NodeJS : Reading a part of a file bottom to up and return the lines

readFileBottomUp.js

/*
reads a file upwards from the given 'startPosition' to the specified length and
returns an object with the lines and the final position read
if 'startPosition' is not specified it will be the very bottom of the file
*/
async function readFileBottomUp(filename, length, startPosition) {

    return new Promise((resolve, reject) => {

        // file does not exist
        if (!fs.existsSync(filename)) {
            reject(new Error(`File ${filename} does not exist.`))
            return
        }

        // calculate boundaries to read
        let endPosition = (startPosition > -1) ? startPosition : fs.statSync(filename).size - 1
        startPosition = endPosition - length

        // read the chunk
        let buffer = ''
        let fileStream = fs.createReadStream(
            filename,
            {
                'start': startPosition,
                'end': endPosition,
                'encoding': 'utf8'
            }
        )

        fileStream.on('readable', () => {

            let data = fileStream.read()
            // data chunk available => add to buffer
            if (data) {
                buffer += data
            }
            // end of stream => process buffer
            else {
                let firstLineStart = buffer.indexOf('\n') + 1
                let lastLineEnd = (endPosition == fs.statSync(filename).size - 1) ? buffer.length : buffer.lastIndexOf('\n')
                let lines = buffer.substring(firstLineStart, lastLineEnd)
                resolve({ 'lines': lines, 'endByte': startPosition + firstLineStart })
            }

        })

    })

}