Generate an accurate S3 ETAG in Node.js for any file (including multipart).

Please Note, this only works for unencrypted buckets.

npm install s3-etag

import { generateETag } from 's3-etag';

// Simple MD5 hash of contents for non-multipart files
const etag = generateETag(absoluteFilePath);

// MD5 hash of combined contents & part number (see below) for multipart files
const partSizeInBytes = 10 * 1024 * 1024; // 10mb
const etag = generateETag(absoluteFilePath, partSizeInBytes);

This is a Node.js implementation of this algorithm.

At a high level:

• If no partSizeInBytes is specified, return MD5 hash of file contents
• If partSizeInBytes is specified:
  • Generate parts by comparing partSizeInBytes to the file size
  • Read each part from the file, MD5 hash the part, and append it to a global combined hash
  • Once all parts are processed, generate a new MD5 from the global combined hash, and suffix with the amount of parts

If the partSizeInBytes is unknown, you can find it by using AWS CLI:

1. Use head-object to retrieve the object's metadata using the following command, where raw-files is the bucket name, and IMG2345.CR2 is the key aws s3api head-object --bucket raw-files --key IMG2345.CR2 this will return

     {
     "AcceptRanges": "bytes",
     "ContentType": "text/html",
     "LastModified": "2024-05-22T08:45:10+00:00"
     "ContentLength": 28333464,
     "ETag": "\"85ae33db28930d3afe594da14cd190bb-2\"",
     "VersionId": "28P4UkX5sCO.8vbyMojvecHndkHDwf",
     "ContentType": "binary/octet-stream",
     "ServerSideEncryption": "AES256",
     "Metadata": {}
     }
   

2. Look for a -n at the end of the eTag, where n is a number >= 2, representing the number of parts/chunks. For single part objects, the eTag will simply be the MD5 of the object.
3. Use aws s3api head-object --bucket raw-files --key IMG2345.CR2 --part-number 1to get the metadata of the first part/chunk. This will return

      {
      "AcceptRanges": "bytes",
      "ContentType": "text/html",
      "LastModified": "2024-05-22T08:45:10+00:00"
      "ContentLength": 17179870,
      "ETag": "\"85ae33db28930d3afe594da14cd190bb-2\"",
      "VersionId": "28P4UkX5sCO.8vbyMojvecHndkHDwf",
      "ContentType": "binary/octet-stream",
      "ServerSideEncryption": "AES256",
      "Metadata": {},
      "PartsCount": 2
      }
   

4. Use the ContentLength value as the partSizeInBytes

See LICENSE.md.