Intersection Observer using React
Tags: react, webdev, javascript, html
Published: 2021-02-15 07:57:50

Today we are gonna explore how to use the intersection observer API in React and see some useful examples, you can find the code in the following [repository](https://github.com/zygisS22/intersectionObserverApi), let's begin.

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[Mozilla web documentation](https://developer.mozilla.org/en-US/docs/Web/API/Intersection_Observer_API) describes the intersection observer API as:

> lets code register a callback function that is executed whenever an element they wish to monitor enters or exits another element (or the viewport), or when the amount by which the two intersect changes by a requested amount. This way, sites no longer need to do anything on the main thread to watch for this kind of element intersection, and the browser is free to optimize the management of intersections as it sees fit.

In plain english, it allows us to detect when certain elements are visible in our [viewport](https://developer.mozilla.org/en-US/docs/Web/CSS/Viewport_concepts), this only happens when the element meets your desired intersection ratio.

![threshold explained](https://dev-to-uploads.s3.amazonaws.com/i/sxx2evqd3c4k9g5skrco.png)

As you can see, if we scroll down the page the intersection ratio will be going up until it meets the designed threshold and that's when the callback function is executed.

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## Initialization

![Constructor](https://dev-to-uploads.s3.amazonaws.com/i/dcwkrxkxqvl258qdtllf.png)

The intersection observer object constructor requires two arguments:

1. Callback function
2. Options

Just like that we are ready to see some action but first we need to know what each option means, the options argument is an object with the following values:

![Options](https://dev-to-uploads.s3.amazonaws.com/i/usak6zescrooj99s71kn.png)

- **root**: The element that is used as the viewport for checking visibility of the target. Must be the ancestor of the target. Defaults to the browser viewport if not specified or if null.
- **rootMargin**: This set of values serves to grow or shrink each side of the root element's bounding box before computing intersections, the options are similar to those of margin in CSS.
- **threshold**: Either a single number or an array of numbers which indicate at what percentage of the target's visibility the observer's callback should be executed, ranges from 0 to 1.0, where 1.0 means every pixel is visible in the viewport.

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## Using React.js

![viewport implementation gif](https://dev-to-uploads.s3.amazonaws.com/i/yd1l1b6hldo8xxwz9y0o.gif)

Now let's see an implementation of the intersection observer API using react.

![Basic example of use in react](https://dev-to-uploads.s3.amazonaws.com/i/lp8nfhr242anv1a18j7d.png)

1. Start with a reference to the element we want to observe, use the react hook **useRef**.
2. Create a state variable <code>isVisible</code>, we are gonna use it to display a message whenever our box is in the viewport.
2. Declare a **callback** function that receives an array of  IntersectionObserverEntries as a parameter, inside this function we take the first and only entry and check if is intersecting with the viewport and if it is then we call <code>setIsVisible</code> with the value of <code>entry.isIntersecting</code> (true/false).
3. Create the options object with the same values as the image.
4. Add the react hook **useEffect** and create an observer contructor using the callback function and the options we just created before, it's **optional** in our case but you can return a cleanup function to unobserve our target when the component **unmounts**.
5. Set the **useRef** variable on the element we want to observe.
![useRef element](https://dev-to-uploads.s3.amazonaws.com/i/zgcplfssai6w3h9026qa.png)
6. Let's add a background and some properties to our HTML elements
![css styles](https://dev-to-uploads.s3.amazonaws.com/i/7am7ap8chudxva0vhzy3.png)
7. It's done, simple and easy!

remember this is just a basic implementation and there are many different ways of doing it.

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## Hooking it up

![in viewport gif 2](https://dev-to-uploads.s3.amazonaws.com/i/x25932zltsj5j8ech36y.gif)

Let's now implement the same code we just did before but separating all the logic into a new hook called <code>useElementOnScreen</code>.

![hook code 1](https://dev-to-uploads.s3.amazonaws.com/i/eopf1h7rv6kfe2denonb.png)

1. Create a new function called <code>useElementOnScreen</code> with the parameter **options**
2. Move **useRef**, **useState** and the entire **useEffect** inside our new shiny hook.
3. Now the only thing missing in our hook is the return statement, we pass <code>isVisible</code> and <code>containerRef</code> as an array.
4. okay, we are almost there, we just need to call it in our component and see if it works!

![hook code 2](https://dev-to-uploads.s3.amazonaws.com/i/z7b35o1llozq11csstoz.png)

1. Import the recently created hook file into our component.
2. Initialize it with the options object.
3. Just like that we are done.

Congratulations we have successfully used the intersection observer API and we even made a hook for it!

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## Final words

Thanks for reading this, Hopefully it helped someone getting started with the IO API using react, stay safe ❤️!

![good bye gif](https://dev-to-uploads.s3.amazonaws.com/i/gm6dgrp7fm6oza6m73sv.gif)