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:

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(

, document.body);

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

By default - yes.

There is a method boolean shouldComponentUpdate(object nextProps, object nextState), each component has this method and it's responsible to determine "should component update (run render function)?" every time you change state or pass new props from parent component.

You can write your own implementation of shouldComponentUpdate method for your component, but default implementation always returns true - meaning always re-run render function.

Quote from official docs http://facebook.github.io/react/docs/component-specs.html#updating-shouldcomponentupdate

> By default, shouldComponentUpdate always returns true to prevent > subtle bugs when the state is mutated in place, but if you are careful to > always treat the state as immutable and to read-only from props and state > in render() then you can override shouldComponentUpdate with an > implementation that compares the old props and state to their > replacements.

Next part of your question: > If so, why? I thought the idea was that React only rendered as little as needed - when the state changed.

There are two steps of what we may call "render":

1. Virtual DOM renders: when render method is called it returns a new virtual dom structure of the component. As I mentioned before, this render method is called always when you call setState(), because shouldComponentUpdate always returns true by default. So, by default, there is no optimization here in React.

2. Native DOM renders: React changes real DOM nodes in your browser only if they were changed in the Virtual DOM and as little as needed - this is that great React's feature which optimizes real DOM mutation and makes React fast.

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.

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.

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'});
    },

    shouldComponentUpdate: function(nextProps, nextState) {
      if (this.state == null)
        return true;
  
      if (this.state.test == nextState.test)
        return false;
        
      return true;
  },

    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);

<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>

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

It seems that the accepted answers are no longer the case when using React hooks (with primitive values, see comments on this answer for details). 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

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.)

React 18 and beyond

Starting from React 18 all state updates are automatically batched to groups multiple state updates into a single re-render for better performance.

So when you update your state, React always try to batch these updates in a group update, causing fewer render than setState calls. The behaviour is the same when using hooks.

You can read the very long explanation in the Automatic batching for React 18 announcement.

React 17 and below

In React 17 and below, only updates inside React event handlers are batched. Updates inside of promises, setTimeout, native event handlers, or any other event were not batched in React by default.

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.