Latest and greatest way to do this:

Node supports file and buffer operations with the base64 encoding:

const fs = require('fs');
const contents = fs.readFileSync('/path/to/file.jpg', {encoding: 'base64'});

Or using the new promises API:

const fs = require('fs').promises;
const contents = await fs.readFile('/path/to/file.jpg', {encoding: 'base64'});

This should work. Sample script:

const fs = require('fs')
const filepath = './testfile'
//write "it's" into the file
fs.writeFileSync(filepath,"it's")

//read the file
const file_buffer  = fs.readFileSync(filepath);
//encode contents into base64
const contents_in_base64 = file_buffer.toString('base64');
//write into a new file, specifying base64 as the encoding (decodes)
fs.writeFileSync('./fileB64',contents_in_base64,{encoding:'base64'})
//file fileB64 should now contain "it's"

I suspect your original file does not have utf-8 encoding, looking at your decoding code: fs.writeFile(reportPath, body.buffer, {encoding: 'base64'})

I am guessing your content comes from a http request of some sorts so it is possible that the content is not utf-8 encoded. Take a look at this: https://www.w3.org/International/articles/http-charset/index if charset is not specified Content-Type text/ uses ISO-8859-1.

Here's my very slightly adjusted version of your code:

import fs from "node:fs";

const merged = fs.readFileSync("/Users/emfive/Downloads/mergedData.dat", "utf-8");

const decoded = Buffer.from(merged, "base64");

console.log("original: " + merged.length);

console.log("decoded: " + decoded.length);

fs.writeFileSync("foo.mp4", decoded);

It takes no time to run, reading the source and writing the decoded result in under a second. There's no need for streams or "chunked" operations.

If the code in the question is not working, then there's something about it that has not been shared. For example, when you read in the base64 content, you have to make sure you're reading a string (by adding "utf-8" to the readFileSync() method).