Multer writes file with zero size if originalname changes in the storage.filename prop.

const fs = require('node:fs')
const multer = require('multer')
const path = require('node:path')

const FILE_UPLOAD_PATH = '/code/uploads'
const UPDATE_FILE_URL = /\/file\/update\/[\d]+/

async function generateUserFileName(originalname, request){

    const userId = request.userId
    const fileId = request.params?.id || null
    const url = request.url
    const file = await dao.getFile(fileId, userId)
    const fileDir = path.resolve(FILE_UPLOAD_PATH, userId.toString())
    const filePath = path.resolve(fileDir, originalname)

    if (file){
        const oldFileName = path.resolve(fileDir, file.name)
        if (url.match(UPDATE_FILE_URL)){
            try{
                await fs.promises.access(oldFileName)
                await fs.promises.unlink(oldFileName)
            }
            catch (error){
                //pass
            }
        }
    }

    try{ 
        await fs.promises.access(filePath) //if filename is exist then it generates a new name:
        return path.parse(originalname).name + `_${Date.now()}` + path.extname(originalname)
    }
    catch (error){
        return originalname //if file is not exist then filename is the same as it is in request.originalname
    }
}

const storage = multer.diskStorage({
    destination: async (request, file, callback) => {
        console.log(`Userid: ${request.userId}`)

        const fileDir = path.resolve(FILE_UPLOAD_PATH, request.userId.toString())

        try{
            await fs.promises.access(fileDir)
        }
        catch (error){
            await fs.promises.mkdir(path.resolve(fileDir))
        }
        callback(null, fileDir)
    },

    filename: async (request, file, callback) => {
        const fn = await generateUserFileName(file.originalname, request)
        
        console.log(`File name ${fn} was generated successfully`)// Always, here my filename is generated successfully
        callback(null, fn)
    }
})

const upload = multer({ 
    storage: storage
})
const uploadStorage = upload.single('file')


module.exports = {
    uploadStorage,
    deleteFile
}