React: Do Children Always Rerender When the Parent Component Rerenders

React: Do children always rerender when the parent component rerenders?

I'll post your actual code for context:

class Application extends React.Component {
  render() {
    return (
      <div>
        {/* 
          Clicking this component only logs 
          the parents render function 
        */}
        <DynamicParent>
          <Child />
        </DynamicParent>

        {/* 
          Clicking this component logs both the 
          parents and child render functions 
        */}
        <StaticParent />
      </div>
    );
  }
}

class DynamicParent extends React.Component {
  state = { x: false };
  render() {
    console.log("DynamicParent");
    return (
      <div onClick={() => this.setState({ x: !this.state.x })}>
        {this.props.children}
      </div>
    );
  }
}

class StaticParent extends React.Component {
  state = { x: false };
  render() {
    console.log("StaticParent");
    return (
      <div onClick={() => this.setState({ x: !this.state.x })}>
        <Child />
      </div>
    );
  }
}

function Child(props) {
  console.log("child");
  return <div>Child Text</div>;
}

When you write this code in your Application render:

<StaticParent />

What's rendered is this:

 <div onClick={() => this.setState({ x: !this.state.x })}>
    <Child />
 </div>

And in reality, what happens (roughly) is this:

function StaticParent(props) {
  return React.createElement(
    "div",
    { onClick: () => this.setState({ x: !this.state.x }) },
    React.createElement(Child, null)
  );
}

React.createElement(StaticParent, null);

When you render your DynamicParent like this:

<DynamicParent>
    <Child />
</DynamicParent>

This is what actually happens (again, roughly speaking)

function DynamicParent(props) {
    return React.createElement(
        "div",
        { 
            onClick: () => this.setState({ x: !this.state.x }), 
            children: props.children 
        }
    );
}

React.createElement(
      DynamicParent,
      { children: React.createElement(Child, null) },
);

And this is the Child in both cases:

function Child(props) {
    return React.createElement("div", props, "Child Text");
}

What does this mean? Well, in your StaticParent component you're calling React.createElement(Child, null) every time the render method of StaticParent is called. In the DynamicParent case, the Child gets created once and passed as a prop. And since React.createElement is a pure function, then it's probably memoized somewhere for performance.

What would make Child's render run again in the DynamicParent case is a change in Child's props. If the parent's state was used as a prop to the Child, for example, that would trigger a re-render in both cases.

I really hope Dan Abramov doesn't show up on the comments to trash this answer, it was a pain to write (but entertaining)

does parent component re-renders when changes occur in child components?

No, it will not re-render. If you pass any props to the component from the parent component and you update that prop in children or that prop update in the parent component so both will re-render. But if the data or state has no dependency on the parent component so it will not cause a re-render in the parent component.

React: Parent component re-renders all children, even those that haven't changed on state change

If a parent component is updated, does React always update all the direct children within that component?

No. React will only re-render a component if shouldComponentUpdate() returns true. By default, that method always returns true to avoid any subtle bugs for newcomers (and as William B pointed out, the DOM won't actually update unless something changed, lowering the impact).

To prevent your sub-component from re-rendering unnecessarily, you need to implement the shouldComponentUpdate method in such a way that it only returns true when the data has actually changed. If this.props.messages is always the same array, it could be as simple as this:

shouldComponentUpdate(nextProps) {
    return (this.props.messages !== nextProps.messages);
}

You may also want to do some sort of deep comparison or comparison of the message IDs or something, it depends on your requirements.

EDIT: After a few years many people are using functional components. If that's the case for you then you'll want to check out React.memo. By default functional components will re-render every time just like the default behavior of class components. To modify that behavior you can use React.memo() and optionally provide an areEqual() function.

Does react hook component re-render when parent component re-renders?

Yes, the standard behavior in react is that when a component renders, its children render too. To improve performance you can skip some renders with shouldComponentUpdate on a class component or React.memo on a functional component, but this is a performance optimization, not something you should rely on for skipping effects.

If you're fetching data in a component, you often only want to do it on mount, or only when certain relevant data changes. This is controlled with useEffect's dependency array. To run it only on mount, provide an empty array []. The component will still rerender when the parent component changes, but its effect will not rerun.

How does React re-use child components / keep the state of child components when re-rendering the parent component?

createElement vs render vs mount

When a React Component such as your Button is rendered, a number of children are created with createElement. createElement(Timer, props, children) does not create an instance of the Timer component, or even render it, it only creates a "React element" which represents the fact that the component should be rendered.

When your Button is rendered, react will reconcile the result with the previous result, to decide what needs to be done with each child element:

• If the element is not matched to one in the previous result, then a component instance is created then mounted then rendered (recursively applying this same process). Note that when Button is rendered for the first time, all of the children will be new (because there is no previous result to match against).
• If the element is matched to one in the previous result, then the component instance is reused: its props are updated, then the component is re-rendered (again, recursively applying this same process). If the props did not change, React might even choose not to re-render as an efficiency.
• Any elements in the previous result that was not matched to an element in the new result will be unmounted and destroyed.

React's diffing algorithm

An element "matches" another one if React compares them and they have the same type.

The default way for React to compare children, is to simply iterate over both lists of children at the same time, comparing the first elements with each other, then the second, etc.

If the children have keys, then each child in the new list is compared to the child in the old list that has the same key.

See the React Reconciliation Docs for a more detailed explanation.

Examples

Your Button always returns exactly one element: a button. So, when your Button re-renders, the button matches, and its DOM element is re-used, then the children of the button are compared.

The first child is always a Timer, so the type matches and the component instance is reused. The Timer props did not change, so React might re-render it (calling render on the instance with the same state), or it might not re-render it, leaving that part of the tree untouched. Both of these cases would result in the same result in your case - because you have no side-effects in render - and React deliberately leaves the decision of when to re-render as an implementation detail.

The second child is always the string "Clicks: " so react leaves that DOM element alone too.

If this.state.click has changed since the last render, then the third child will be a different string, maybe changing from "0" to "1", so the text node will be replaced in the DOM.

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If Buttons render were to return a root element of a different type like so:

  render() {
    return createElement(this.state.clicks % 2 ? 'button' : 'a', { onClick: () => this.click() },
      createElement(Timer, null),
      'Clicks: ',
      this.state.clicks
    );
  }

then in the first step, the a would be compared to the button and because they are different types, the old element and all of its children would be removed from the DOM, unmounted, and destroyed. Then the new element would be created with no previous render result, and so a new Timer instance would be created with fresh state, and the timer would be back at 0.

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