Instances of Buffer are also instances of Uint8Array in node.js 4.x and higher. Thus, the most efficient solution is to access the buffer's own .buffer property directly, as per https://stackoverflow.com/a/31394257/1375574. The Buffer constructor also takes an ArrayBufferView argument if you need to go the other direction.

Note that this will not create a copy, which means that writes to any ArrayBufferView will write through to the original Buffer instance.

From Buffer to ArrayBuffer:

function toArrayBuffer(buffer) {
  const arrayBuffer = new ArrayBuffer(buffer.length);
  const view = new Uint8Array(arrayBuffer);
  for (let i = 0; i < buffer.length; ++i) {
    view[i] = buffer[i];
  }
  return arrayBuffer;
}

From ArrayBuffer to Buffer:

function toBuffer(arrayBuffer) {
  const buffer = Buffer.alloc(arrayBuffer.byteLength);
  const view = new Uint8Array(arrayBuffer);
  for (let i = 0; i < buffer.length; ++i) {
    buffer[i] = view[i];
  }
  return buffer;
}

As ArrayBufferLike can be at least either an ArrayBuffer | Shared ArrayBuffer | ..., I found that we can wrap the Buffer<ArrayBufferLike> into an Uint8Array to guarantee an ArrayBuffer:

import fs from 'fs';
import path from 'path';
import * as kdbxweb from 'kdbxweb';

async getClientSecret(): Promise<string> {

    const fileBuffer: Buffer<ArrayBufferLike> = fs.readFileSync(KeePassFilePath);
    const keePassFileBuffer: ArrayBuffer = new Uint8Array(fileBuffer).buffer;
    const keePassDB: kdbxweb.Kdbx = await kdbxweb.Kdbx.load(keePassFileBuffer, new kdbxweb.Credentials(kdbxweb.ProtectedValue.fromString('123')));
    // ...
}