React: update one item in a list without recreating all items

Question

Let's say I have a list of 1000 items. And I rendering it with React, like this:

class Parent extends React.Component {
  render() {
    // this.state.list is a list of 1000 items
    return <List list={this.state.list} />;
  }
}

class List extends React.Component {
  render() {
    // here we're looping through this.props.list and creating 1000 new Items
    var list = this.props.list.map(item => {
      return <Item key={item.key} item={item} />;
    });
    return <div>{list}</div>;
  }
}

class Item extends React.Component {
  shouldComponentUpdate() {
    // here I comparing old state/props with new
  }
  render() {
    // some rendering here...
  }
}

With a relatively long list map() takes about 10-20ms and I can notice a small lag in the interface.

Can I prevent recreation of 1000 React objects every time when I only need to update one?

Answer 1

There is a way to do this, but I don't think the React team would call it Kosher so to speak. Basically, instead of the parent having state, it has lists of refs. Your components have their own state. Your components are created once in the parent and stored in a parent's ref property which holds an array of all those components. So the components are never recreated on each rerender, and instead are persisted. You also would need a list of refs that attach to a function in each component to allow the parent to call individual components (in hooks you can use imperativehandle to do this).

Now, when the user does something that would cause the data to change for a specific component in that list, you would find that component's ref in the list of attached functions and call the function on it. The component could then update and rerender itself based off that function call from its parent, without other components being affected/recreated/rerendered.

I believe this is called imperative programming rather than declarative, and React doesn't like it. I personally have used this technique in my own projects for similar reasons to you, and it worked for me.

Answer 2

You can do it by using any state management library, so that your Parent doesn't keep track of this.state.list => your List only re-renders when new Item is added. And the individual Item will re-render when they are updated.

Lets say you use redux.

Your code will become something like this:

// Parent.js
class Parent extends React.Component {
  render() {        
    return <List />;
  }
}

// List.js
class List extends React.Component {
  render() {        
    var list = this.props.list.map(item => {
      return <Item key={item.key} uniqueKey={item.key} />;
    });
    return <div>{list}</div>;
  }
}

const mapStateToProps = (state) => ({
  list: getList(state)
});

export default connect(mapStateToProps)(List);
 

// Item.js
class Item extends React.Component {
  shouldComponentUpdate() {
  }
  render() {
  }
}

const mapStateToProps = (state, ownProps) => ({
  item: getItemByKey(ownProps.uniqueKey)
});

export default connect(mapStateToProps)(Item);

Of course, you have to implement the reducer and the two selectors getList and getItemByKey.

With this, you List re-render will be trigger if new elements added, or if you change item.key (which you shouldn't)

Answer 3

EDIT:

My inital suggestions only addressed possible efficiency improvements to rendered lists and did not address the question about limiting the re-rendering of components as a result of the list changing.

See @xiaofan2406's answer for a clean solution to the original question.

---

Libraries that help make rendering long lists more efficient and easy:

React Infinite

React-Virtualized

Answer 4

When you change your data, react default operation is to render all children components, and creat virtual dom to judge which component is need to be rerender.

So, if we can let react know there is only one component need to be rerender. It can save times.

You can use shouldComponentsUpdate in your list component.

If in this function return false, react will not create vitual dom to judge.

I assume your data like this [{name: 'name_1'}, {name: 'name_2'}]

class Item extends React.Component {
  // you need judge if props and state have been changed, if not
  // execute return false;
  shouldComponentUpdate(nextProps, nextState) { 
    if (nextProps.name === this.props.name) return false;

    return true;
  }
  render() {
    return (
      <li>{this.props.name}</li>
   )
  }
}

As react just render what have been changed component. So if you just change one item's data, others will not do render.

Answer 5

There are a few things you can do:

1. When you build, make sure you are setting NODE_ENV to production. e.g. NODE_ENV=production npm run build or similar. ReactJS performs a lot of safety checks when NODE_ENV is not set to production, such as PropType checks. Switching these off should give you a >2x performance improvement for React rendering, and is vital for your production build (though leave it off during development - those safety checks help prevent bugs!). You may find this is good enough for the number of items you need to support.
2. If the elements are in a scrollable panel, and you can only see a few of them, you can set things up only to render the visible subset of elements. This is easiest when the items have fixed height. The basic approach is to add firstRendered/lastRendered props to your List state (that's first inclusive and last exclusive of course). In List.render, render a filler blank div (or tr if applicable) of the correct height (firstRendered * itemHeight), then your rendered range of items [firstRendered, lastRendered), then another filler div with the remaining height ((totalItems - lastRendered) * itemHeight). Make sure you give your fillers and items fixed keys. You then just need to handle onScroll on the scrollable div, and work out what the correct range to render is (generally you want to render a decent overlap off the top and bottom, also you want to only trigger a setState to change the range when you get near to the edge of it). A crazier alternative is to render and implement your own scrollbar (which is what Facebook's own FixedDataTable does I think - https://facebook.github.io/fixed-data-table/). There are lots of examples of this general approach here https://react.rocks/tag/InfiniteScroll
3. Use a sideways loading approach using a state management library. For larger apps this is essential anyway. Rather than passing the Items' state down from the top, have each Item retrieve its own state, either from 'global' state (as in classical Flux), or via React context (as in modern Flux implementations, MobX, etc.). That way, when an item changes, only that item needs to re-render.

Answer 6

One way to avoid looping through the component list every render would be to do it outside of render function and save it to a variable.

class Item extends React.Component {
    shouldComponentUpdate(nextProps, nextState) {
        return this.props.item != nextProps.item;
    }

    render() {
        return <li>{this.props.item}</li>;
    }
}

class List extends React.Component {
    constructor(props) {
        super(props);
        this.items = [];
        this.update = this.update.bind(this);
    }

    componentWillMount() {
        this.props.items.forEach((item, index) => { this.items[index] = <Item key={index} item={item} /> });
    }

    update(index) {

        this.items[index] = <Item key={index} item={'item changed'} />
        this.forceUpdate();
    }

    render() {
        return <div>
            <button onClick={() => { this.update(199); }}>Update</button>
            <ul>{this.items}</ul>
        </div>
    }
}