Add track-specific conversion options, fix esbuild race condition

docs/guide/converting-media-files.md

@@ -92,22 +92,18 @@ This automatically frees up all resources used by the conversion process.
 
 ## Video options
 
-You can set the `video` property in the conversion options to configure the converter's behavior for video tracks:
+You can set the `video` property in the conversion options to configure the converter's behavior for video tracks. The options are:
 ```ts
-type ConversionOptions = {
-	// ...
-	video?: {
-		discard?: boolean;
-		width?: number;
-		height?: number;
-		fit?: 'fill' | 'contain' | 'cover';
-		rotate?: 0 | 90 | 180 | 270;
-		frameRate?: number;
-		codec?: VideoCodec;
-		bitrate?: number | Quality;
-		forceTranscode?: boolean;
-	};
-	// ...
+type ConversionVideoOptions = {
+	discard?: boolean;
+	width?: number;
+	height?: number;
+	fit?: 'fill' | 'contain' | 'cover';
+	rotate?: 0 | 90 | 180 | 270;
+	frameRate?: number;
+	codec?: VideoCodec;
+	bitrate?: number | Quality;
+	forceTranscode?: boolean;
 };
 ```
 
@@ -125,7 +121,7 @@ const conversion = await Conversion.init({
 ```
 
 ::: info
-The provided configuration will apply equally to all video tracks of the input.
+The provided configuration will apply equally to all video tracks of the input. If you want to apply a separate configuration to each video track, check [track-specific options](#track-specific-options).
 :::
 
 ### Discarding video
@@ -143,6 +139,8 @@ The `width`, `height` and `fit` properties control how the video is resized. If
 
 If `width` or `height` is used in conjunction with `rotation`, they control the post-rotation dimensions.
 
+If you want to apply max/min constraints to a video's dimensions, check out [track-specific options](#track-specific-options).
+
 ### Adjusting frame rate
 
 The `frameRate` property can be used to set the frame rate of the output video in Hz. If not specified, the original input frame rate will be used (which may be variable).
@@ -157,19 +155,15 @@ If you want to prevent direct copying of media data and force a transcoding step
 
 ## Audio options
 
-You can set the `audio` property in the conversion options to configure the converter's behavior for audio tracks:
+You can set the `audio` property in the conversion options to configure the converter's behavior for audio tracks. The options are:
 ```ts
-type ConversionOptions = {
-	// ...
-	audio?: {
-		discard?: boolean;
-		codec?: AudioCodec;
-		bitrate?: number | Quality;
-		numberOfChannels?: number;
-		sampleRate?: number;
-		forceTranscode?: boolean;
-	};
-	// ...
+type ConversionAudioOptions = {
+	discard?: boolean;
+	codec?: AudioCodec;
+	bitrate?: number | Quality;
+	numberOfChannels?: number;
+	sampleRate?: number;
+	forceTranscode?: boolean;
 };
 ```
 
@@ -186,7 +180,7 @@ const conversion = await Conversion.init({
 ```
 
 ::: info
-The provided configuration will apply equally to all audio tracks of the input.
+The provided configuration will apply equally to all audio tracks of the input. If you want to apply a separate configuration to each audio track, check [track-specific options](#track-specific-options).
 :::
 
 ### Discarding audio
@@ -207,6 +201,46 @@ Use the `bitrate` property to control the bitrate of the output audio. For examp
 
 If you want to prevent direct copying of media data and force a transcoding step, use `forceTranscode: true`.
 
+## Track-specific options
+
+You may want to configure your video and audio options differently depending on the specifics of the input track. Or, in case a media file has multiple video or audio tracks, you may want to discard only specific tracks or configure each track separately.
+
+For this, instead of passing an object for `video` and `audio`, you can instead pass a function:
+
+```ts
+const conversion = await Conversion.init({
+	input,
+	output,
+
+	// Function gets invoked for each video track:
+	video: (videoTrack, n) => {
+		if (n > 1) {
+			// Keep only the first video track
+			return { discard: true };
+		}
+
+		return {
+			// Shrink width to 640 only if the track is wider
+			width: Math.min(videoTrack.displayWidth, 640),
+		};
+	},
+
+	// Async functions work too:
+	audio: async (audioTrack, n) => {
+		if (audioTrack.languageCode !== 'rus') {
+			// Keep only Russian audio tracks
+			return { discard: true };
+		}
+
+		return {
+			codec: 'aac',
+		};
+	},
+});
+```
+
+For documentation about the properties of video and audio tracks, refer to [Reading track metadata](./reading-media-files#reading-track-metadata).
+
 ## Trimming
 
 Use the `trim` property in the conversion options to extract only a section of the input file into the output file: