Lourenci's Blog

What is the difference between a Component and a PureComponent?

August 05, 2019

In the class components land, you can define a component either by extending it from React.Component or from React.PureComponent.

Whatever component you use, when it changes its state or it receives a new prop from its parent, React will call the render lifecycle method of the component and of the all children of the component, the famous VDOM (kind of DOM in memory) will be generated and React will compare the generated VDOM to the previous generated VDOM (render phase), if any changes are detected React will apply them to the real DOM (commit phase). It’s much cheaper and faster to deal with the VDOM than to the real DOM, that’s the magic behind the “modern” front-end libs to be very fast.

Even React doing a lot of work under the hood to keep this process fast, the render method is the most expensive “routine” in it. We should avoid it whenever it’s possible.

shouldComponentUpdate lifecycle method

Just before to call the render method, React will call the shouldComponentUpdate lifecycle method passing the prevProps and the prevState as parameters. You can use this hook to prevent your component to be rendered. If you return false on it, React will not perform the render, thus it will not call the render lifecycle method. The <Counter /> below will never be shown updated:

class Counter extends React.Component {
  constructor (props) {
    super(props)
    this.state = { value: 0 }
  }

  shouldComponentUpdate (_prevProps, _prevState) {
    // This will make the render to never be called.
    return false
  }

  render () {
    return (
      <div>
        <span>{this.state.value}</span>
        <button
          type='button'
          onClick={() => this.setState(({ value }) => ({ value: value + 1 }))}
        >
          Increment
        </button>
      </div>
    )
  }
}

Where does the PureComponent differ from the Component?

When a component receives a new value for the prop or for the state, React will call the render method even when the change is to the same value.

With that in mind, the PureComponent has an implementation of the shouldComponentUpdate hook that performs a shallow comparison between the prevProps with the new props and the prevState with the new state. When the change is to the same values, the React shouldComponentUpdate implementation will return false preventing the render method to be called.

To demonstrate that, I made the component below that keeps a state for a value and it renders two components to show this value: one through the PureComponent and another with the Component. When the button is clicked, the state is changed to the same value. Every time the render is called on these components, a log appears in the console to show their updates. Click on the button and look at the console.

A shallow comparison is about to not be deep in some array or object comparison. If an object has others objects inside it, this inner objects will not be compared. Furthermore, objects (including arrays and functions) are compared by their references and primitives values by their values. You can see the shallow code on React’s github.

The use of the PureComponent is a good strategy to get a better performance on your project.

Leveraging the PureComponent

Don’t mutate

You shouldn’t mutate any parent’s object that you pass through a prop to its pure children, once they will perform a shallow comparison before to call the render, your UI children won’t get updated.

Try to bind functions in constructor whenever it’s possible

Every time the render of the parent of this PureComponent

<PureCounter
  fn={this.callback.bind(this, 'arg')}
  value={1}
/>

is called, a new function will be created and the shouldComponentUpdate will fail. This happens with arrow functions too.

In order to avoid that, you could pass the handle as one prop and its argument as another prop. Something like that:

// this.callback is binded in constructor.
<PureCounter
  fn={this.callback}
  fnArg='arg'
  value={1}
/>

// PureCounter/index.js
onClick () {
  this.props.fn(this.props.fnArg)
}

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