好的，这挽救了我的生命：https :
//youtu.be/pXHOF4GWuZQ。这是最终代码：
downloadFile(file_id) { return new Promise((resolve, reject) => { var mongoose = require('mongoose'); var Grid = require('gridfs-stream'); var fs = require('fs'); mongoose.connect(config.db, {useNewUrlParser: true},).catch(e => console.log(e)); var conn = mongoose.connection; Grid.mongo = mongoose.mongo; var gfs = Grid(conn.db); console.log('downloadfile', file_id); var read_stream = gfs.createReadStream({_id: file_id}); let file = []; read_stream.on('data', function (chunk) { file.push(chunk); }); read_stream.on('error', e => { console.log(e); reject(e); }); return read_stream.on('end', function () { file = Buffer.concat(file); const img = `data:image/png;base64,${Buffer(file).toString('base64')}`; resolve(img); }); });}
