const fs = require('fs');
const readable = getReadableStreamSomehow();
const writable = fs.createWriteStream('file.txt');
// All the data from readable goes into 'file.txt'
readable.pipe(writable);

https://nodejs.org/api/stream...

sftp.getFile(fileName).then(data => {
// data 是ReadStream類型  data.pipe(require('fs').createWriteStream(`files/download/${fileName}.csv`));
  console.log(data)
}).catch(err => {
  console.log(err);
});