cloneElement

const clonedElement = cloneElement(element, props, ...children)

• Viite
  • cloneElement(element, props, ...children)
• Käyttö
  • Overriding props of an element
• Alternatives
  • Passing data with a render prop
  • Passing data through context
  • Extracting logic into a custom Hook

Viite

cloneElement(element, props, ...children)

Call cloneElement to create a React element based on the element, but with different props and children:

import { cloneElement } from 'react';

// ...
const clonedElement = cloneElement(
  <Row title="Cabbage">
    Hello
  </Row>,
  { isHighlighted: true },
  'Goodbye'
);

console.log(clonedElement); // <Row title="Cabbage">Goodbye</Row>

See more examples below.

Parametrit

• element: The element argument must be a valid React element. For example, it could be a JSX node like <Something />, the result of calling createElement, or the result of another cloneElement call.

• props: The props argument must either be an object or null. If you pass null, the cloned element will retain all of the original element.props. Otherwise, for every prop in the props object, the returned element will “prefer” the value from props over the value from element.props. The rest of the props will be filled from the original element.props. If you pass props.key or props.ref, they will replace the original ones.

• optional ...children: Zero or more child nodes. They can be any React nodes, including React elements, strings, numbers, portals, empty nodes (null, undefined, true, and false), and arrays of React nodes. If you don’t pass any ...children arguments, the original element.props.children will be preserved.

Palautukset

cloneElement returns a React element object with a few properties:

• type: Same as element.type.
• props: The result of shallowly merging element.props with the overriding props you have passed.
• ref: The original element.ref, unless it was overridden by props.ref.
• key: The original element.key, unless it was overridden by props.key.

Usually, you’ll return the element from your component or make it a child of another element. Although you may read the element’s properties, it’s best to treat every element as opaque after it’s created, and only render it.

Rajoitukset

• Cloning an element does not modify the original element.

• You should only pass children as multiple arguments to cloneElement if they are all statically known, like cloneElement(element, null, child1, child2, child3). If your children are dynamic, pass the entire array as the third argument: cloneElement(element, null, listItems). This ensures that React will warn you about missing keys for any dynamic lists. For static lists this is not necessary because they never reorder.

• cloneElement makes it harder to trace the data flow, so try the alternatives instead.

Käyttö

Overriding props of an element

To override the props of some React element, pass it to cloneElement with the props you want to override:

import { cloneElement } from 'react';

// ...
const clonedElement = cloneElement(
  <Row title="Cabbage" />,
  { isHighlighted: true }
);

Here, the resulting cloned element will be <Row title="Cabbage" isHighlighted={true} />.

Let’s walk through an example to see when it’s useful.

Imagine a List component that renders its children as a list of selectable rows with a “Next” button that changes which row is selected. The List component needs to render the selected Row differently, so it clones every <Row> child that it has received, and adds an extra isHighlighted: true or isHighlighted: false prop:

export default function List({ children }) {
  const [selectedIndex, setSelectedIndex] = useState(0);
  return (
    <div className="List">
      {Children.map(children, (child, index) =>
        cloneElement(child, {
          isHighlighted: index === selectedIndex 
        })
      )}

Let’s say the original JSX received by List looks like this:

<List>
  <Row title="Cabbage" />
  <Row title="Garlic" />
  <Row title="Apple" />
</List>

By cloning its children, the List can pass extra information to every Row inside. The result looks like this:

<List>
  <Row
    title="Cabbage"
    isHighlighted={true} 
  />
  <Row
    title="Garlic"
    isHighlighted={false} 
  />
  <Row
    title="Apple"
    isHighlighted={false} 
  />
</List>

Notice how pressing “Next” updates the state of the List, and highlights a different row:

To summarize, the List cloned the <Row /> elements it received and added an extra prop to them.

Alternatives

Passing data with a render prop

Instead of using cloneElement, consider accepting a render prop like renderItem. Here, List receives renderItem as a prop. List calls renderItem for every item and passes isHighlighted as an argument:

export default function List({ items, renderItem }) {
  const [selectedIndex, setSelectedIndex] = useState(0);
  return (
    <div className="List">
      {items.map((item, index) => {
        const isHighlighted = index === selectedIndex;
        return renderItem(item, isHighlighted);
      })}

The renderItem prop is called a “render prop” because it’s a prop that specifies how to render something. For example, you can pass a renderItem implementation that renders a <Row> with the given isHighlighted value:

<List
  items={products}
  renderItem={(product, isHighlighted) =>
    <Row
      key={product.id}
      title={product.title}
      isHighlighted={isHighlighted}
    />
  }
/>

The end result is the same as with cloneElement:

<List>
  <Row
    title="Cabbage"
    isHighlighted={true} 
  />
  <Row
    title="Garlic"
    isHighlighted={false} 
  />
  <Row
    title="Apple"
    isHighlighted={false} 
  />
</List>

However, you can clearly trace where the isHighlighted value is coming from.

This pattern is preferred to cloneElement because it is more explicit.

Passing data through context

Another alternative to cloneElement is to pass data through context.

For example, you can call createContext to define a HighlightContext:

export const HighlightContext = createContext(false);

Your List component can wrap every item it renders into a HighlightContext provider:

export default function List({ items, renderItem }) {
  const [selectedIndex, setSelectedIndex] = useState(0);
  return (
    <div className="List">
      {items.map((item, index) => {
        const isHighlighted = index === selectedIndex;
        return (
          <HighlightContext.Provider key={item.id} value={isHighlighted}>
            {renderItem(item)}
          </HighlightContext.Provider>
        );
      })}

With this approach, Row does not need to receive an isHighlighted prop at all. Instead, it reads the context:

export default function Row({ title }) {
  const isHighlighted = useContext(HighlightContext);
  // ...

This allows the calling component to not know or worry about passing isHighlighted to <Row>:

<List
  items={products}
  renderItem={product =>
    <Row title={product.title} />
  }
/>

Instead, List and Row coordinate the highlighting logic through context.

Learn more about passing data through context.

Extracting logic into a custom Hook

Another approach you can try is to extract the “non-visual” logic into your own Hook, and use the information returned by your Hook to decide what to render. For example, you could write a useList custom Hook like this:

import { useState } from 'react';

export default function useList(items) {
  const [selectedIndex, setSelectedIndex] = useState(0);

  function onNext() {
    setSelectedIndex(i =>
      (i + 1) % items.length
    );
  }

  const selected = items[selectedIndex];
  return [selected, onNext];
}

Then you could use it like this:

export default function App() {
  const [selected, onNext] = useList(products);
  return (
    <div className="List">
      {products.map(product =>
        <Row
          key={product.id}
          title={product.title}
          isHighlighted={selected === product}
        />
      )}
      <hr />
      <button onClick={onNext}>
        Next
      </button>
    </div>
  );
}

The data flow is explicit, but the state is inside the useList custom Hook that you can use from any component:

This approach is particularly useful if you want to reuse this logic between different components.