A tiny React hook for rendering large datasets like a breeze.

Features

♻️ Renders millions of items with highly performant way, using DOM recycling.
🎣 Easy to use, based on React hook.
💅🏼 Apply styles without hassle, just few setups.
🧱 Supports fixed, variable, dynamic, and real-time heights/widths.
🖥 Supports responsive web design (RWD) for better UX.
📌 Supports sticky headers for building on-trend lists.
🚚 Built-ins load more callback for you to deal with infinite scroll + skeleton screens.
🖱 Imperative scroll-to methods for offset, items, and alignment.
🛹 Out-of-the-box smooth scrolling and the effect is DIY-able.
💬 It's possible to implement stick to bottom and pre-pending items for chat, feeds, etc.
⛳ Provides isScrolling indicator to you for UI placeholders or performance optimization.
🗄️ Supports server-side rendering (SSR) for a fast FP + FCP and better SEO.
📜 Supports TypeScript type definition.
🎛 Super flexible API design, built with DX in mind.
🦔 Tiny size (~ 3.1kB gzipped). No external dependencies, aside from the react.

Why?

When rendering a large set of data (e.g. list, table, etc.) in React, we all face performance/memory troubles. There're some great libraries already available but most of them are component-based solutions that provide well-defineded way of using but increase a lot of bundle size. However, a library comes out as a hook-based solution that is flexible and headless but using and styling it can be verbose (because it's a low-level hook). Furthermore, it lacks many of the useful features.

React Cool Virtual is a tiny React hook that gives you a better DX and modern way for virtualizing a large amount of data without struggle 🤯.

Docs

Getting Started

Requirement

To use React Cool Virtual, you must use react@16.8.0 or greater which includes hooks.

Installation

This package is distributed via npm.

$ yarn add react-cool-virtual
# or
$ npm install --save react-cool-virtual

⚠️ This package using ResizeObserver API under the hook. Most modern browsers support it natively, you can also add polyfill for full browser support.

CDN

If you're not using a module bundler or package manager. We also provide a UMD build which is available over the unpkg.com CDN. Simply use a <script> tag to add it after React CDN links as below:

<script crossorigin src="https://unpkg.com/react/umd/react.production.min.js"></script>
<script crossorigin src="https://unpkg.com/react-dom/umd/react-dom.production.min.js"></script>
<!-- react-cool-virtual comes here -->
<script crossorigin src="https://unpkg.com/react-cool-virtual/dist/index.umd.production.min.js"></script>

Once you've added this you will have access to the window.ReactCoolVirtual.useVirtual variable.

Basic Usage

Here's the basic concept of how it rocks:

import useVirtual from "react-cool-virtual";

const List = () => {
  const { outerRef, innerRef, items } = useVirtual({
    itemCount: 10000, // Provide the total number for the list items
    itemSize: 50, // The size of each item (default = 50)
  });

  return (
    <div
      ref={outerRef} // Attach the `outerRef` to the scroll container
      style={{ width: "300px", height: "500px", overflow: "auto" }}
    >
      {/* Attach the `innerRef` to the wrapper of the items */}
      <div ref={innerRef}>
        {items.map(({ index, size }) => (
          // You can set the item's height with the `size` property
          <div key={index} style={{ height: `${size}px` }}>
            ⭐️ {index}
          </div>
        ))}
      </div>
    </div>
  );
};

✨ Pretty easy right? React Cool Virtual is more powerful than you think. Let's explore more use cases through the examples!

Examples

Fixed Size

This example demonstrates how to create a fixed size row. For column or grid, please refer to CodeSandbox.

import useVirtual from "react-cool-virtual";

const List = () => {
  const { outerRef, innerRef, items } = useVirtual({
    itemCount: 1000,
  });

  return (
    <div
      style={{ width: "300px", height: "300px", overflow: "auto" }}
      ref={outerRef}
    >
      <div ref={innerRef}>
        {items.map(({ index, size }) => (
          <div key={index} style={{ height: `${size}px` }}>
            ⭐️ {index}
          </div>
        ))}
      </div>
    </div>
  );
};

Variable Size

This example demonstrates how to create a variable size row. For column or grid, please refer to CodeSandbox.

import useVirtual from "react-cool-virtual";

const List = () => {
  const { outerRef, innerRef, items } = useVirtual({
    itemCount: 1000,
    itemSize: (idx) => (idx % 2 ? 100 : 50),
  });

  return (
    <div
      style={{ width: "300px", height: "300px", overflow: "auto" }}
      ref={outerRef}
    >
      <div ref={innerRef}>
        {items.map(({ index, size }) => (
          <div key={index} style={{ height: `${size}px` }}>
            ⭐️ {index}
          </div>
        ))}
      </div>
    </div>
  );
};

Dynamic Size

This example demonstrates how to create a dynamic (unknown) size row. For column or grid, please refer to CodeSandbox.

import useVirtual from "react-cool-virtual";

const List = () => {
  const { outerRef, innerRef, items } = useVirtual({
    itemCount: 1000,
    itemSize: 75, // The unmeasured item sizes will refer to this value (default = 50)
  });

  return (
    <div
      style={{ width: "300px", height: "300px", overflow: "auto" }}
      ref={outerRef}
    >
      <div ref={innerRef}>
        {items.map(({ index, measureRef }) => (
          // Use the `measureRef` to measure the item size
          <div key={index} ref={measureRef}>
            {/* Some data... */}
          </div>
        ))}
      </div>
    </div>
  );
};

💡 The scrollbar is jumping (or unexpected position)? It's because the total size of the items is gradually corrected along with an item that has been measured. You can tweak the itemSize to reduce the phenomenon.

Real-time Resize

This example demonstrates how to create a real-time resize row (e.g. accordion, collapse, etc.). For column or grid, please refer to CodeSandbox.

import { useState, forwardRef } from "react";
import useVirtual from "react-cool-virtual";

const AccordionItem = forwardRef(({ children, height, ...rest }, ref) => {
  const [h, setH] = useState(height);

  return (
    <div
      {...rest}
      style={{ height: `${h}px` }}
      ref={ref}
      onClick={() => setH((prevH) => (prevH === 50 ? 100 : 50))}
    >
      {children}
    </div>
  );
});

const List = () => {
  const { outerRef, innerRef, items } = useVirtual({
    itemCount: 50,
  });

  return (
    <div
      style={{ width: "300px", height: "300px", overflow: "auto" }}
      ref={outerRef}
    >
      <div ref={innerRef}>
        {items.map(({ index, size, measureRef }) => (
          // Use the `measureRef` to measure the item size
          <AccordionItem key={index} height={size} ref={measureRef}>
            👋🏻 Click Me
          </AccordionItem>
        ))}
      </div>
    </div>
  );
};

Responsive Web Design (RWD)

This example demonstrates how to create a list with RWD to provide a better UX for the user.

import useVirtual from "react-cool-virtual";

const List = () => {
  const { outerRef, innerRef, items } = useVirtual({
    itemCount: 1000,
    // Use the outer's width (2nd parameter) to adjust the item's size
    itemSize: (_, width) => (width > 400 ? 50 : 100),
    // The event will be triggered on outer's size changes
    onResize: (size) => console.log("Outer's size: ", size),
  });

  return (
    <div
      style={{ width: "100%", height: "400px", overflow: "auto" }}
      ref={outerRef}
    >
      <div ref={innerRef}>
        {/* We can also access the outer's width here */}
        {items.map(({ index, size, width }) => (
          <div key={index} style={{ height: `${size}px` }}>
            ⭐️ {index} ({width})
          </div>
        ))}
      </div>
    </div>
  );
};

💡 If the item size is specified through the function of itemSize, please ensure there's no the measureRef on the item element. Otherwise, the hook will use the measured (cached) size for the item. When working with RWD, we can only use either of the two.

Sticky Headers

This example demonstrates how to make sticky headers with React Cool Virtual.

import useVirtual from "react-cool-virtual";

const List = () => {
  const { outerRef, innerRef, items } = useVirtual({
    itemCount: 1000,
    itemSize: 75,
    stickyIndices: [0, 10, 20, 30, 40, 50], // The values must be provided in ascending order
  });

  return (
    <div
      style={{ width: "300px", height: "300px", overflow: "auto" }}
      ref={outerRef}
    >
      <div ref={innerRef}>
        {items.map(({ index, size, isSticky }) => {
          let style = { height: `${size}px` };
          // Use the `isSticky` property to style the sticky item, that's it ✨
          style = isSticky ? { ...style, position: "sticky", top: "0" } : style;

          return (
            <div key={someData[index].id} style={style}>
              {someData[index].content}
            </div>
          );
        })}
      </div>
    </div>
  );
};

💡 For better performance & accessibility. We encourage you to add will-change:transform to the positioned elements to render the element in its own layer, improving repaint speed and therefore improving performance and accessibility.

💡 The scrollbar disappears when using Chrome in Mac? If you encounter this issue, you can add will-change:transform to the outer element to workaround this problem.

Scroll to Offset / Items

You can imperatively scroll to offset or items as follows:

const { scrollTo, scrollToItem } = useVirtual();

const scrollToOffset = () => {
  // Scrolls to 500px
  scrollTo(500, () => {
    // 🤙🏼 Do whatever you want through the callback
  });
};

const scrollToItem = () => {
  // Scrolls to the 500th item
  scrollToItem(500, () => {
    // 🤙🏼 Do whatever you want through the callback
  });

  // We can control the alignment of the item with the `align` option
  // Acceptable values are: "auto" (default) | "start" | "center" | "end"
  // Using "auto" will scroll the item into the view at the start or end, depending on which is closer
  scrollToItem({ index: 10, align: "auto" });
};

Smooth Scrolling

React Cool Virtual provides the smooth scrolling feature out of the box, all you need to do is turn the smooth option on.

const { scrollTo, scrollToItem } = useVirtual();

// Smoothly scroll to 500px
const scrollToOffset = () => scrollTo({ offset: 500, smooth: true });

// Smoothly scroll to the 500th item
const scrollToItem = () => scrollToItem({ index: 10, smooth: true });

💡 When working with dynamic size, the scroll position will be automatically corrected along with the items are measured. To optimize it, we can provide an estimated item size to the itemSize option.

The default easing effect is easeInOutSine, and the duration is 100ms <= distance * 0.075 <= 500ms. You can easily customize your own effect as follows:

const { scrollTo } = useVirtual({
  // For 500 milliseconds
  scrollDuration: 500,
  // Or whatever duration you want based on the scroll distance
  scrollDuration: (distance) => distance * 0.05,
  // Using "easeInOutBack" effect (default = easeInOutSine), see: https://easings.net/#easeInOutSine
  scrollEasingFunction: (t) => {
    const c1 = 1.70158;
    const c2 = c1 * 1.525;

    return t < 0.5
      ? (Math.pow(2 * t, 2) * ((c2 + 1) * 2 * t - c2)) / 2
      : (Math.pow(2 * t - 2, 2) * ((c2 + 1) * (t * 2 - 2) + c2) + 2) / 2;
  },
});

const scrollToOffset = () => scrollTo({ offset: 500, smooth: true });

💡 For more cool easing effects, please check it out.

Infinite Scroll

It's possible to make a complicated infinite scroll logic simple by just using a hook, no kidding! Let's see how possible 🤔.

Working with Skeleton Screens

import { useState } from "react";
import useVirtual from "react-cool-virtual";
import axios from "axios";

const TOTAL_COMMENTS = 500;
const BATCH_COMMENTS = 5;
const isItemLoadedArr = [];

const loadData = async ({ loadIndex }, setComments) => {
  // Set the state of a batch items as `true`
  // to avoid the callback from being invoked repeatedly
  isItemLoadedArr[loadIndex] = true;

  try {
    const { data: comments } = await axios(`/comments?postId=${loadIndex + 1}`);

    setComments((prevComments) => {
      const nextComments = [...prevComments];

      comments.forEach((comment) => {
        nextComments[comment.id - 1] = comment;
      });

      return nextComments;
    });
  } catch (err) {
    // If there's an error set the state back to `false`
    isItemLoadedArr[loadIndex] = false;
    // Then try again
    loadData({ loadIndex }, setComments);
  }
};

const List = () => {
  const [comments, setComments] = useState([]);
  const { outerRef, innerRef, items } = useVirtual({
    itemCount: TOTAL_COMMENTS,
    // Estimated item size (with padding)
    itemSize: 122,
    // The number of items that you want to load/or pre-load, it will trigger the `loadMore` callback
    // when the user scrolls within every items, e.g. 1 - 5, 6 - 10, and so on (default = 15)
    loadMoreCount: BATCH_COMMENTS,
    // Provide the loaded state of a batch items to the callback for telling the hook
    // whether the `loadMore` should be triggered or not
    isItemLoaded: (loadIndex) => isItemLoadedArr[loadIndex],
    // We can fetch the data through the callback, it's invoked when more items need to be loaded
    loadMore: (e) => loadData(e, setComments),
  });

  return (
    <div
      style={{ width: "300px", height: "500px", overflow: "auto" }}
      ref={outerRef}
    >
      <div ref={innerRef}>
        {items.map(({ index, measureRef }) => (
          <div
            key={comments[index]?.id || `fb-${index}`}
            style={{ padding: "16px", minHeight: "122px" }}
            ref={measureRef} // Used to measure the unknown item size
          >
            {comments[index]?.body || "⏳ Loading..."}
          </div>
        ))}
      </div>
    </div>
  );
};

Working with A Loading Indicator

import { Fragment, useState } from "react";
import useVirtual from "react-cool-virtual";
import axios from "axios";

const TOTAL_COMMENTS = 500;
const BATCH_COMMENTS = 5;
const isItemLoadedArr = [];
// We only have 50 (500 / 5) batches of items, so set the 51th (index = 50) batch as `true`
// to avoid the `loadMore` callback from being invoked, yep it's a trick 😉
isItemLoadedArr[50] = true;

const loadData = async ({ loadIndex }, setComments) => {
  isItemLoadedArr[loadIndex] = true;

  try {
    const { data: comments } = await axios(`/comments?postId=${loadIndex + 1}`);

    setComments((prevComments) => [...prevComments, ...comments]);
  } catch (err) {
    isItemLoadedArr[loadIndex] = false;
    loadData({ loadIndex }, setComments);
  }
};

const Loading = () => <div>⏳ Loading...</div>;

const List = () => {
  const [comments, setComments] = useState([]);
  const { outerRef, innerRef, items } = useVirtual({
    itemCount: comments.length, // Provide the number of comments
    loadMoreCount: BATCH_COMMENTS,
    isItemLoaded: (loadIndex) => isItemLoadedArr[loadIndex],
    loadMore: (e) => loadData(e, setComments),
  });

  return (
    <div
      style={{ width: "300px", height: "500px", overflow: "auto" }}
      ref={outerRef}
    >
      <div ref={innerRef}>
        {items.length ? (
          items.map(({ index, measureRef }) => {
            const showLoading =
              index === comments.length - 1 && comments.length < TOTAL_COMMENTS;

            return (
              <Fragment key={comments[index].id}>
                <div ref={measureRef}>{comments[index].body}</div>
                {showLoading && <Loading />}
              </Fragment>
            );
          })
        ) : (
          <Loading />
        )}
      </div>
    </div>
  );
};

Pre-pending Items

This example demonstrates how to pre-pend items and maintain scroll position for the user.

import { useEffect, useLayoutEffect, useState } from "react";

import useVirtual from "react-cool-virtual";
import axios from "axios";

const TOTAL_COMMENTS = 500;
const BATCH_COMMENTS = 5;
let shouldFetchData = true;
let postId = 100;

const fetchData = async (postId, setComments) => {
  try {
    const { data: comments } = await axios(`/comments?postId=${postId}`);

    // Pre-pend new items
    setComments((prevComments) => [...comments, ...prevComments]);
  } catch (err) {
    // Try again
    fetchData(postId, setComments);
  }
};

const List = () => {
  const [comments, setComments] = useState([]);
  const { outerRef, innerRef, items, startItem } = useVirtual({
    // Provide the number of comments
    itemCount: comments.length,
    onScroll: ({ scrollForward, scrollOffset }) => {
      // Tweak the threshold of data fetching that you want
      if (!scrollForward && scrollOffset < 50 && shouldFetchData) {
        fetchData(--postId, setComments);
        shouldFetchData = false;
      }
    },
  });

  useEffect(() => fetchData(postId, setComments), []);

  // Execute the `startItem` through `useLayoutEffect` before the browser to paint
  // See https://reactjs.org/docs/hooks-reference.html#uselayouteffect to learn more
  useLayoutEffect(() => {
    // After the list updated, maintain the previous scroll position for the user
    startItem(BATCH_COMMENTS, () => {
      // After the scroll position updated, re-allow data fetching
      if (comments.length < TOTAL_COMMENTS) shouldFetchData = true;
    });
  }, [comments.length, startItem]);

  return (
    <div
      style={{ width: "300px", height: "500px", overflow: "auto" }}
      ref={outerRef}
    >
      <div ref={innerRef}>
        {items.length ? (
          items.map(({ index, measureRef }) => (
            // Used to measure the unknown item size
            <div key={comments[index].id} ref={measureRef}>
              {comments[index].body}
            </div>
          ))
        ) : (
          <div className="item">⏳ Loading...</div>
        )}
      </div>
    </div>
  );
};

Filtering Items

When working with filtering items, we can reset the scroll position when the itemCount is changed by enabling the resetScroll option.

import { useState } from "react";
import useVirtual from "react-cool-virtual";

const List = () => {
  const [itemCount, setItemCount] = useState(100);
  const { outerRef, innerRef, items } = useVirtual({
    itemCount,
    // Resets the scroll position when the `itemCount` is changed (default = false)
    resetScroll: true,
  });

  return (
    <div
      style={{ width: "300px", height: "300px", overflow: "auto" }}
      ref={outerRef}
    >
      <div ref={innerRef}>
        {items.map(({ index, size }) => (
          <div key={index} style={{ height: `${size}px` }}>
            ⭐️ {index}
          </div>
        ))}
      </div>
    </div>
  );
};

Sticking to Bottom

This example demonstrates the scenario of sticking/unsticking the scroll position to the bottom for a chatroom.

import { useState, useEffect } from "react";
import useVirtual from "react-cool-virtual";
import axios from "axios";

const TOTAL_MESSAGES = 200;
let isScrolling = false; // Used to prevent UX conflict
let id = 0;

const loadData = async (id, setMessages) => {
  try {
    const { data: messages } = await axios(`/messages/${id}`);

    setMessages((prevMessages) => [...prevMessages, messages]);
  } catch (err) {
    loadData(id, setMessages);
  }
};

const Chatroom = () => {
  const [shouldSticky, setShouldSticky] = useState(true);
  const [messages, setMessages] = useState([]);
  const { outerRef, innerRef, items, scrollToItem } = useVirtual({
    // Provide the number of messages
    itemCount: messages.length,
    // You can speed up smooth scrolling
    scrollDuration: 50,
    onScroll: ({ userScroll }) => {
      // If the user scrolls and isn't automatically scrolling, cancel stick to bottom
      if (userScroll && !isScrolling) setShouldSticky(false);
    },
  });

  useEffect(() => {
    // Mock messages service
    if (id <= TOTAL_MESSAGES)
      setTimeout(
        () => loadData(++id, setMessages),
        Math.floor(500 + Math.random() * 2000)
      );
  }, [messages.length]);

  useEffect(() => {
    // Automatically stick to bottom, using smooth scrolling for better UX
    if (shouldSticky) {
      isScrolling = true;
      scrollToItem({ index: messages.length - 1, smooth: true }, () => {
        isScrolling = false;
      });
    }
  }, [messages.length, shouldSticky, scrollToItem]);

  return (
    <div>
      <div
        style={{ width: "300px", height: "400px", overflow: "auto" }}
        ref={outerRef}
      >
        <div ref={innerRef}>
          {items.map(({ index, measureRef }) => (
            // Used to measure the unknown item size
            <div key={`${messages[index].id}`} ref={measureRef}>
              <div>{messages[index].content}</div>
            </div>
          ))}
        </div>
      </div>
      {!shouldSticky && (
        <button onClick={() => setShouldSticky(true)}>Stick to Bottom</button>
      )}
    </div>
  );
};

Working with Input Elements

This example demonstrates how to handle input elements (or form fields) in a virtualized list.

import { useState } from "react";
import useVirtual from "react-cool-virtual";

const defaultValues = new Array(20).fill(false);

const Form = () => {
  const [formData, setFormData] = useState({ todo: defaultValues });
  const { outerRef, innerRef, items } = useVirtual({
    itemCount: defaultValues.length,
  });

  const handleInputChange = ({ target }, index) => {
    // Store the input values in React state
    setFormData((prevData) => {
      const todo = [...prevData.todo];
      todo[index] = target.checked;
      return { todo };
    });
  };

  const handleSubmit = (e) => {
    e.preventDefault();
    alert(JSON.stringify(formData, undefined, 2));
  };

  return (
    <form onSubmit={handleSubmit}>
      <div
        style={{ width: "300px", height: "300px", overflow: "auto" }}
        ref={outerRef}
      >
        <div ref={innerRef}>
          {items.map(({ index, size }) => (
            <div key={index} style={{ height: `${size}px` }}>
              <input
                id={`todo-${index}`}
                type="checkbox"
                // Populate the corresponding state to the default value
                defaultChecked={formData.todo[index]}
                onChange={(e) => handleInputChange(e, index)}
              />
              <label htmlFor={`todo-${index}`}>{index}. I'd like to...</label>
            </div>
          ))}
        </div>
      </div>
      <input type="submit" />
    </form>
  );
};

When dealing with forms, we can use React Cool Form to handle the form state and boost performance for use.

import useVirtual from "react-cool-virtual";
import { useForm } from "react-cool-form";

const defaultValues = new Array(20).fill(false);

const Form = () => {
  const { outerRef, innerRef, items } = useVirtual({
    itemCount: defaultValues.length,
  });
  const { form } = useForm({
    defaultValues: { todo: defaultValues },
    removeOnUnmounted: false, // To keep the value of unmounted fields
    onSubmit: (formData) => alert(JSON.stringify(formData, undefined, 2)),
  });

  return (
    <form ref={form}>
      <div
        style={{ width: "300px", height: "300px", overflow: "auto" }}
        ref={outerRef}
      >
        <div ref={innerRef}>
          {items.map(({ index, size }) => (
            <div key={index} style={{ height: `${size}px` }}>
              <input
                id={`todo-${index}`}
                name={`todo[${index}]`}
                type="checkbox"
              />
              <label htmlFor={`todo-${index}`}>{index}. I'd like to...</label>
            </div>
          ))}
        </div>
      </div>
      <input type="submit" />
    </form>
  );
};

Dealing with Dynamic Items

React requires keys for array items. I'd recommend using an unique id as the key as possible as we can, especially when working with reordering, filtering, etc. Refer to this article to learn more.

const List = () => {
  const { outerRef, innerRef, items } = useVirtual();

  return (
    <div
      ref={outerRef}
      style={{ width: "300px", height: "300px", overflow: "auto" }}
    >
      <div ref={innerRef}>
        {items.map(({ index, size }) => (
          // Use IDs from your data as keys
          <div key={someData[index].id} style={{ height: `${size}px` }}>
            {someData[index].content}
          </div>
        ))}
      </div>
    </div>
  );
};

Server-side Rendering (SSR)

Server-side rendering allows us to provide a fast FP and FCP, it also benefits for SEO. React Cool Virtual supplies you a seamless DX between SSR and CSR.

const List = () => {
  const { outerRef, innerRef, items } = useVirtual({
    itemCount: 1000,
    ssrItemCount: 30, // Renders 0th - 30th items on SSR
    // Or
    ssrItemCount: [50, 80], // Renders 50th - 80th items on SSR
  });

  return (
    <div
      style={{ width: "300px", height: "300px", overflow: "auto" }}
      ref={outerRef}
    >
      <div ref={innerRef}>
        {/* The items will be rendered both on SSR and CSR, depending on our settings */}
        {items.map(({ index, size }) => (
          <div key={someData[index].id} style={{ height: `${size}px` }}>
            {someData[index].content}
          </div>
        ))}
      </div>
    </div>
  );
};

💡 Please note, when using the ssrItemCount, the initial items will be the SSR items but it has no impact to the UX. In addition, you might notice that some styles (i.e. width, start) of the SSR items are 0. It's by design, because there's no way to know the outer's size on SSR. However, you can make up these styles based on the environments if you need.

API

React Cool Virtual is a custom React hook that supplies you with all the features for building highly performant virtualized datasets easily 🚀. It takes options parameters and returns useful methods as follows.

const returnValues = useVirtual(options);

Options

An object with the following options:

itemCount (Required)

number

The total number of items. It can be an arbitrary number if actual number is unknown, see the example to learn more.

ssrItemCount

number | [number, number]

The number of items that are rendered on server-side, see the example to learn more.

itemSize

number | (index: number, width: number) => number

The size of an item (default = 50). When working with dynamic size, it will be the default/or estimated size of the unmeasured items.

For number use case, please refer to the fixed size example.
For index callback use case, please refer to the variable size example.
For width callback use case, please refer to the RWD example.

horizontal

boolean

The layout/orientation of the list (default = false). When true means left/right scrolling, so the hook will use width as the item size and use the left as the start position.

resetScroll

boolean

It's used to tell the hook to reset the scroll position when the itemCount is changed (default = false). It's useful for filtering items.

overscanCount

number

The number of items to render behind and ahead of the visible area (default = 1). That can be used for two reasons:

To slightly reduce/prevent a flash of empty screen while the user is scrolling. Please note, too many can negatively impact performance.
To allow the tab key to focus on the next (invisible) item for better accessibility.

useIsScrolling

boolean

To enable/disable the isScrolling indicator of an item (default = false). It's useful for UI placeholders or performance optimization when the list is being scrolled. Please note, using it will result in an additional render after scrolling has stopped.

stickyIndices

number[]

An array of indexes to make certain items in the list sticky. See the example to learn more.

The values must be provided in ascending order, i.e. [0, 10, 20, 30, ...].

scrollDuration

number | (distance: number) => number

The duration of smooth scrolling, the unit is milliseconds (default = 100ms <= distance * 0.075 <= 500ms).

scrollEasingFunction

(time: number) => number

A function that allows us to customize the easing effect of smooth scrolling (default = easeInOutSine).

loadMoreCount

number

How many number of items that you want to load/or pre-load (default = 15), it's used for infinite scroll. A number 15 means the loadMore callback will be invoked when the user scrolls within every 15 items, e.g. 1 - 15, 16 - 30, and so on.

isItemLoaded

(index: number) => boolean

A callback for us to provide the loaded state of a batch items, it's used for infinite scroll. It tells the hook whether the loadMore should be triggered or not.

loadMore

(event: Object) => void

A callback for us to fetch (more) data, it's used for infinite scroll. It's invoked when more items need to be loaded, which based on the mechanism of loadMoreCount and isItemLoaded.

const loadMore = ({
  startIndex, // (number) The index of the first batch item
  stopIndex, // (number) The index of the last batch item
  loadIndex, // (number) The index of the current batch items (e.g. 1 - 15 as `0`, 16 - 30 as `1`, and so on)
  scrollOffset, // (number) The scroll offset from top/left, depending on the `horizontal` option
  userScroll, // (boolean) Tells you the scrolling is through the user or not
}) => {
  // Fetch data...
};

const props = useVirtual({ loadMore });

onScroll

(event: Object) => void

This event will be triggered when scroll position is being changed by the user scrolls or scrollTo/scrollToItem methods.

const onScroll = ({
  overscanStartIndex, // (number) The index of the first overscan item
  overscanStopIndex, // (number) The index of the last overscan item
  visibleStartIndex, // (number) The index of the first visible item
  visibleStopIndex, // (number) The index of the last visible item
  scrollOffset, // (number) The scroll offset from top/left, depending on the `horizontal` option
  scrollForward, // (boolean) The scroll direction of up/down or left/right, depending on the `horizontal` option
  userScroll, // (boolean) Tells you the scrolling is through the user or not
}) => {
  // Do something...
};

const props = useVirtual({ onScroll });

onResize

(event: Object) => void

This event will be triggered when the size of the outer element changes.

const onResize = ({
  width, // (number) The content width of the outer element
  height, // (number) The content height of the outer element
}) => {
  // Do something...
};

const props = useVirtual({ onResize });

Return Values

An object with the following properties:

outerRef

React.useRef<HTMLElement>

A ref to attach to the outer element. We must apply it for using this hook.

innerRef

React.useRef<HTMLElement>

A ref to attach to the inner element. We must apply it for using this hook.

items

Object[]

The virtualized items for rendering rows/columns. Each item is an object that contains the following properties:

Name	Type	Description
index	number	The index of the item.
size	number	The fixed/variable/measured size of the item.
width	number	The current content width of the outer element. It's useful for a RWD row/column.
start	number	The starting position of the item. We might only need this when working with grids.
isScrolling	true \| undefined	An indicator to show a placeholder or optimize performance for the item.
isSticky	true \| undefined	An indicator to make certain items become sticky in the list.
measureRef	Function	It's used to measure an item with dynamic or real-time heights/widths.

scrollTo

(offsetOrOptions: number | Object, callback?: () => void) => void

This method allows us to scroll to the specified offset from top/left, depending on the horizontal option.

// Basic usage
scrollTo(500);

// Using options
scrollTo({
  offset: 500,
  smooth: true, // Enable/disable smooth scrolling (default = false)
});

💡 It's possible to customize the easing effect of the smoothly scrolling, see the example to learn more.

scrollToItem

(indexOrOptions: number | Object, callback?: () => void) => void

This method allows us to scroll to the specified item.

// Basic usage
scrollToItem(10);

// Using options
scrollToItem({
  index: 10,
  // Control the alignment of the item, acceptable values are: "auto" (default) | "start" | "center" | "end"
  // Using "auto" will scroll the item into the view at the start or end, depending on which is closer
  align: "auto",
  // Enable/disable smooth scrolling (default = false)
  smooth: true,
});

💡 It's possible to customize the easing effect of the smoothly scrolling, see the example to learn more.

startItem

(index: number, callback?: () => void) => void

This method is used to work with pre-pending items. It allows us to main the previous scroll position for the user.

Others

Performance Optimization

Items are re-rendered whenever the user scrolls. If your item is a heavy data component, there're two strategies for performance optimization.

Use React.memo

When working with non-dynamic size, we can extract the item to it's own component and wrap it with React.memo. It shallowly compares the current props and the next props to avoid unnecessary re-renders.

import { memo } from "react";
import useVirtual from "react-cool-virtual";

const MemoizedItem = memo(({ height, ...rest }) => {
  // A lot of heavy computing here... 🤪

  return (
    <div {...rest} style={{ height: `${height}px` }}>
      🐳 Am I heavy?
    </div>
  );
});

const List = () => {
  const { outerRef, innerRef, items } = useVirtual({
    itemCount: 1000,
    itemSize: 75,
  });

  return (
    <div
      style={{ width: "300px", height: "300px", overflow: "auto" }}
      ref={outerRef}
    >
      <div ref={innerRef}>
        {items.map(({ index, size }) => (
          <MemoizedItem key={index} height={size} />
        ))}
      </div>
    </div>
  );
};

Use `isScrolling` Indicator

If the above solution can't meet your case or you're working with dynamic size. React Cool Virtual supplies you an isScrolling indicator that allows you to replace the heavy component with a light one while the user is scrolling.

import { forwardRef } from "react";
import useVirtual from "react-cool-virtual";

const HeavyItem = forwardRef((props, ref) => {
  // A lot of heavy computing here... 🤪

  return (
    <div {...props} ref={ref}>
      🐳 Am I heavy?
    </div>
  );
});

const LightItem = (props) => <div {...props}>🦐 I believe I can fly...</div>;

const List = () => {
  const { outerRef, innerRef, items } = useVirtual({
    itemCount: 1000,
    useIsScrolling: true, // Just use it (default = false)
    // Or
    useIsScrolling: (speed) => speed > 50, // Use it based on the scroll speed (more user friendly)
  });

  return (
    <div
      style={{ width: "300px", height: "300px", overflow: "auto" }}
      ref={outerRef}
    >
      <div ref={innerRef}>
        {items.map(({ index, isScrolling, measureRef }) =>
          isScrolling ? (
            <LightItem key={index} />
          ) : (
            <HeavyItem key={index} ref={measureRef} />
          )
        )}
      </div>
    </div>
  );
};

💡 Well... the isScrolling can also be used in many other ways, please use your imagination 🤗.

How to Share A `ref`?

You can share a ref as follows, here we take the outerRef as the example:

import { useRef } from "react";
import useVirtual from "react-cool-virtual";

const App = () => {
  const ref = useRef();
  const { outerRef } = useVirtual();

  return (
    <div
      ref={(el) => {
        outerRef.current = el; // Set the element to the `outerRef`
        ref.current = el; // Share the element for other purposes
      }}
    />
  );
};

Layout Items

React Cool Virtual is designed to simplify the styling and keep all the items in the document flow for rows/columns. However, when working with grids, we need to layout the items in two-dimensional. For that reason, we also provide the start property for you to achieve it.

import { Fragment } from "react";
import useVirtual from "react-cool-virtual";

const Grid = () => {
  const row = useVirtual({
    itemCount: 1000,
  });
  const col = useVirtual({
    horizontal: true,
    itemCount: 1000,
    itemSize: 100,
  });

  return (
    <div
      style={{ width: "400px", height: "400px", overflow: "auto" }}
      ref={(el) => {
        row.outerRef.current = el;
        col.outerRef.current = el;
      }}
    >
      <div
        style={{ position: "relative" }}
        ref={(el) => {
          row.innerRef.current = el;
          col.innerRef.current = el;
        }}
      >
        {row.items.map((rowItem) => (
          <Fragment key={rowItem.index}>
            {col.items.map((colItem) => (
              <div
                key={colItem.index}
                style={{
                  position: "absolute",
                  height: `${rowItem.size}px`,
                  width: `${colItem.size}px`,
                  // The `start` property can be used for positioning the items
                  transform: `translateX(${colItem.start}px) translateY(${rowItem.start}px)`,
                }}
              >
                ⭐️ {rowItem.index}, {colItem.index}
              </div>
            ))}
          </Fragment>
        ))}
      </div>
    </div>
  );
};

Working in TypeScript

React Cool Virtual is built with TypeScript, you can tell the hook what type of your outer and inner elements are as follows.

If the outer element and inner element are the different types:

const App = () => {
  // 1st is the `outerRef`, 2nd is the `innerRef`
  const { outerRef, innerRef } = useVirtual<HTMLDivElement, HTMLUListElement>();

  return (
    <div ref={outerRef}>
      <ul ref={innerRef}>{/* Rendering items... */}</ul>
    </div>
  );
};

If the outer element and inner element are the same types:

const App = () => {
  // By default, the `innerRef` will refer to the type of the `outerRef`
  const { outerRef, innerRef } = useVirtual<HTMLDivElement>();

  return (
    <div ref={outerRef}>
      <div ref={innerRef}>{/* Rendering items... */}</div>
    </div>
  );
};

💡 For more available types, please check it out.

ResizeObserver Polyfill

ResizeObserver has good support amongst browsers, but it's not universal. You'll need to use polyfill for browsers that don't support it. Polyfills is something you should do consciously at the application level. Therefore React Cool Virtual doesn't include it.

We recommend using @juggle/resize-observer:

$ yarn add @juggle/resize-observer
# or
$ npm install --save @juggle/resize-observer

Then pollute the window object:

import { ResizeObserver } from "@juggle/resize-observer";

if (!("ResizeObserver" in window)) window.ResizeObserver = ResizeObserver;

You could use dynamic imports to only load the file when the polyfill is required:

(async () => {
  if (!("ResizeObserver" in window)) {
    const module = await import("@juggle/resize-observer");
    window.ResizeObserver = module.ResizeObserver;
  }
})();