var FormData = require('form-data');
var fs = require('fs');

var form = new FormData();
form.append('my_file', fs.createReadStream('/foo/bar.jpg'));
form.submit('example.org/upload', function(err, res) {
 console.log(res.statusCode);
});

const FormData = require('form-data');

const filename = 'my-file.txt';
const content = 'balalalalala...变身';

const formData = new FormData();
// 1.先将字符串转换成Buffer
const fileContent = Buffer.from(content);
// 2.补上文件meta信息
formData.append('file', fileContent, {
 filename,
 contentType: 'text/plain',
 knownLength: fileContent.byteLength
});

function ReadStream(path, options) {
 // 1.打开path指定的文件
 if (typeof this.fd !== 'number')
  this.open();
}

ReadStream.prototype.open = function() {
 fs.open(this.path, this.flags, this.mode, (er, fd) => {
  // 2.拿到文件描述符并持有
  this.fd = fd;
  this.emit('open', fd);
  this.emit('ready');
  // 3.开始流式读取数据
  // read来自父类Readable，主要调用内部方法_read
  // ref: https://github.com/nodejs/node/blob/v10.16.3/lib/_stream_readable.js#L390
  this.read();
 });
};

ReadStream.prototype._read = function(n) {
 // 4.从文件中读取一个chunk
 fs.read(this.fd, pool, pool.used, toRead, this.pos, (er, bytesRead) => {
  let b = null;
  if (bytesRead > 0) {
   this.bytesRead += bytesRead;
   b = thisPool.slice(start, start + bytesRead);
  }
  // 5.（通过触发data事件）吐出一个chunk，如果还有数据，process.nextTick再次this.read，直至this.push(null)触发'end'事件
  // ref: https://github.com/nodejs/node/blob/v10.16.3/lib/_stream_readable.js#L207
  this.push(b);
 });
};

// 从文件中读取一个chunk
fs.read(this.fd, pool, pool.used, toRead, this.pos, (er, bytesRead) => {
 /* ... */
});

// 换成
this._fakeReadFile(this.fd, pool, pool.used, toRead, this.pos, (bytesRead) => {
 /* ... */
});

// 从输入字符串对应的Buffer中copy出一个chunk
ReadStream.prototype._fakeReadFile = function(_, buffer, offset, length, position, cb) {
 position = position || this.input._position;
 // fake read file async
 setTimeout(() => {
  let bytesRead = 0;
  if (position < this.input.byteLength) {
   bytesRead = this.input.copy(buffer, offset, position, position + length - 1);
   this.input._position += bytesRead;
  }
  cb(bytesRead);
 }, 0);
}

string2fileStream('string-content') === fs.createReadStream(/* path to a text file with content 'string-content' */)`

const string2fileStream = require('string-to-file-stream');

const input = 'Oh, my great data!';
const s = string2fileStream(input);
s.on('data', (chunk) => {
 assert.equal(chunk.toString(), input);
});
生成的流同样能够具有文件 meta 信息：

const string2fileStream = require('string-to-file-stream');

const formData = new FormData();
formData.append('file', string2fileStream('my-string-data', { path: './abc.txt' }));
form.submit('example.org/upload', function(err, res) {
 console.log(res.statusCode);
});