move file first string to the end of file: node.js, stream

move-first-string-to-eof.js

#!/usr/bin/env node

const fs = require('fs');
const {Buffer} = require('buffer');

const filepath = process.argv[2];
if (!filepath) {
    console.log('\nДолжен быть указан путь к существующему файлу!\n');
    process.exit(0);
}
console.log(`FILE: ${filepath}`);

let buffer;
const readStream = fs.createReadStream(filepath);//, {highWaterMark: 7});
readStream.on('error', (err) => {
    console.error('READING ERROR!!!');
    console.trace(err);
    process.exit(1);
})
.on('data', (b) => {
    buffer = buffer ? Buffer.concat([buffer, b]) : b;
    readStream.pause();
    const pos = buffer.indexOf('\n');
    if (pos === -1) {
        readStream.resume();
        return;
    }
    const firstLine = buffer.slice(0, pos);
    const restLine = buffer.slice(pos+1, buffer.length);

    const writeStream = fs.createWriteStream(filepath, {start: 0, flags: 'a'});
    writeStream.on('error', (err) => {
        console.error('WRITING ERROR!!!');
        console.trace(err);
        process.exit(1);
    });
    readStream.removeAllListeners('data');
    readStream.once('end', () => {
        writeStream.write('\n');
        writeStream.end(firstLine);
    });
    writeStream.write(restLine);
    readStream.pipe(writeStream);
});