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prefetch()

Available in v3.2.23

By calling prefetch(), an asset will be fetched and kept in memory so it is ready when you want to play it in a <Player>.

If you pass the original URL into either an <Audio>, <Video>, <OffthreadVideo> or <Img> tag and the asset is fully fetched, those components will use Blob URL instead.

note

Remote assets need to support CORS.

More info
tsx
import {prefetch} from 'remotion';
 
const {free, waitUntilDone} = prefetch('https://example.com/video.mp4', {
  method: 'blob-url',
});
 
waitUntilDone().then(() => {
  console.log('Video has finished loading');
});
 
// Call free() if you want to un-prefetch and free up the memory:
free();
tsx
import {prefetch} from 'remotion';
 
const {free, waitUntilDone} = prefetch('https://example.com/video.mp4', {
  method: 'blob-url',
});
 
waitUntilDone().then(() => {
  console.log('Video has finished loading');
});
 
// Call free() if you want to un-prefetch and free up the memory:
free();

This API is only useful if you are using the <Player /> component and you want to display a media asset and want to be sure it is fully loaded before it appears.

An alternative to prefetch() are the @remotion/preload APIs. See @remotion/preload vs prefetch() to decide which one is better for your usecase.

API​

Arguments​

src​

The function takes a src, which can be a remote URL, an imported asset or an asset loaded using staticFile() (see: Importing assets). Once called, prefetching starts and an object with two properties are returned:

options​

method?v3.2.35​

previously defaulted to blob-url

Either blob-url or base64.

  • With blob-url, the asset is turned into a Blob URL using URL.createObjectURL()
  • With base64, the asset is turned into a Base 64 URL.

Read below for which option is suitable for you.

contentType?v4.0.40​

Set a content type for the blob. By default, the content type from the HTTP response is taken. We recommend setting an appropriate content type for the media, e.g video/mp4 only if the HTTP resource does not contain the right one by default.

onProgress?v4.0.85​

Callback for progress events.

tsx
import {prefetch} from 'remotion';
import type {PrefetchOnProgress} from 'remotion';
 
const onProgress: PrefetchOnProgress = (progress) => {
  if (progress.totalBytes === null) {
    // HTTP response has no "Content-Length" header,
    // therefore no relative progress can be calculated.
    console.log('Loaded bytes:', progress.loadedBytes);
    return;
  }
 
  console.log(
    'Loading progress:',
    Math.round(progress.loadedBytes / progress.totalBytes / 100) + '%',
  );
};
 
prefetch('https://example.com/video.mp4', {
  onProgress,
});
tsx
import {prefetch} from 'remotion';
import type {PrefetchOnProgress} from 'remotion';
 
const onProgress: PrefetchOnProgress = (progress) => {
  if (progress.totalBytes === null) {
    // HTTP response has no "Content-Length" header,
    // therefore no relative progress can be calculated.
    console.log('Loaded bytes:', progress.loadedBytes);
    return;
  }
 
  console.log(
    'Loading progress:',
    Math.round(progress.loadedBytes / progress.totalBytes / 100) + '%',
  );
};
 
prefetch('https://example.com/video.mp4', {
  onProgress,
});

Return value​

An object with:

free()​

Calling this function will abort the prefetching is still in progress and free up the memory. Components using the asset might re-request the asset.

waitUntilDone()​

Calling this will return a Promise<string> that resolves once asset has finished downloading and is ready to be used.

The string that is being returned is either:

  • A blob: URL or data: Base 64 data respectively
  • The same src you passed as input, if in a rendering environment.

You don't need to use this URL in <Audio>, <Video>, <OffthreadVideo> or <Img>, which will automatically use it.

If you neeed the URL somewhere (like for mask-image), you can grab it from waitUntilDone().

blob-url or base64?​

Turning the asset into a blob URL is much faster and efficient in general.
In Safari, even though it is a local URL, Safari may not be instantly playing the audio and have slight delays buffering audio.

Use base64 in Safari if you notice that your audio file plays with hickups even though the <Player> requests a blob: URL.

Setting the right content type​

Files loaded from the internet should be served with a proper content type, e.g. video/mp4 for an MP4 file. When the file gets loaded into a blob URL, the file extension gets removed and the browser loses the information about the content type.

This can lead to the video file not playing correctly in the browser. If you cannot influence the server, you can also set the content type manually using the contentType option.

This issue may arise in Safari specifically. We recommend testing your implementation in Safari after you coded it.

See also​