[javascript] ReactJS - Does render get called any time "setState" is called?

Does React re-render all components and sub components every time setState() is called?

If so, why? I thought the idea was that React only rendered as little as needed - when state changed.

In the following simple example, both classes render again when the text is clicked, despite the fact that the state doesn't change on subsequent clicks, as the onClick handler always sets the state to the same value:

this.setState({'test':'me'});

I would've expected that renders would only happen if state data had changed.

Here's the code of the example, as a JS Fiddle, and embedded snippet:

_x000D_
_x000D_
var TimeInChild = React.createClass({
    render: function() {
        var t = new Date().getTime();

        return (
            <p>Time in child:{t}</p>
        );
    }
});

var Main = React.createClass({
    onTest: function() {
        this.setState({'test':'me'});
    },

    render: function() {
        var currentTime = new Date().getTime();

        return (
            <div onClick={this.onTest}>
            <p>Time in main:{currentTime}</p>
            <p>Click me to update time</p>
            <TimeInChild/>
            </div>
        );
    }
});

ReactDOM.render(<Main/>, document.body);
_x000D_
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/15.0.0/react.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/15.0.0/react-dom.min.js"></script>
_x000D_
_x000D_
_x000D_

This question is related to javascript reactjs

The answer is


No, React doesn't render everything when the state changes.

  • Whenever a component is dirty (its state changed), that component and its children are re-rendered. This, to some extent, is to re-render as little as possible. The only time when render isn't called is when some branch is moved to another root, where theoretically we don't need to re-render anything. In your example, TimeInChild is a child component of Main, so it also gets re-rendered when the state of Main changes.

  • React doesn't compare state data. When setState is called, it marks the component as dirty (which means it needs to be re-rendered). The important thing to note is that although render method of the component is called, the real DOM is only updated if the output is different from the current DOM tree (a.k.a diffing between the Virtual DOM tree and document's DOM tree). In your example, even though the state data hasn't changed, the time of last change did, making Virtual DOM different from the document's DOM, hence why the HTML is updated.


Not All Components.

the state in component looks like the source of the waterfall of state of the whole APP.

So the change happens from where the setState called. The tree of renders then get called from there. If you've used pure component, the render will be skipped.


Another reason for "lost update" can be the next:

  • If the static getDerivedStateFromProps is defined then it is rerun in every update process according to official documentation https://reactjs.org/docs/react-component.html#updating.
  • so if that state value comes from props at the beginning it is overwrite in every update.

If it is the problem then U can avoid setting the state during update, you should check the state parameter value like this

static getDerivedStateFromProps(props: TimeCorrectionProps, state: TimeCorrectionState): TimeCorrectionState {
   return state ? state : {disable: false, timeCorrection: props.timeCorrection};
}

Another solution is add a initialized property to state, and set it up in the first time (if the state is initialized to non null value.)


Yes. It calls the render() method every time we call setState only except when "shouldComponentUpdate" returns false.


Even though it's stated in many of the other answers here, the component should either:

  • implement shouldComponentUpdate to render only when state or properties change

  • switch to extending a PureComponent, which already implements a shouldComponentUpdate method internally for shallow comparisons.

Here's an example that uses shouldComponentUpdate, which works only for this simple use case and demonstration purposes. When this is used, the component no longer re-renders itself on each click, and is rendered when first displayed, and after it's been clicked once.

_x000D_
_x000D_
var TimeInChild = React.createClass({_x000D_
    render: function() {_x000D_
        var t = new Date().getTime();_x000D_
_x000D_
        return (_x000D_
            <p>Time in child:{t}</p>_x000D_
        );_x000D_
    }_x000D_
});_x000D_
_x000D_
var Main = React.createClass({_x000D_
    onTest: function() {_x000D_
        this.setState({'test':'me'});_x000D_
    },_x000D_
_x000D_
    shouldComponentUpdate: function(nextProps, nextState) {_x000D_
      if (this.state == null)_x000D_
        return true;_x000D_
  _x000D_
      if (this.state.test == nextState.test)_x000D_
        return false;_x000D_
        _x000D_
      return true;_x000D_
  },_x000D_
_x000D_
    render: function() {_x000D_
        var currentTime = new Date().getTime();_x000D_
_x000D_
        return (_x000D_
            <div onClick={this.onTest}>_x000D_
            <p>Time in main:{currentTime}</p>_x000D_
            <p>Click me to update time</p>_x000D_
            <TimeInChild/>_x000D_
            </div>_x000D_
        );_x000D_
    }_x000D_
});_x000D_
_x000D_
ReactDOM.render(<Main/>, document.body);
_x000D_
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/15.0.0/react.min.js"></script>_x000D_
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/15.0.0/react-dom.min.js"></script>
_x000D_
_x000D_
_x000D_


It seems that the accepted answers are no longer the case when using React hooks. You can see in this code sandbox that the class component is rerendered when the state is set to the same value, while in the function component, setting the state to the same value doesn't cause a rerender.

https://codesandbox.io/s/still-wave-wouk2?file=/src/App.js