Add ducktyped Promise checks (fixes #472)

src/adts/adts-demuxer.ts

@@ -23,6 +23,7 @@ import {
 	AsyncMutex,
 	binarySearchExact,
 	binarySearchLessOrEqual,
+	isThenable,
 	UNDETERMINED_LANGUAGE,
 } from '../misc';
 import { EncodedPacket, PLACEHOLDER_DATA } from '../packet';
@@ -84,7 +85,7 @@ export class AdtsDemuxer extends Demuxer {
 			// Skip all ID3v2 tags at the start of the file
 			while (true) {
 				let slice = this.reader.requestSlice(this.lastLoadedPos, ID3_V2_HEADER_SIZE);
-				if (slice instanceof Promise) slice = await slice;
+				if (isThenable(slice)) slice = await slice;
 
 				if (!slice) {
 					this.lastSampleLoaded = true;
@@ -105,7 +106,7 @@ export class AdtsDemuxer extends Demuxer {
 			MIN_ADTS_FRAME_HEADER_SIZE,
 			MAX_ADTS_FRAME_HEADER_SIZE,
 		);
-		if (slice instanceof Promise) slice = await slice;
+		if (isThenable(slice)) slice = await slice;
 		if (!slice) {
 			this.lastSampleLoaded = true;
 			return;
@@ -167,7 +168,7 @@ export class AdtsDemuxer extends Demuxer {
 
 			while (true) {
 				let headerSlice = this.reader.requestSlice(currentPos, ID3_V2_HEADER_SIZE);
-				if (headerSlice instanceof Promise) headerSlice = await headerSlice;
+				if (isThenable(headerSlice)) headerSlice = await headerSlice;
 				if (!headerSlice) break;
 
 				const id3V2Header = readId3V2Header(headerSlice);
@@ -176,7 +177,7 @@ export class AdtsDemuxer extends Demuxer {
 				}
 
 				let contentSlice = this.reader.requestSlice(headerSlice.filePos, id3V2Header.size);
-				if (contentSlice instanceof Promise) contentSlice = await contentSlice;
+				if (isThenable(contentSlice)) contentSlice = await contentSlice;
 				if (!contentSlice) break;
 
 				parseId3V2Tag(contentSlice, id3V2Header, this.metadataTags);
@@ -306,7 +307,7 @@ class AdtsAudioTrackBacking implements InputAudioTrackBacking {
 			data = PLACEHOLDER_DATA;
 		} else {
 			let slice = this.demuxer.reader.requestSlice(rawSample.dataStart, rawSample.dataSize);
-			if (slice instanceof Promise) slice = await slice;
+			if (isThenable(slice)) slice = await slice;
 
 			if (!slice) {
 				return null; // Data didn't fit into the rest of the file

src/aes.ts

@@ -6,7 +6,7 @@
  * file, You can obtain one at https://mozilla.org/MPL/2.0/.
  */
 
-import { assert, MaybePromise } from './misc';
+import { assert, isThenable, MaybePromise } from './misc';
 import { readBytes, Reader } from './reader';
 
 // Inspired in part by https://github.com/halloweeks/AES-128-CBC/blob/main/AES_128_CBC.h
@@ -258,7 +258,7 @@ export const createAes128CbcDecryptStream = (
 			const requestedLength = CHUNK_SIZE + BLOCK_SIZE;
 
 			let nextSlice = reader.requestSliceRange(pos, 0, requestedLength);
-			if (nextSlice instanceof Promise) nextSlice = await nextSlice;
+			if (isThenable(nextSlice)) nextSlice = await nextSlice;
 			if (!nextSlice || nextSlice.length === 0) {
 				// Due to padding, this should never happen
 				throw new Error('Invalid ciphertext.');

src/flac/flac-demuxer.ts

@@ -15,6 +15,7 @@ import {
 	assert,
 	AsyncMutex,
 	binarySearchLessOrEqual,
+	isThenable,
 	textDecoder,
 	UNDETERMINED_LANGUAGE,
 } from '../misc';
@@ -108,7 +109,7 @@ export class FlacDemuxer extends Demuxer {
 			let currentPos = 0;
 			while (true) {
 				let headerSlice = this.reader.requestSlice(currentPos, ID3_V2_HEADER_SIZE);
-				if (headerSlice instanceof Promise) headerSlice = await headerSlice;
+				if (isThenable(headerSlice)) headerSlice = await headerSlice;
 
 				if (!headerSlice) {
 					this.lastSampleLoaded = true;
@@ -121,7 +122,7 @@ export class FlacDemuxer extends Demuxer {
 				}
 
 				let contentSlice = this.reader.requestSlice(headerSlice.filePos, id3V2Header.size);
-				if (contentSlice instanceof Promise) contentSlice = await contentSlice;
+				if (isThenable(contentSlice)) contentSlice = await contentSlice;
 				assert(contentSlice);
 
 				parseId3V2Tag(contentSlice, id3V2Header, this.metadataTags);
@@ -136,7 +137,7 @@ export class FlacDemuxer extends Demuxer {
 				|| currentPos < this.reader.fileSize
 			) {
 				let sizeSlice = this.reader.requestSlice(currentPos, 4);
-				if (sizeSlice instanceof Promise) sizeSlice = await sizeSlice;
+				if (isThenable(sizeSlice)) sizeSlice = await sizeSlice;
 				currentPos += 4;
 
 				if (sizeSlice === null) {
@@ -160,7 +161,7 @@ export class FlacDemuxer extends Demuxer {
 							currentPos,
 							size,
 						);
-						if (streamInfoBlock instanceof Promise) streamInfoBlock = await streamInfoBlock;
+						if (isThenable(streamInfoBlock)) streamInfoBlock = await streamInfoBlock;
 
 						assert(streamInfoBlock);
 						if (streamInfoBlock === null) {
@@ -219,7 +220,7 @@ export class FlacDemuxer extends Demuxer {
 							currentPos,
 							size,
 						);
-						if (vorbisCommentBlock instanceof Promise) vorbisCommentBlock = await vorbisCommentBlock;
+						if (isThenable(vorbisCommentBlock)) vorbisCommentBlock = await vorbisCommentBlock;
 
 						assert(vorbisCommentBlock);
 
@@ -237,7 +238,7 @@ export class FlacDemuxer extends Demuxer {
 							currentPos,
 							size,
 						);
-						if (pictureBlock instanceof Promise) pictureBlock = await pictureBlock;
+						if (isThenable(pictureBlock)) pictureBlock = await pictureBlock;
 
 						assert(pictureBlock);
 						const pictureType = readU32Be(pictureBlock);
@@ -775,7 +776,7 @@ class FlacAudioTrackBacking implements InputAudioTrackBacking {
 				rawSample.byteOffset,
 				rawSample.byteSize,
 			);
-			if (slice instanceof Promise) slice = await slice;
+			if (isThenable(slice)) slice = await slice;
 
 			if (!slice) {
 				return null; // Data didn't fit into the rest of the file

src/input-format.ts

@@ -36,7 +36,7 @@ import { TS_PACKET_SIZE } from './mpeg-ts/mpeg-ts-misc';
 import { HlsDemuxer } from './hls/hls-demuxer';
 import { HLS_MIME_TYPE } from './hls/hls-misc';
 import { PathedSource } from './source';
-import { MaybePromise } from './misc';
+import { isThenable, MaybePromise } from './misc';
 
 /**
  * Base class representing an input media file format.
@@ -75,7 +75,7 @@ export abstract class IsobmffInputFormat extends InputFormat {
 	/** @internal */
 	protected async _getMajorBrand(input: Input) {
 		let slice = input._reader.requestSlice(0, 12);
-		if (slice instanceof Promise) slice = await slice;
+		if (isThenable(slice)) slice = await slice;
 		if (!slice) return null;
 
 		slice.skip(4);
@@ -117,7 +117,7 @@ export class Mp4InputFormat extends IsobmffInputFormat {
 		}
 
 		let slice = input._reader.requestSlice(4, 4);
-		if (slice instanceof Promise) slice = await slice;
+		if (isThenable(slice)) slice = await slice;
 		if (!slice) return false;
 
 		const fourCc = readAscii(slice, 4);
@@ -169,7 +169,7 @@ export class MatroskaInputFormat extends InputFormat {
 	/** @internal */
 	protected async isSupportedEBMLOfDocType(input: Input, desiredDocType: string) {
 		let headerSlice = input._reader.requestSlice(0, MAX_HEADER_SIZE);
-		if (headerSlice instanceof Promise) headerSlice = await headerSlice;
+		if (isThenable(headerSlice)) headerSlice = await headerSlice;
 		if (!headerSlice) return false;
 
 		const varIntSize = readVarIntSize(headerSlice);
@@ -192,7 +192,7 @@ export class MatroskaInputFormat extends InputFormat {
 		}
 
 		let dataSlice = input._reader.requestSlice(headerSlice.filePos, dataSize);
-		if (dataSlice instanceof Promise) dataSlice = await dataSlice;
+		if (isThenable(dataSlice)) dataSlice = await dataSlice;
 		if (!dataSlice) return false;
 
 		const startPos = headerSlice.filePos;
@@ -295,7 +295,7 @@ export class Mp3InputFormat extends InputFormat {
 
 		while (true) {
 			let slice = input._reader.requestSlice(currentPos, ID3_V2_HEADER_SIZE);
-			if (slice instanceof Promise) slice = await slice;
+			if (isThenable(slice)) slice = await slice;
 			if (!slice) break;
 
 			const id3V2Header = readId3V2Header(slice);
@@ -315,7 +315,7 @@ export class Mp3InputFormat extends InputFormat {
 		const xingOffset = getXingOffset(firstHeader.mpegVersionId, firstHeader.channel);
 
 		let slice = input._reader.requestSlice(firstResult.startPos + xingOffset, 4);
-		if (slice instanceof Promise) slice = await slice;
+		if (isThenable(slice)) slice = await slice;
 		if (!slice) return false;
 
 		const word = readU32Be(slice);
@@ -376,7 +376,7 @@ export class WaveInputFormat extends InputFormat {
 	/** @internal */
 	async _canReadInput(input: Input) {
 		let slice = input._reader.requestSlice(0, 12);
-		if (slice instanceof Promise) slice = await slice;
+		if (isThenable(slice)) slice = await slice;
 		if (!slice) return false;
 
 		const riffType = readAscii(slice, 4);
@@ -416,7 +416,7 @@ export class OggInputFormat extends InputFormat {
 	/** @internal */
 	async _canReadInput(input: Input) {
 		let slice = input._reader.requestSlice(0, 4);
-		if (slice instanceof Promise) slice = await slice;
+		if (isThenable(slice)) slice = await slice;
 		if (!slice) return false;
 
 		return readAscii(slice, 4) === 'OggS';
@@ -451,7 +451,7 @@ export class FlacInputFormat extends InputFormat {
 		// There might be ID3v2 headers at the start, skip 'em
 		while (true) {
 			let slice = input._reader.requestSlice(currentPos, ID3_V2_HEADER_SIZE);
-			if (slice instanceof Promise) slice = await slice;
+			if (isThenable(slice)) slice = await slice;
 			if (!slice) break;
 
 			const id3V2Header = readId3V2Header(slice);
@@ -463,7 +463,7 @@ export class FlacInputFormat extends InputFormat {
 		}
 
 		let slice = input._reader.requestSlice(currentPos, 4);
-		if (slice instanceof Promise) slice = await slice;
+		if (isThenable(slice)) slice = await slice;
 		if (!slice) return false;
 
 		return readAscii(slice, 4) === 'fLaC';
@@ -498,7 +498,7 @@ export class AdtsInputFormat extends InputFormat {
 
 		while (true) {
 			let slice = input._reader.requestSlice(currentPos, ID3_V2_HEADER_SIZE);
-			if (slice instanceof Promise) slice = await slice;
+			if (isThenable(slice)) slice = await slice;
 			if (!slice) break;
 
 			const id3V2Header = readId3V2Header(slice);
@@ -514,7 +514,7 @@ export class AdtsInputFormat extends InputFormat {
 			MIN_ADTS_FRAME_HEADER_SIZE,
 			MAX_ADTS_FRAME_HEADER_SIZE,
 		);
-		if (slice instanceof Promise) slice = await slice;
+		if (isThenable(slice)) slice = await slice;
 		if (!slice) return false;
 
 		const firstHeader = readAdtsFrameHeader(slice);
@@ -529,7 +529,7 @@ export class AdtsInputFormat extends InputFormat {
 			MIN_ADTS_FRAME_HEADER_SIZE,
 			MAX_ADTS_FRAME_HEADER_SIZE,
 		);
-		if (slice instanceof Promise) slice = await slice;
+		if (isThenable(slice)) slice = await slice;
 		if (!slice) return false;
 
 		const secondHeader = readAdtsFrameHeader(slice);
@@ -573,7 +573,7 @@ export class MpegTsInputFormat extends InputFormat {
 	async _canReadInput(input: Input) {
 		const lengthToCheck = TS_PACKET_SIZE + 16 + 1;
 		let slice = input._reader.requestSlice(0, lengthToCheck);
-		if (slice instanceof Promise) slice = await slice;
+		if (isThenable(slice)) slice = await slice;
 		if (!slice) return false;
 
 		const bytes = readBytes(slice, lengthToCheck);
@@ -618,7 +618,7 @@ export class HlsInputFormat extends InputFormat {
 	/** @internal */
 	async _canReadInput(input: Input) {
 		let slice = input._reader.requestSlice(0, 7);
-		if (slice instanceof Promise) slice = await slice;
+		if (isThenable(slice)) slice = await slice;
 		if (!slice) return false;
 
 		const isM3u8 = readAscii(slice, 7) === '#EXTM3U';

src/input-track.ts

@@ -12,7 +12,7 @@ import { customAudioDecoders, customVideoDecoders } from './custom-coder';
 import { Input } from './input';
 import { Logging } from './logging';
 import { EncodedPacketSink, PacketRetrievalOptions } from './media-sink';
-import { assert, MaybePromise, Rational, Rotation, roundToDivisor, simplifyRational } from './misc';
+import { assert, isThenable, MaybePromise, Rational, Rotation, roundToDivisor, simplifyRational } from './misc';
 import { TrackType } from './output';
 import { EncodedPacket, PacketType } from './packet';
 import { TrackDisposition } from './metadata';
@@ -528,7 +528,7 @@ export abstract class InputTrack {
 }
 
 const requireSync = <T>(value: MaybePromise<T>, getterName: string, asyncName: string): T => {
-	if (value instanceof Promise) {
+	if (isThenable(value)) {
 		throw new Error(
 			`'${getterName}' is deprecated and not available synchronously for this track. Use the preferred`
 			+ ` '${asyncName}()' instead.`,
@@ -554,7 +554,7 @@ const toValidatedPredicate = <T extends InputTrack>(
 				};
 
 				const result = predicate(track);
-				if (result instanceof Promise) {
+				if (isThenable(result)) {
 					return result.then(handle);
 				}
 				return handle(result);
@@ -1225,7 +1225,7 @@ export const toValidatedInputTrackQuery = <T extends InputTrack>(
 					};
 
 					const result = query.filter!(track);
-					if (result instanceof Promise) {
+					if (isThenable(result)) {
 						return result.then(handle);
 					} else {
 						return handle(result);
@@ -1248,7 +1248,7 @@ export const toValidatedInputTrackQuery = <T extends InputTrack>(
 					};
 
 					const result = query.sortBy!(track);
-					if (result instanceof Promise) {
+					if (isThenable(result)) {
 						return result.then(handle);
 					} else {
 						return handle(result);
@@ -1274,7 +1274,7 @@ export const mergeInputTrackQueries = <T extends InputTrack>(
 						return queryB?.filter?.(track) ?? true;
 					};
 
-					if (resultA instanceof Promise) {
+					if (isThenable(resultA)) {
 						return resultA.then(handleResultA);
 					} else {
 						return handleResultA(resultA);
@@ -1294,7 +1294,7 @@ export const mergeInputTrackQueries = <T extends InputTrack>(
 						];
 					};
 
-					if (resultA instanceof Promise || resultB instanceof Promise) {
+					if (isThenable(resultA) || isThenable(resultB)) {
 						return Promise.all([resultA, resultB]).then(([resultA, resultB]) => {
 							return join(resultA, resultB);
 						});
@@ -1313,7 +1313,7 @@ export const queryInputTracks = async <T extends InputTrack>(
 	let matched = tracks;
 	if (query?.filter) {
 		const filterMatches = tracks.map(t => query.filter!(t));
-		const hasAsyncFilter = filterMatches.some(x => x instanceof Promise);
+		const hasAsyncFilter = filterMatches.some(x => isThenable(x));
 		if (hasAsyncFilter) {
 			// eslint-disable-next-line @typescript-eslint/await-thenable
 			const resolvedFilterMatches = await Promise.all(filterMatches);
@@ -1328,7 +1328,7 @@ export const queryInputTracks = async <T extends InputTrack>(
 	}
 
 	const sortValues = matched.map(t => query.sortBy!(t));
-	const hasAsyncSort = sortValues.some(x => x instanceof Promise);
+	const hasAsyncSort = sortValues.some(x => isThenable(x));
 	const resolvedSortValues = hasAsyncSort
 		// eslint-disable-next-line @typescript-eslint/await-thenable
 		? await Promise.all(sortValues)

src/isobmff/isobmff-demuxer.ts

@@ -58,6 +58,7 @@ import {
 	COLOR_PRIMARIES_MAP_INVERSE,
 	findLastIndex,
 	isIso639Dash2LanguageCode,
+	isThenable,
 	last,
 	MATRIX_COEFFICIENTS_MAP_INVERSE,
 	normalizeRotation,
@@ -364,7 +365,7 @@ export class IsobmffDemuxer extends Demuxer {
 
 			while (true) {
 				let slice = this.reader.requestSliceRange(currentPos, MIN_BOX_HEADER_SIZE, MAX_BOX_HEADER_SIZE);
-				if (slice instanceof Promise) slice = await slice;
+				if (isThenable(slice)) slice = await slice;
 				if (!slice) break;
 
 				const startPos = currentPos;
@@ -380,7 +381,7 @@ export class IsobmffDemuxer extends Demuxer {
 					// Found moov, load it
 
 					let moovSlice = this.reader.requestSlice(slice.filePos, boxInfo.contentSize);
-					if (moovSlice instanceof Promise) moovSlice = await moovSlice;
+					if (isThenable(moovSlice)) moovSlice = await moovSlice;
 					if (!moovSlice) break;
 
 					this.moovSlice = moovSlice;
@@ -431,7 +432,7 @@ export class IsobmffDemuxer extends Demuxer {
 
 				// The last 4 bytes may contain the size of the mfra box at the end of the file
 				let lastWordSlice = this.reader.requestSlice(this.reader.fileSize - 4, 4);
-				if (lastWordSlice instanceof Promise) lastWordSlice = await lastWordSlice;
+				if (isThenable(lastWordSlice)) lastWordSlice = await lastWordSlice;
 				assert(lastWordSlice);
 
 				const lastWord = readU32Be(lastWordSlice);
@@ -443,7 +444,7 @@ export class IsobmffDemuxer extends Demuxer {
 						MIN_BOX_HEADER_SIZE,
 						MAX_BOX_HEADER_SIZE,
 					);
-					if (mfraHeaderSlice instanceof Promise) mfraHeaderSlice = await mfraHeaderSlice;
+					if (isThenable(mfraHeaderSlice)) mfraHeaderSlice = await mfraHeaderSlice;
 
 					if (mfraHeaderSlice) {
 						const boxInfo = readBoxHeader(mfraHeaderSlice);
@@ -451,7 +452,7 @@ export class IsobmffDemuxer extends Demuxer {
 						if (boxInfo && boxInfo.name === 'mfra') {
 							// We found the mfra box, allowing for much better random access. Let's parse it.
 							let mfraSlice = this.reader.requestSlice(mfraHeaderSlice.filePos, boxInfo.contentSize);
-							if (mfraSlice instanceof Promise) mfraSlice = await mfraSlice;
+							if (isThenable(mfraSlice)) mfraSlice = await mfraSlice;
 
 							if (mfraSlice) {
 								this.readContiguousBoxes(mfraSlice);
@@ -678,14 +679,14 @@ export class IsobmffDemuxer extends Demuxer {
 		}
 
 		let headerSlice = this.reader.requestSliceRange(startPos, MIN_BOX_HEADER_SIZE, MAX_BOX_HEADER_SIZE);
-		if (headerSlice instanceof Promise) headerSlice = await headerSlice;
+		if (isThenable(headerSlice)) headerSlice = await headerSlice;
 		assert(headerSlice);
 
 		const moofBoxInfo = readBoxHeader(headerSlice);
 		assert(moofBoxInfo?.name === 'moof');
 
 		let entireSlice = this.reader.requestSlice(startPos, moofBoxInfo.totalSize);
-		if (entireSlice instanceof Promise) entireSlice = await entireSlice;
+		if (isThenable(entireSlice)) entireSlice = await entireSlice;
 		assert(entireSlice);
 
 		this.traverseBox(entireSlice);
@@ -3144,7 +3145,7 @@ abstract class IsobmffTrackBacking implements InputTrackBacking {
 				sampleInfo.sampleOffset,
 				sampleInfo.sampleSize,
 			);
-			if (slice instanceof Promise) slice = await slice;
+			if (isThenable(slice)) slice = await slice;
 			if (!slice) {
 				return null; // Data is outside
 			}
@@ -3207,7 +3208,7 @@ abstract class IsobmffTrackBacking implements InputTrackBacking {
 				fragmentSample.byteOffset,
 				fragmentSample.byteSize,
 			);
-			if (slice instanceof Promise) slice = await slice;
+			if (isThenable(slice)) slice = await slice;
 			if (!slice) {
 				return null; // Data is outside
 			}
@@ -3321,7 +3322,7 @@ abstract class IsobmffTrackBacking implements InputTrackBacking {
 
 			// Load the header
 			let slice = demuxer.reader.requestSliceRange(currentPos, MIN_BOX_HEADER_SIZE, MAX_BOX_HEADER_SIZE);
-			if (slice instanceof Promise) slice = await slice;
+			if (isThenable(slice)) slice = await slice;
 			if (!slice) break;
 
 			const boxStartPos = currentPos;
@@ -3748,7 +3749,7 @@ const resolveEncryptionAuxInfo = async (
 	}
 
 	let slice = reader.requestSlice(aux.offset, totalSize);
-	if (slice instanceof Promise) slice = await slice;
+	if (isThenable(slice)) slice = await slice;
 	if (!slice) {
 		throw new Error('Failed to read auxiliary encryption info.');
 	}

src/matroska/ebml.ts

@@ -7,7 +7,7 @@
  */
 
 import { MediaCodec } from '../codec';
-import { assert, assertNever, textDecoder, textEncoder } from '../misc';
+import { assert, assertNever, isThenable, textDecoder, textEncoder } from '../misc';
 import { FileSlice, readBytes, Reader, readF32Be, readF64Be, readU8 } from '../reader';
 import { Writer } from '../writer';
 
@@ -688,7 +688,7 @@ export const searchForNextElementId = async (
 
 	while (until === null || currentPos < until) {
 		let slice = reader.requestSliceRange(currentPos, MIN_HEADER_SIZE, MAX_HEADER_SIZE);
-		if (slice instanceof Promise) slice = await slice;
+		if (isThenable(slice)) slice = await slice;
 		if (!slice) break;
 
 		const elementHeader = readElementHeader(slice);
@@ -716,7 +716,7 @@ export const resync = async (reader: Reader, startPos: number, ids: EBMLId[], un
 
 	while (currentPos < until) {
 		let slice = reader.requestSliceRange(currentPos, 0, Math.min(CHUNK_SIZE, until - currentPos));
-		if (slice instanceof Promise) slice = await slice;
+		if (isThenable(slice)) slice = await slice;
 		if (!slice) break;
 		if (slice.length < MAX_VAR_INT_SIZE) break;
 

src/matroska/matroska-demuxer.ts

@@ -42,6 +42,7 @@ import {
 	COLOR_PRIMARIES_MAP_INVERSE,
 	findLastIndex,
 	isIso639Dash2LanguageCode,
+	isThenable,
 	last,
 	MATRIX_COEFFICIENTS_MAP_INVERSE,
 	normalizeRotation,
@@ -334,7 +335,7 @@ export class MatroskaDemuxer extends Demuxer {
 			// Loop over all top-level elements in the file
 			while (true) {
 				let slice = this.reader.requestSliceRange(currentPos, MIN_HEADER_SIZE, MAX_HEADER_SIZE);
-				if (slice instanceof Promise) slice = await slice;
+				if (isThenable(slice)) slice = await slice;
 				if (!slice) break;
 
 				const header = readElementHeader(slice);
@@ -350,7 +351,7 @@ export class MatroskaDemuxer extends Demuxer {
 					assertDefinedSize(size);
 
 					let slice = this.reader.requestSlice(dataStartPos, size);
-					if (slice instanceof Promise) slice = await slice;
+					if (isThenable(slice)) slice = await slice;
 					if (!slice) break;
 
 					this.readContiguousElements(slice);
@@ -433,7 +434,7 @@ export class MatroskaDemuxer extends Demuxer {
 
 		while (this.currentSegment.elementEndPos === null || currentPos < this.currentSegment.elementEndPos) {
 			let slice = this.reader.requestSliceRange(currentPos, MIN_HEADER_SIZE, MAX_HEADER_SIZE);
-			if (slice instanceof Promise) slice = await slice;
+			if (isThenable(slice)) slice = await slice;
 			if (!slice) break;
 
 			const elementStartPos = currentPos;
@@ -468,7 +469,7 @@ export class MatroskaDemuxer extends Demuxer {
 				assertDefinedSize(size);
 
 				let slice = this.reader.requestSlice(dataStartPos, size);
-				if (slice instanceof Promise) slice = await slice;
+				if (isThenable(slice)) slice = await slice;
 
 				if (slice) {
 					this.readContiguousElements(slice);
@@ -484,7 +485,7 @@ export class MatroskaDemuxer extends Demuxer {
 				assertDefinedSize(size);
 
 				let slice = this.reader.requestSlice(dataStartPos, size);
-				if (slice instanceof Promise) slice = await slice;
+				if (isThenable(slice)) slice = await slice;
 
 				if (slice) {
 					this.readContiguousElements(slice);
@@ -519,7 +520,7 @@ export class MatroskaDemuxer extends Demuxer {
 					MIN_HEADER_SIZE,
 					MAX_HEADER_SIZE,
 				);
-				if (slice instanceof Promise) slice = await slice;
+				if (isThenable(slice)) slice = await slice;
 				if (!slice) continue;
 
 				const header = readElementHeader(slice);
@@ -533,7 +534,7 @@ export class MatroskaDemuxer extends Demuxer {
 				this.currentSegment[target.flag] = true;
 
 				let dataSlice = this.reader.requestSlice(slice.filePos, size);
-				if (dataSlice instanceof Promise) dataSlice = await dataSlice;
+				if (isThenable(dataSlice)) dataSlice = await dataSlice;
 				if (!dataSlice) continue;
 
 				this.readContiguousElements(dataSlice);
@@ -608,7 +609,7 @@ export class MatroskaDemuxer extends Demuxer {
 		}
 
 		let headerSlice = this.reader.requestSliceRange(startPos, MIN_HEADER_SIZE, MAX_HEADER_SIZE);
-		if (headerSlice instanceof Promise) headerSlice = await headerSlice;
+		if (isThenable(headerSlice)) headerSlice = await headerSlice;
 		assert(headerSlice);
 
 		const elementStartPos = startPos;
@@ -637,7 +638,7 @@ export class MatroskaDemuxer extends Demuxer {
 
 		// Load the entire cluster
 		let dataSlice = this.reader.requestSlice(dataStartPos, size);
-		if (dataSlice instanceof Promise) dataSlice = await dataSlice;
+		if (isThenable(dataSlice)) dataSlice = await dataSlice;
 
 		const cluster: Cluster = {
 			segment,
@@ -894,7 +895,7 @@ export class MatroskaDemuxer extends Demuxer {
 				MIN_HEADER_SIZE,
 				MAX_HEADER_SIZE,
 			);
-			if (slice instanceof Promise) slice = await slice;
+			if (isThenable(slice)) slice = await slice;
 			if (!slice) continue;
 
 			const header = readElementHeader(slice);
@@ -907,7 +908,7 @@ export class MatroskaDemuxer extends Demuxer {
 			this.currentSegment = segment;
 
 			let dataSlice = this.reader.requestSlice(slice.filePos, size);
-			if (dataSlice instanceof Promise) dataSlice = await dataSlice;
+			if (isThenable(dataSlice)) dataSlice = await dataSlice;
 			if (dataSlice) {
 				this.readContiguousElements(dataSlice);
 			}
@@ -2347,7 +2348,7 @@ abstract class MatroskaTrackBacking implements InputTrackBacking {
 
 			// Load the header
 			let slice = demuxer.reader.requestSliceRange(currentPos, MIN_HEADER_SIZE, MAX_HEADER_SIZE);
-			if (slice instanceof Promise) slice = await slice;
+			if (isThenable(slice)) slice = await slice;
 			if (!slice) break;
 
 			const elementStartPos = currentPos;
@@ -2418,7 +2419,7 @@ abstract class MatroskaTrackBacking implements InputTrackBacking {
 				// the first segment.
 
 				let slice = demuxer.reader.requestSliceRange(endPos, MIN_HEADER_SIZE, MAX_HEADER_SIZE);
-				if (slice instanceof Promise) slice = await slice;
+				if (isThenable(slice)) slice = await slice;
 				if (!slice) break;
 
 				const elementId = readElementId(slice);

src/media-source.ts

@@ -28,6 +28,7 @@ import {
 	clamp,
 	clearIntervalUnthrottled,
 	floorToDivisor,
+	isThenable,
 	last,
 	missingWebCodecsClassMessage,
 	promiseWithResolvers,
@@ -426,7 +427,7 @@ class VideoEncoderWrapper {
 		// Apply the user-defined process function, if any
 		if (config.transform?.process) {
 			let processed = config.transform.process(videoSample);
-			if (processed instanceof Promise) {
+			if (isThenable(processed)) {
 				processed = await processed;
 			}
 
@@ -1937,7 +1938,7 @@ class AudioEncoderWrapper {
 		if (config.transform?.process) {
 			try {
 				let processed = config.transform.process(audioSample);
-				if (processed instanceof Promise) {
+				if (isThenable(processed)) {
 					processed = await processed;
 				}
 

src/misc.ts

@@ -738,6 +738,20 @@ export const missingWebCodecsClassMessage = (className: string) => {
  */
 export type MaybePromise<T> = T | Promise<T>;
 
+const NativePromiseConstructor = /* #__PURE__ */ (async () => {})().constructor as PromiseConstructor;
+
+/**
+ * Needed to properly deal with custom Promise implementations and because this is closer to how the JS spec does it.
+ */
+export const isThenable = <T>(value: MaybePromise<T>): value is Promise<T> => {
+	if (value instanceof NativePromiseConstructor || value instanceof Promise) {
+		return true;
+	}
+
+	// Fall back to a crude duck typing check
+	return typeof (value as PromiseLike<T> | null | undefined)?.then === 'function';
+};
+
 /** Acts like `??` except the condition is -1 and not null/undefined. */
 export const coalesceIndex = (a: number, b: number) => {
 	return a !== -1 ? a : b;

src/mp3/mp3-demuxer.ts

@@ -17,6 +17,7 @@ import {
 	AsyncMutex,
 	binarySearchExact,
 	binarySearchLessOrEqual,
+	isThenable,
 	toDataView,
 	UNDETERMINED_LANGUAGE,
 } from '../misc';
@@ -102,7 +103,7 @@ export class Mp3Demuxer extends Demuxer {
 			// Let's skip all ID3v2 tags at the start of the file
 			while (true) {
 				let slice = this.reader.requestSlice(this.lastLoadedPos, ID3_V2_HEADER_SIZE);
-				if (slice instanceof Promise) slice = await slice;
+				if (isThenable(slice)) slice = await slice;
 
 				if (!slice) {
 					this.lastSampleLoaded = true;
@@ -136,7 +137,7 @@ export class Mp3Demuxer extends Demuxer {
 		const xingOffset = getXingOffset(header.mpegVersionId, header.channel);
 
 		let slice = this.reader.requestSlice(result.startPos + xingOffset, 4);
-		if (slice instanceof Promise) slice = await slice;
+		if (isThenable(slice)) slice = await slice;
 		if (slice) {
 			const word = readU32Be(slice);
 			const isXing = word === XING || word === INFO;
@@ -151,7 +152,7 @@ export class Mp3Demuxer extends Demuxer {
 
 				if (!this.xingData) {
 					let xingDataSlice = this.reader.requestSlice(result.startPos + xingOffset + 4, 12);
-					if (xingDataSlice instanceof Promise) xingDataSlice = await xingDataSlice;
+					if (isThenable(xingDataSlice)) xingDataSlice = await xingDataSlice;
 					if (xingDataSlice) {
 						const xingData = readBytes(xingDataSlice, 12);
 						const view = toDataView(xingData);
@@ -216,7 +217,7 @@ export class Mp3Demuxer extends Demuxer {
 
 			while (true) {
 				let headerSlice = this.reader.requestSlice(currentPos, ID3_V2_HEADER_SIZE);
-				if (headerSlice instanceof Promise) headerSlice = await headerSlice;
+				if (isThenable(headerSlice)) headerSlice = await headerSlice;
 				if (!headerSlice) break;
 
 				const id3V2Header = readId3V2Header(headerSlice);
@@ -227,7 +228,7 @@ export class Mp3Demuxer extends Demuxer {
 				id3V2HeaderFound = true;
 
 				let contentSlice = this.reader.requestSlice(headerSlice.filePos, id3V2Header.size);
-				if (contentSlice instanceof Promise) contentSlice = await contentSlice;
+				if (isThenable(contentSlice)) contentSlice = await contentSlice;
 				if (!contentSlice) break;
 
 				parseId3V2Tag(contentSlice, id3V2Header, this.metadataTags);
@@ -238,7 +239,7 @@ export class Mp3Demuxer extends Demuxer {
 			if (!id3V2HeaderFound && this.reader.fileSize !== null && this.reader.fileSize >= ID3_V1_TAG_SIZE) {
 				// Try reading an ID3v1 tag at the end of the file
 				let slice = this.reader.requestSlice(this.reader.fileSize - ID3_V1_TAG_SIZE, ID3_V1_TAG_SIZE);
-				if (slice instanceof Promise) slice = await slice;
+				if (isThenable(slice)) slice = await slice;
 				assert(slice);
 
 				const tag = readAscii(slice, 3);
@@ -388,7 +389,7 @@ class Mp3AudioTrackBacking implements InputAudioTrackBacking {
 			data = PLACEHOLDER_DATA;
 		} else {
 			let slice = this.demuxer.reader.requestSlice(rawSample.dataStart, rawSample.dataSize);
-			if (slice instanceof Promise) slice = await slice;
+			if (isThenable(slice)) slice = await slice;
 
 			if (!slice) {
 				return null; // Data didn't fit into the rest of the file

src/mp3/mp3-reader.ts

@@ -7,6 +7,7 @@
  */
 
 import { MP3_FRAME_HEADER_SIZE, getMp3ChannelCount, Mp3FrameHeader, readMp3FrameHeader } from '../../shared/mp3-misc';
+import { isThenable } from '../misc';
 import { Reader, readU32Be } from '../reader';
 
 export const readNextMp3FrameHeader = async (
@@ -27,7 +28,7 @@ export const readNextMp3FrameHeader = async (
 			: CHUNK_SIZE;
 
 		let slice = reader.requestSliceRange(currentPos, MP3_FRAME_HEADER_SIZE, maxLength);
-		if (slice instanceof Promise) slice = await slice;
+		if (isThenable(slice)) slice = await slice;
 		if (!slice || slice.length < MP3_FRAME_HEADER_SIZE) break;
 
 		while (slice.remainingLength >= MP3_FRAME_HEADER_SIZE) {

src/mpeg-ts/mpeg-ts-demuxer.ts

@@ -63,6 +63,7 @@ import {
 	COLOR_PRIMARIES_MAP_INVERSE,
 	findLastIndex,
 	floorToMultiple,
+	isThenable,
 	last,
 	MATRIX_COEFFICIENTS_MAP_INVERSE,
 	readExpGolomb,
@@ -190,7 +191,7 @@ export class MpegTsDemuxer extends Demuxer {
 		return this.metadataPromise ??= (async () => {
 			const lengthToCheck = TS_PACKET_SIZE + 16 + 1;
 			let startingSlice = this.reader.requestSlice(0, lengthToCheck);
-			if (startingSlice instanceof Promise) startingSlice = await startingSlice;
+			if (isThenable(startingSlice)) startingSlice = await startingSlice;
 			assert(startingSlice);
 
 			const startingBytes = readBytes(startingSlice, lengthToCheck);
@@ -951,7 +952,7 @@ export class MpegTsDemuxer extends Demuxer {
 
 	async readPacketHeader(pos: number): Promise<TsPacketHeader | null> {
 		let slice = this.reader.requestSlice(pos, 4);
-		if (slice instanceof Promise) slice = await slice;
+		if (isThenable(slice)) slice = await slice;
 
 		if (!slice) {
 			return null;
@@ -987,7 +988,7 @@ export class MpegTsDemuxer extends Demuxer {
 	async readPacket(pos: number): Promise<TsPacket | null> {
 		// Code in here is duplicated from readPacketHeader for performance reasons
 		let slice = this.reader.requestSlice(pos, TS_PACKET_SIZE);
-		if (slice instanceof Promise) slice = await slice;
+		if (isThenable(slice)) slice = await slice;
 
 		if (!slice) {
 			return null;
@@ -2099,7 +2100,7 @@ class PacketReadingContext {
 
 			while (true) {
 				let remaining = this.ensureBuffered(CHUNK_SIZE);
-				if (remaining instanceof Promise) remaining = await remaining;
+				if (isThenable(remaining)) remaining = await remaining;
 
 				if (remaining === 0) {
 					break;
@@ -2253,7 +2254,7 @@ class PacketReadingContext {
 
 			while (true) {
 				let remaining = this.ensureBuffered(CHUNK_SIZE);
-				if (remaining instanceof Promise) remaining = await remaining;
+				if (isThenable(remaining)) remaining = await remaining;
 
 				const startPos = this.currentPos;
 
@@ -2269,7 +2270,7 @@ class PacketReadingContext {
 						const possibleHeaderStartPos = this.currentPos;
 
 						let remaining = this.ensureBuffered(MAX_ADTS_FRAME_HEADER_SIZE);
-						if (remaining instanceof Promise) remaining = await remaining;
+						if (isThenable(remaining)) remaining = await remaining;
 
 						if (remaining < MAX_ADTS_FRAME_HEADER_SIZE) {
 							return;
@@ -2282,7 +2283,7 @@ class PacketReadingContext {
 							this.seekTo(possibleHeaderStartPos);
 
 							let remaining = this.ensureBuffered(header.frameLength);
-							if (remaining instanceof Promise) remaining = await remaining;
+							if (isThenable(remaining)) remaining = await remaining;
 
 							return this.supplyPacket(
 								remaining,
@@ -2300,7 +2301,7 @@ class PacketReadingContext {
 						const possibleHeaderStartPos = this.currentPos;
 
 						let remaining = this.ensureBuffered(MP3_FRAME_HEADER_SIZE);
-						if (remaining instanceof Promise) remaining = await remaining;
+						if (isThenable(remaining)) remaining = await remaining;
 
 						if (remaining < MP3_FRAME_HEADER_SIZE) {
 							return;
@@ -2314,7 +2315,7 @@ class PacketReadingContext {
 							this.seekTo(possibleHeaderStartPos);
 
 							let remaining = this.ensureBuffered(result.header.totalSize);
-							if (remaining instanceof Promise) remaining = await remaining;
+							if (isThenable(remaining)) remaining = await remaining;
 
 							const duration = result.header.audioSamplesInFrame * TIMESCALE
 								/ elementaryStream.info.sampleRate;
@@ -2332,7 +2333,7 @@ class PacketReadingContext {
 
 						// Need at least 5 bytes for sync word + CRC + fscod/frmsizecod
 						let remaining = this.ensureBuffered(5);
-						if (remaining instanceof Promise) remaining = await remaining;
+						if (isThenable(remaining)) remaining = await remaining;
 
 						if (remaining < 5) {
 							return;
@@ -2361,7 +2362,7 @@ class PacketReadingContext {
 						this.seekTo(possibleSyncPos);
 
 						remaining = this.ensureBuffered(frameSize);
-						if (remaining instanceof Promise) remaining = await remaining;
+						if (isThenable(remaining)) remaining = await remaining;
 
 						const duration = Math.round(
 							AC3_SAMPLES_PER_FRAME * TIMESCALE / elementaryStream.info.sampleRate,
@@ -2377,7 +2378,7 @@ class PacketReadingContext {
 
 						// Need at least 5 bytes for E-AC-3 header parsing (sync word + frmsiz + fscod/numblkscod)
 						let remaining = this.ensureBuffered(5);
-						if (remaining instanceof Promise) remaining = await remaining;
+						if (isThenable(remaining)) remaining = await remaining;
 
 						if (remaining < 5) {
 							return;
@@ -2399,7 +2400,7 @@ class PacketReadingContext {
 						this.seekTo(possibleSyncPos);
 
 						remaining = this.ensureBuffered(frameSize);
-						if (remaining instanceof Promise) remaining = await remaining;
+						if (isThenable(remaining)) remaining = await remaining;
 
 						// Duration = numblks * 256 samples per block
 						const samplesPerFrame = numblks * 256;
@@ -2416,7 +2417,7 @@ class PacketReadingContext {
 						const possibleSyncPos = this.currentPos;
 
 						let remaining = this.ensureBuffered(DTS_CORE_FRAME_HEADER_SIZE);
-						if (remaining instanceof Promise) remaining = await remaining;
+						if (isThenable(remaining)) remaining = await remaining;
 
 						if (remaining < DTS_CORE_FRAME_HEADER_SIZE) {
 							return;
@@ -2438,7 +2439,7 @@ class PacketReadingContext {
 
 							const headerBound = Math.min(leadingExss.frameSize, DTS_EXSS_MAX_HEADER_SIZE);
 							let remaining = this.ensureBuffered(headerBound);
-							if (remaining instanceof Promise) remaining = await remaining;
+							if (isThenable(remaining)) remaining = await remaining;
 
 							leadingExss = parseDtsExssHeader(this.readBytes(remaining)) ?? leadingExss;
 						}
@@ -2456,7 +2457,7 @@ class PacketReadingContext {
 
 								const neededBytes = nextSubstreamPos + DTS_EXSS_HEADER_PREFIX_SIZE;
 								let remaining = this.ensureBuffered(neededBytes);
-								if (remaining instanceof Promise) remaining = await remaining;
+								if (isThenable(remaining)) remaining = await remaining;
 
 								if (remaining < neededBytes) {
 									break;
@@ -2484,7 +2485,7 @@ class PacketReadingContext {
 						this.seekTo(possibleSyncPos);
 
 						remaining = this.ensureBuffered(frameSize);
-						if (remaining instanceof Promise) remaining = await remaining;
+						if (isThenable(remaining)) remaining = await remaining;
 
 						const duration = Math.round(
 							sampleCount * TIMESCALE / elementaryStream.info.sampleRate,

src/ogg/ogg-demuxer.ts

@@ -18,6 +18,7 @@ import {
 	AsyncMutex,
 	binarySearchLessOrEqual,
 	findLast,
+	isThenable,
 	last,
 	roundIfAlmostInteger,
 	toDataView,
@@ -72,7 +73,7 @@ export class OggDemuxer extends Demuxer {
 
 			while (true) {
 				let slice = this.reader.requestSliceRange(currentPos, MIN_PAGE_HEADER_SIZE, MAX_PAGE_HEADER_SIZE);
-				if (slice instanceof Promise) slice = await slice;
+				if (isThenable(slice)) slice = await slice;
 				if (!slice) break;
 
 				const page = readPageHeader(slice);
@@ -274,7 +275,7 @@ export class OggDemuxer extends Demuxer {
 		while (true) {
 			// Load the entire page data
 			let pageSlice = this.reader.requestSlice(currentPage.dataStartPos, currentPage.dataSize);
-			if (pageSlice instanceof Promise) pageSlice = await pageSlice;
+			if (isThenable(pageSlice)) pageSlice = await pageSlice;
 			assert(pageSlice);
 			const pageData = readBytes(pageSlice, currentPage.dataSize);
 
@@ -299,7 +300,7 @@ export class OggDemuxer extends Demuxer {
 			let currentPos = currentPage.headerStartPos + currentPage.totalSize;
 			while (true) {
 				let headerSlice = this.reader.requestSliceRange(currentPos, MIN_PAGE_HEADER_SIZE, MAX_PAGE_HEADER_SIZE);
-				if (headerSlice instanceof Promise) headerSlice = await headerSlice;
+				if (isThenable(headerSlice)) headerSlice = await headerSlice;
 				if (!headerSlice) {
 					return null;
 				}
@@ -358,7 +359,7 @@ export class OggDemuxer extends Demuxer {
 		let currentPos = lastPacket.endPage.headerStartPos + lastPacket.endPage.totalSize;
 		while (true) {
 			let slice = this.reader.requestSliceRange(currentPos, MIN_PAGE_HEADER_SIZE, MAX_PAGE_HEADER_SIZE);
-			if (slice instanceof Promise) slice = await slice;
+			if (isThenable(slice)) slice = await slice;
 			if (!slice) {
 				return null;
 			}
@@ -658,7 +659,7 @@ class OggAudioTrackBacking implements InputAudioTrackBacking {
 				);
 
 				let searchSlice = this.demuxer.reader.requestSlice(searchStartPos, until - searchStartPos);
-				if (searchSlice instanceof Promise) searchSlice = await searchSlice;
+				if (isThenable(searchSlice)) searchSlice = await searchSlice;
 				assert(searchSlice);
 
 				const found = findNextPageHeader(searchSlice, until);
@@ -672,7 +673,7 @@ class OggAudioTrackBacking implements InputAudioTrackBacking {
 					MIN_PAGE_HEADER_SIZE,
 					MAX_PAGE_HEADER_SIZE,
 				);
-				if (headerSlice instanceof Promise) headerSlice = await headerSlice;
+				if (isThenable(headerSlice)) headerSlice = await headerSlice;
 				assert(headerSlice);
 
 				const page = readPageHeader(headerSlice);
@@ -685,7 +686,7 @@ class OggAudioTrackBacking implements InputAudioTrackBacking {
 					pageValid = true;
 				} else {
 					let pageSlice = this.demuxer.reader.requestSlice(page.headerStartPos, page.totalSize);
-					if (pageSlice instanceof Promise) pageSlice = await pageSlice;
+					if (isThenable(pageSlice)) pageSlice = await pageSlice;
 					assert(pageSlice);
 
 					// Validate the page by checking checksum
@@ -759,7 +760,7 @@ class OggAudioTrackBacking implements InputAudioTrackBacking {
 
 			const nextPos = currentPage.headerStartPos + currentPage.totalSize;
 			let slice = this.demuxer.reader.requestSliceRange(nextPos, MIN_PAGE_HEADER_SIZE, MAX_PAGE_HEADER_SIZE);
-			if (slice instanceof Promise) slice = await slice;
+			if (isThenable(slice)) slice = await slice;
 			assert(slice);
 
 			const nextPage = readPageHeader(slice);

src/output.ts

@@ -6,7 +6,16 @@
  * file, You can obtain one at https://mozilla.org/MPL/2.0/.
  */
 
-import { assert, AsyncMutex, EventEmitter, isIso639Dash2LanguageCode, MaybePromise, Rotation, toArray } from './misc';
+import {
+	assert,
+	AsyncMutex,
+	EventEmitter,
+	isIso639Dash2LanguageCode,
+	isThenable,
+	MaybePromise,
+	Rotation,
+	toArray,
+} from './misc';
 import { MetadataTags, TrackDisposition, validateMetadataTags, validateTrackDisposition } from './metadata';
 import { Muxer } from './muxer';
 import { OutputFormat } from './output-format';
@@ -451,7 +460,7 @@ export class Output<
 		}
 
 		const rootTargetResult = this._getRootTarget();
-		if (rootTargetResult instanceof Promise) {
+		if (isThenable(rootTargetResult)) {
 			throw new TypeError(errorMessage);
 		}
 
@@ -516,7 +525,7 @@ export class Output<
 			return result;
 		};
 
-		if (result instanceof Promise) {
+		if (isThenable(result)) {
 			return result.then(handleResult);
 		} else {
 			return handleResult(result);
@@ -604,7 +613,7 @@ export class Output<
 			return target;
 		};
 
-		if (result instanceof Promise) {
+		if (isThenable(result)) {
 			return this._rootTargetPromise = result.then(handleResult);
 		} else {
 			return handleResult(result);

src/reader.ts

@@ -7,7 +7,7 @@
  */
 
 import { InputDisposedError } from './input';
-import { assert, clamp, getUint24, MaybePromise, textDecoder, toDataView } from './misc';
+import { assert, clamp, getUint24, isThenable, MaybePromise, textDecoder, toDataView } from './misc';
 import { DEFAULT_MAX_READ_POSITION, DEFAULT_MIN_READ_POSITION, Source } from './source';
 
 export class Reader {
@@ -47,7 +47,7 @@ export class Reader {
 		const end = start + length;
 		const result = this.source._read(start, end, DEFAULT_MIN_READ_POSITION, DEFAULT_MAX_READ_POSITION);
 
-		if (result instanceof Promise) {
+		if (isThenable(result)) {
 			return result.then((x) => {
 				if (!x) {
 					return null;
@@ -95,7 +95,7 @@ export class Reader {
 				);
 			};
 
-			if (promisedAttempt instanceof Promise) {
+			if (isThenable(promisedAttempt)) {
 				return promisedAttempt.then(handleAttempt);
 			} else {
 				return handleAttempt(promisedAttempt);
@@ -121,7 +121,7 @@ export class Reader {
 				}
 
 				let slice = this.requestSliceRange(currentSize, 0, CHUNK_SIZE);
-				if (slice instanceof Promise) slice = await slice;
+				if (isThenable(slice)) slice = await slice;
 
 				if (!slice || slice.length === 0) {
 					break;

src/sample.ts

@@ -11,6 +11,7 @@ import {
 	clamp,
 	COLOR_PRIMARIES_MAP,
 	isAllowSharedBufferSource,
+	isThenable,
 	MATRIX_COEFFICIENTS_MAP,
 	Rotation,
 	SECOND_TO_MICROSECOND_FACTOR,
@@ -864,7 +865,7 @@ export class VideoSample implements Disposable {
 
 		if (this._data instanceof VideoSampleResource) {
 			let result = this._data.getDataPlanes();
-			if (result instanceof Promise) result = await result;
+			if (isThenable(result)) result = await result;
 
 			if (
 				!Array.isArray(result)
@@ -1022,7 +1023,7 @@ export class VideoSample implements Disposable {
 			}
 
 			const planes = this._data.getDataPlanes();
-			if (planes instanceof Promise) {
+			if (isThenable(planes)) {
 				throw new Error(
 					'Cannot convert a VideoSampleResource-backed VideoSample to VideoFrame if getDataPlanes() returns'
 					+ ' a promise.',
@@ -1586,7 +1587,7 @@ export class VideoSample implements Disposable {
 		// Description's finalized; let's see if a registered transformer wants to handle it
 		for (const transformer of registeredVideoSampleTransformers) {
 			let result = transformer(this, description);
-			if (result instanceof Promise) result = await result;
+			if (isThenable(result)) result = await result;
 
 			if (result !== null) {
 				return result;

src/source.ts

@@ -14,6 +14,7 @@ import {
 	closedIntervalsOverlap,
 	FilePath,
 	isNumber,
+	isThenable,
 	isWebKit,
 	MaybePromise,
 	mergeRequestInit,
@@ -328,7 +329,7 @@ export abstract class PathedSource extends Source {
 			return ref;
 		};
 
-		if (result instanceof Promise) {
+		if (isThenable(result)) {
 			return result.then(handle);
 		} else {
 			return handle(result);
@@ -387,7 +388,7 @@ export class CustomPathedSource extends PathedSource {
 					return ref;
 				};
 
-				if (result instanceof Promise) {
+				if (isThenable(result)) {
 					this._rootRequest = result.then(handle);
 				} else {
 					handle(result);
@@ -894,7 +895,7 @@ export class UrlSource extends PathedSource {
 			return result;
 		};
 
-		if (result instanceof Promise) {
+		if (isThenable(result)) {
 			return result.then(processResult);
 		} else {
 			return processResult(result);
@@ -1198,7 +1199,7 @@ export class UrlSource extends PathedSource {
 		for (const slice of uniqueSlices) {
 			const result = backing._read(slice.start, slice.start + slice.bytes.length);
 
-			if (result instanceof Promise) {
+			if (isThenable(result)) {
 				result.then((readResult) => {
 					if (readResult) {
 						// The backing's cache is empty at this point, so the read is guaranteed to produce
@@ -1459,7 +1460,7 @@ export class CustomSource extends Source {
 
 		const result = this._options.getSize();
 
-		if (result instanceof Promise) {
+		if (isThenable(result)) {
 			return result.then((size) => {
 				if (!Number.isInteger(size) || size < 0) {
 					throw new TypeError('options.getSize must return or resolve to a non-negative integer.');
@@ -1485,7 +1486,7 @@ export class CustomSource extends Source {
 			const originalTargetPos = worker.targetPos;
 
 			let data = this._options.read(worker.currentPos, originalTargetPos);
-			if (data instanceof Promise) data = await data;
+			if (isThenable(data)) data = await data;
 
 			if (worker.aborted) {
 				break;
@@ -2706,7 +2707,7 @@ export class RangedSource extends Source {
 			return result;
 		};
 
-		if (result instanceof Promise) {
+		if (isThenable(result)) {
 			return result.then(processResult);
 		} else {
 			return processResult(result);

src/wave/wave-demuxer.ts

@@ -12,7 +12,7 @@ import { Input } from '../input';
 import { InputAudioTrackBacking } from '../input-track';
 import { PacketRetrievalOptions } from '../media-sink';
 import { DEFAULT_TRACK_DISPOSITION, MetadataTags } from '../metadata';
-import { assert, UNDETERMINED_LANGUAGE } from '../misc';
+import { assert, isThenable, UNDETERMINED_LANGUAGE } from '../misc';
 import { EncodedPacket, PLACEHOLDER_DATA } from '../packet';
 import { readAscii, readBytes, Reader, readU16, readU32, readU64 } from '../reader';
 import { ID3_V2_HEADER_SIZE, parseId3V2Tag, readId3V2Header } from '../id3';
@@ -52,7 +52,7 @@ export class WaveDemuxer extends Demuxer {
 	async readMetadata() {
 		return this.metadataPromise ??= (async () => {
 			let slice = this.reader.requestSlice(0, 12);
-			if (slice instanceof Promise) slice = await slice;
+			if (isThenable(slice)) slice = await slice;
 			assert(slice);
 
 			const riffType = readAscii(slice, 4);
@@ -77,7 +77,7 @@ export class WaveDemuxer extends Demuxer {
 
 			while (totalFileSize === null || currentPos < totalFileSize) {
 				let slice = this.reader.requestSlice(currentPos, 8);
-				if (slice instanceof Promise) slice = await slice;
+				if (isThenable(slice)) slice = await slice;
 				if (!slice) break;
 
 				const chunkId = readAscii(slice, 4);
@@ -103,7 +103,7 @@ export class WaveDemuxer extends Demuxer {
 					// File and data chunk sizes are defined in here instead
 
 					let ds64Slice = this.reader.requestSlice(startPos, chunkSize);
-					if (ds64Slice instanceof Promise) ds64Slice = await ds64Slice;
+					if (isThenable(ds64Slice)) ds64Slice = await ds64Slice;
 					if (!ds64Slice) break;
 
 					const riffChunkSize = readU64(ds64Slice, littleEndian);
@@ -136,7 +136,7 @@ export class WaveDemuxer extends Demuxer {
 
 	private async parseFmtChunk(startPos: number, size: number, littleEndian: boolean) {
 		let slice = this.reader.requestSlice(startPos, size);
-		if (slice instanceof Promise) slice = await slice;
+		if (isThenable(slice)) slice = await slice;
 		if (!slice) return; // File too short
 
 		let formatTag = readU16(slice, littleEndian);
@@ -206,7 +206,7 @@ export class WaveDemuxer extends Demuxer {
 
 	private async parseListChunk(startPos: number, size: number, littleEndian: boolean) {
 		let slice = this.reader.requestSlice(startPos, size);
-		if (slice instanceof Promise) slice = await slice;
+		if (isThenable(slice)) slice = await slice;
 		if (!slice) return; // File too short
 
 		const infoType = readAscii(slice, 4);
@@ -303,7 +303,7 @@ export class WaveDemuxer extends Demuxer {
 	private async parseId3Chunk(startPos: number, size: number) {
 		// Parse ID3 tag embedded in WAV file (non-default, but used a lot in practice anyway)
 		let slice = this.reader.requestSlice(startPos, size);
-		if (slice instanceof Promise) slice = await slice;
+		if (isThenable(slice)) slice = await slice;
 		if (!slice) return; // File too short
 
 		const id3V2Header = readId3V2Header(slice);
@@ -489,7 +489,7 @@ class WaveAudioTrackBacking implements InputAudioTrackBacking {
 			// requested slice actually exists.
 
 			let slice = this.demuxer.reader.requestSlice(this.demuxer.dataStart + startOffset, sizeInBytes);
-			if (slice instanceof Promise) slice = await slice;
+			if (isThenable(slice)) slice = await slice;
 
 			if (!slice) {
 				return null;
@@ -501,7 +501,7 @@ class WaveAudioTrackBacking implements InputAudioTrackBacking {
 			data = PLACEHOLDER_DATA;
 		} else {
 			let slice = this.demuxer.reader.requestSlice(this.demuxer.dataStart + startOffset, sizeInBytes);
-			if (slice instanceof Promise) slice = await slice;
+			if (isThenable(slice)) slice = await slice;
 			assert(slice);
 
 			data = readBytes(slice, sizeInBytes);