.

Summary

An element is a plain object describing what you want to appear on the screen in terms of the DOM nodes or other components. Elements can contain other elements in their props. Creating a React element is cheap. Once an element is created, it is never mutated.

A component can be declared in several different ways. It can be a class with a render() method. Alternatively, in simple cases, it can be defined as a function. In either case, it takes props as an input, and returns an element tree as the output.

When a component receives some props as an input, it is because a particular parent component returned an element with its type and these props. This is why people say that the props flows one way in React: from parents to children.

An instance is what you refer to as this in the component class you write. It is useful for storing local state and reacting to the lifecycle events.

Function components don’t have instances at all. Class components have instances, but you never need to create a component instance directly—React takes care of this.

Finally, to create elements, use React.createElement(), JSX, or an element factory helper. Don’t write elements as plain objects in the real code—just know that they are plain objects under the hood.