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React.memo and React.PureComponent

nibble profile image Joseph Mawa Updated on ・2 min read

Outline

React.PureComponent

In react, components can be defined as functions or ES6 classes. All class components are sub classes of either Component or PureComponent. To understand when PureComponent base class is used for declaring your class component, it is important to understand when a react component defined as a sub class of Component re-renders.

When a class component is a sub class of Component, it is re-rendered whenever:

  • state changes
  • context changes
  • parent re-renders
  • props change

You can see from the above that a component is re-rendered whenever its parent re-renders though its context, state or props haven't changed. To prevent a component from re-rendering because of its parent's re-render, you can declare it as a sub class of PureComponent. PureComponent implements shouldComponentUpdate out of the box. It does a shallow comparison of props and state. If the state and props haven't changed, react doesn't re-render the component even if the parent re-renders. Since this is a shallow comparison, pureComponent should be avoided if your component is deeply nested because react can produce false-negatives for deeper differences. It should only be used if the component is pure.
According to react documentation:

If your React component’s render() function renders the same result given the same props and state, you can use React.PureComponent for a performance boost in some cases.

Below is how a PureComponent is defined using ES6 class.

class App extends React.PureComponent{
    constructor(props){
       super(props);
       this.state = { };
   }
   render(){
    return <div className = "app"> Hello World! </div>
   }
}
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React.memo

React.memo is the functional component equivalent of React.PureComponent. It is a higher-order component. If React.memo wraps a component, it memoizes the rendered output and skips subsequent renders if state, props, or context have not changed. It is worth pointing out that React.memo checks for props changes. If the component's state or context change, the component is re-rendered even if the props haven't. React.memo makes a shallow comparison of props. It also takes a function as second argument if you want to have control over the comparison.

React.memo(App, function comparator(prevProps, nextProps){
});
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The second argument, comparator, returns true if its arguments are equal and false otherwise. This helps React.memo determine whether to re-render the component or not.


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