Generate an S3 ETAG for multipart uploads in Node.js

s3Etag.ts

	/**
	* Generate an S3 ETAG for multipart uploads in Node.js
	* An implementation of this algorithm: https://stackoverflow.com/a/19896823/492325
	* Author: Richard Willis <willis.rh@gmail.com>
	*/
	import fs from 'node:fs';
	import crypto, { BinaryLike } from 'node:crypto';

	const defaultPartSizeInBytes = 5 * 1024 * 1024; // 5MB

	function md5(contents: string \| BinaryLike): string {
	return crypto.createHash('md5').update(contents).digest('hex');
	}

	export function getS3Etag(
	filePath: string,
	partSizeInBytes = defaultPartSizeInBytes
	): string {
	const { size: fileSizeInBytes } = fs.statSync(filePath);
	let parts = Math.floor(fileSizeInBytes / partSizeInBytes);
	if (fileSizeInBytes % partSizeInBytes > 0) {
	parts += 1;
	}
	const fileDescriptor = fs.openSync(filePath, 'r');
	let totalMd5 = '';

	for (let part = 0; part < parts; part++) {
	const skipBytes = partSizeInBytes * part;
	const totalBytesLeft = fileSizeInBytes - skipBytes;
	const bytesToRead = Math.min(totalBytesLeft, partSizeInBytes);
	const buffer = Buffer.alloc(bytesToRead);
	fs.readSync(fileDescriptor, buffer, 0, bytesToRead, skipBytes);
	totalMd5 += md5(buffer);
	}

	const combinedHash = md5(Buffer.from(totalMd5, 'hex'));
	const etag = `${combinedHash}-${parts}`;
	return etag;
	}