# Ad block detection
Detect when an ad fails to load — for example because an ad blocker is active — and decide whether the content may still play.
Tip: open your browser's developer console and use the `player` and `THEOplayer` variables to interact with this demo.
\[ ]Block content if the ad fails (`blockContentIfAdError`)
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When the ad request fails, the player fires an `aderror` event on `player.ads`. Set `blockContentIfAdError` on the source to stop content playback as well — the player then also raises a fatal `error` event. Enable an ad blocker in your browser and press play to reproduce what happens when the ad tag is unreachable — a pre-roll ad is only requested when playback starts, so the error appears after pressing play.
```js
player.ads.addEventListener('aderror', (event) => {
// Custom behavior when the ad could not be loaded.
});
player.source = {
sources: { src: 'https://example.com/video.m3u8' },
ads: [{ sources: 'https://example.com/vast.xml' }],
blockContentIfAdError: true,
};
```
See the [ad block detection guide](https://optiview.dolby.com/docs/theoplayer/how-to-guides/web/ads/block-detection.md).
Code
demo.html
```html
Ad block detection
```