![Milica Mihajlija](https://web.dev/images/authors/mihajlija.jpg) Milica Mihajlija [X](https://twitter.com/bibydigital) [GitHub](https://github.com/mihajlija) [Homepage](https://mihajlija.github.io/)

<br />

> [!CAUTION]
> **Caution:** One or more demos on this page no longer work, as [Glitch has ended hosting](https://blog.glitch.com/post/changes-are-coming-to-glitch).

> [!IMPORTANT]
> To follow this codelab, open [this Glitch](https://glitch.com/edit/#!/adaptive-serving-netinfo-starter) in a second tab.

> [!NOTE]
> **Note:** This codelab uses Chrome DevTools. [Download Chrome](https://www.google.com/chrome) if you don't already have it.

In this codelab, you'll learn how to adapt your content based on the network
quality. This page's background video will load only when users are on a fast
network. On slower networks, an image will load instead.

The
[Network Information API](https://developer.mozilla.org/docs/Web/API/NetworkInformation)
enables you to access information about the user's connection quality. You will
use its `effectiveType` property to decide when to serve a video and when to
serve an image. `effectiveType` can be `'slow-2g'`, `'2g'`, `'3g'`, or `'4g'`.


Browser Support

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[Source](https://developer.mozilla.org/docs/Web/API/NetworkInformation)

<br />

## Step 1: Check connection type

The `index.html` file contains a `<video>` tag to display the background video (line 22). The code in `script.js` loads the video by setting the video tag's `src` attribute. (The video loading code is described in more detail in [Step 2](https://web.dev/articles/codelab-adapt-video-to-image-serving-based-on-network-quality.md.txt#step_2_load_video).)

To load the video conditionally, first check if the Network Information API is available; if it is, check the connection type.

1. In `script.js`, add an `if` statement that tests whether the `navigator.connection` object exists and whether it has the `effectiveType` property.
2. Add an `if` statement to check the `effectiveType` of the network.

```javascript
if (navigator.connection && !!navigator.connection.effectiveType) {
  if (navigator.connection.effectiveType === '4g') {
    // Only load video on the fastest connections.
  } else {
    // In any other case load the image.
  }
}
```

Wrap the existing video loading code in an `else` statement, so that video will
still load in browsers that don't support the Network Information API.

```javascript
if (navigator.connection && !!navigator.connection.effectiveType) {
  if (navigator.connection.effectiveType === '4g') {
    // video loading code
  } else {
    // image loading code
  }
} else {
  const video = document.getElementById('coverVideo');
  const videoSource = video.getAttribute('data-src');
  video.setAttribute('src', videoSource);

  video.setAttribute('style', 'height: 100%; width: 100%; display:inline');
}
```

## Step 2: Load video

If the `effectiveType` is `'4g'`, use the video loading code from the
beginning of the codelab.

```javascript
if (navigator.connection.effectiveType === '4g') {
  const video = document.getElementById('coverVideo');
  const videoSource = video.getAttribute('data-src');
  video.setAttribute('src', videoSource);
  video.setAttribute('style', 'height: 100%; width: 100%; display:inline');
} else {
  // image loading code
}
```

Here's how the video loading code works: the `<video>` tag doesn't download or display anything at first because its `src` attribute is not set. The video URL to load is specified using the `data-src` attribute.

```html
<video id="coverVideo" playsinline autoplay loop muted data-src="https://cdn.glitch.com/b6491350-b058-4eb6-aa6c-55c93122073e%2FMatrix%2C%20Console%2C%20Hacking%2C%20Code.mp4?1551464245607"></video>
```

[Data attributes](https://developer.mozilla.org/docs/Learn/HTML/Howto/Use_data_attributes) allow you to store extra information on standard HTML elements. A data element can be named anything, as long as it starts with "data-".

To actually display the video on the page, you need to get the value from `data-src` and set it as the video element's `src` attribute.

First, get the DOM element that contains the asset:

```javascript
const video = document.getElementById('coverVideo');
```

Then get the resource location from the `data-src` attribute:

```javascript
const videoSource = video.getAttribute('data-src');
```

And finally set that as the `src` attribute of the video element:

```javascript
video.setAttribute('src', videoSource);
```

The last line takes care of CSS positioning:

```javascript
video.setAttribute('style', 'height: 100%; width: 100%; display:inline');
```

## Step 3: Load image

To conditionally load an image on slower networks, use the same strategy as for
the video.

Add an image element to `index.html` (right after the video element), and use
the `data-src` attribute instead of the `src` attribute.

```html
<img id="coverImage" data-src="https://cdn.glitch.com/36529535-5976-40f8-979b-40c898b86bd0%2F36529535-5976-40f8-979b-40c898b86bd0_1_Sn80dgiwpMjBVrqjfiDbnA.jpg?1553003835358" />
```

In `script.js`, add code to set the image's `src` attribute depending on the
`effectiveType` of the network.

```javascript
if (navigator.connection.effectiveType === '4g') {
  const video = document.getElementById('coverVideo');
  const videoSource = video.getAttribute('data-src');
  video.setAttribute('src', videoSource);

  video.setAttribute('style', 'height: 100%; width: 100%; display:inline');
} else {
  const image = document.getElementById('coverImage');
  const imageSource = image.getAttribute('data-src');
  image.setAttribute('src', imageSource);

  image.setAttribute('style', 'height: 100%; width: 100%; display:inline');
}
```

## Try it out

To test it yourself:

1. To preview the site, press **View App** . Then press **Fullscreen** ![fullscreen](https://web.dev/static/images/glitch/fullscreen.svg).
2. Press \`Control+Shift+J\` (or \`Command+Option+J\` on Mac) to open DevTools.
3. Click the **Network** tab.
4. Click the **Throttling** drop-down, which is set to **No throttling** by default. Select **Fast 3G**.

![DevTools Network tab with Fast 3G throttling option highlighted](https://web.dev/static/articles/codelab-adapt-video-to-image-serving-based-on-network-quality/image/devtools-network-tab-fas-2424c4dccd70f.png)

Now reload the page with Fast 3G still enabled. The app loads an image in the background instead of the video:

![Matrix-like image background with 'NETWORK INFORMATION' text overlay](https://web.dev/static/articles/codelab-adapt-video-to-image-serving-based-on-network-quality/image/matrix-image-background-63ffff63d4faf.png)

## Extra credit: Respond to changes

Remember how this API has an `onchange`
[event listener](https://web.dev/articles/adaptive-serving-based-on-network-quality.md.txt#how_it_works)?
You can use it for many things: dynamically adapting content such as video quality; restarting deferred data transfers when a change to a high-bandwidth network type is detected; or notifying users when the network quality changes.

Here's a simple example of how to use this listener. Add it to `script.js`. This
code will call the `displayNetworkInfo` function whenever the network
information changes.

```javascript
navigator.connection.addEventListener('change', displayNetworkInfo);
```

There's already an empty `<h2>` element on the `index.html` page. Now define the
`displayNetworkInfo` function so that it displays the network information in the
`<h2>` element and invoke the function.

```javascript
function displayNetworkInfo() {
  document.getElementById('connection').innerHTML = navigator.connection.effectiveType;
}

displayNetworkInfo();
```

Here's the final state of the [app on Glitch](https://glitch.com/%7Eadaptive-serving-netinfo).

![Matrix-like video background with 'NETWORK INFORMATION 4g' text overlay](https://web.dev/static/articles/codelab-adapt-video-to-image-serving-based-on-network-quality/image/matrix-video-background-c9414754d128d.png)

To test it again:

1. To preview the site, press **View App** . Then press **Fullscreen** ![fullscreen](https://web.dev/static/images/glitch/fullscreen.svg).
2. Press \`Control+Shift+J\` (or \`Command+Option+J\` on Mac) to open DevTools.
3. Click the **Network** tab.
4. Click the **Throttling** drop-down, which is set to **No throttling** by default. Select **Fast 3G**.
5. Reload the app.

The app will update the network information to **3g**:

![Matrix-like video background with 'NETWORK INFORMATION 3g' text overlay](https://web.dev/static/articles/codelab-adapt-video-to-image-serving-based-on-network-quality/image/matrix-video-background-1bc411282c96c.png)