$3

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


> 💡 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.

$3

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

![Edit RCV - Sticky Headers](https://codesandbox.io/s/rcv-sticky-headers-pm79x?fontsize=14&hidenavigation=1&theme=dark)

`js
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 (
style={{ width: "300px", height: "300px", overflow: "auto" }}
ref={outerRef}
>


> 💡 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.

$3

You can imperatively scroll to offset or items as follows:

![Edit RCV - Scroll-to Methods](https://codesandbox.io/s/rcv-scroll-to-methods-cs9x9?fontsize=14&hidenavigation=1&theme=dark)

`js
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" });
};
`

$3

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

![Edit RCV - Smooth Scrolling](https://codesandbox.io/s/rcv-smooth-scrolling-noy4d?fontsize=14&hidenavigation=1&theme=dark)

`js
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:

`js
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.

$3

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

![Edit RCV - Infinite Scroll](https://codesandbox.io/s/rcv-infinite-scroll-3y351?fontsize=14&hidenavigation=1&theme=dark)

#### Working with Skeleton Screens

`js
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 (
style={{ width: "300px", height: "500px", overflow: "auto" }}
ref={outerRef}
>


`js
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 = () =>

`js
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 (
style={{ width: "300px", height: "500px", overflow: "auto" }}
ref={outerRef}
>


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

![Edit RCV - Filter Items](https://codesandbox.io/s/rcv-filter-items-clno9?fontsize=14&hidenavigation=1&theme=dark)

`js
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 (
style={{ width: "300px", height: "300px", overflow: "auto" }}
ref={outerRef}
>




);
};
`

$3

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


`js
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 (


style={{ width: "300px", height: "400px", overflow: "auto" }}
ref={outerRef}
>

{items.map(({ index, measureRef }) => (
// Used to measure the unknown item size

${messages[index].id}} ref={measureRef}>

{messages[index].content}


))}






{!shouldSticky && (
setShouldSticky(true)}>Stick to Bottom
)}

);
};
`

$3

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


`js
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 (


style={{ width: "300px", height: "300px", overflow: "auto" }}
ref={outerRef}
>

{items.map(({ index, size }) => (

${size}px }}>
id={todo-${index}}
type="checkbox"
// Populate the corresponding state to the default value
defaultChecked={formData.todo[index]}
onChange={(e) => handleInputChange(e, index)}
/>
todo-${index}}>{index}. I'd like to...



))}






);
};
`

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


`js
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 (


style={{ width: "300px", height: "300px", overflow: "auto" }}
ref={outerRef}
>

{items.map(({ index, size }) => (

${size}px }}>
id={todo-${index}}
name={todo[${index}]}
type="checkbox"
/>
todo-${index}}>{index}. I'd like to...



))}






);
};
`

$3

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.

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

return (
ref={outerRef}
style={{ width: "300px", height: "300px", overflow: "auto" }}
>


{items.map(({ index, size }) => (
// Use IDs from your data as keys

${size}px }}>
{someData[index].content}



))}




);
};
`

$3

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.

`js
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 (
style={{ width: "300px", height: "300px", overflow: "auto" }}
ref={outerRef}
>


{/ The items will be rendered both on SSR and CSR, depending on our settings /}
{items.map(({ index, size }) => (

${size}px }}>
{someData[index].content}



))}




);
};
`

> 💡 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.

`js
const returnValues = useVirtual(options);
`

$3

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.

`js
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.

`js
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.

`js
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 });
`

$3

An object with the following properties:

#### outerRef

React.useRef

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

#### innerRef

React.useRef

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.

`js
// 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.

`js
// Basic usage
scrollToItem(10);

// Using options
scrollTo({
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

$3

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.

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

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

return (


${height}px }}>
🐳 Am I heavy?


);
});

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

return (
style={{ width: "300px", height: "300px", overflow: "auto" }}
ref={outerRef}
>

{items.map(({ index, size }) => (

))}

);
};
`

#### 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.

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

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

return (



🐳 Am I heavy?


);
});

const LightItem = (props) =>

🦐 I believe I can fly...

;

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 (
style={{ width: "300px", height: "300px", overflow: "auto" }}
ref={outerRef}
>

{items.map(({ index, isScrolling, measureRef }) =>
isScrolling ? (

) : (

)
)}

);
};
`

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

$3

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

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

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

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

$3

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.

`js
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 (
style={{ width: "400px", height: "400px", overflow: "auto" }}
ref={(el) => {
row.outerRef.current = el;
col.outerRef.current = el;
}}
>
style={{ position: "relative" }}
ref={(el) => {
row.innerRef.current = el;
col.innerRef.current = el;
}}
>
{row.items.map((rowItem) => (

{col.items.map((colItem) => (
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}

))}

))}


);
};
`

$3

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:

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

return (


{/ Rendering items... /}


);
};
`

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

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

return (


{/ Rendering items... /}


);
};
`

💡 For more available types, please check it out.

$3

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:

`sh
$ yarn add @juggle/resize-observer


or


$ npm install --save @juggle/resize-observer
`

Then pollute the window object:

`js
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:

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

To Do...

- [ ] Support window scrolling
- [ ] Leverage the power of Offscreen API (maybe...)

Articles / Blog Posts

> 💡 If you have written any blog post or article about React Cool Virtual, please open a PR to add it here.

- Featured on React Status #243.
- Featured on React Newsletter #270.

Contributors ✨

Thanks goes to these wonderful people (emoji key):

Welly 🤔 💻 📖 🚇 🚧

Nikita Pilgrim 💻

Jie Peng 📖

Alex Lyakhnitskiy 💻

This project follows the all-contributors specification. Contributions of any kind welcome!

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