function parallelDownload(files){
    const downloadPromises = files.map(file => download(file));
    return Promise.all(downloadPromises);
}

parallelDownload(mockFiles)
.then(_ => console.log('files were downloaded in parallel mode'))

// ----- output in console -----

// started downloading file 1 ...(immediately)
// started downloading file 2 ...(immediately)
// started downloading file 3 ...(immediately)

// downloaded file 1 in 2000ms ...(after 2s)

// downloaded file 3 in 3000ms ...(after 3s)

// downloaded file 2 in 4000ms ...(after 4s)

// files were downloaded in parallel mode ...(after 4s)