[devote]

function FormField(data) {
  this._d = data;
}

function FormFile(data, opts) {
  FormField.call(this, data);
  this.opts = opts;
}

FormFile.prototype = FormField.prototype;
FormFile.prototype.constructor = FormFile;

var a = new FormFile(1, 2);
var b = new FormFile(10, 22);
console.log(a, a instanceof FormField, b instanceof FormField);
console.log(b, a instanceof FormFile, b instanceof FormFile);