Make the nested fields parsed by multiparty look like req.body from body-parser

multiparty_reformat_fields.js

// Make the nested fields parsed by multiparty look like req.body from body-parser
// e.g. 'metadata[foo]': ['1']            => {metadata: {foo: 1}}
//      'metadata[foo]': ['bar']          => {metadata: {foo: 'bar'}}
//      'metadata[foo][]': ['bar', 'bat'] => {metadata: {foo: ['bar', 'bat']}}

var qs = require('qs');

function reformatFields(fields) {
  // convert numbers to real numbers instead of strings
  function toNumber(i) { 
    return i !== '' && !isNaN(i) ? Number(i) : i;
  }
  
  // remove the extra array wrapper around the values
  for (var f in fields) {
    if (f === 'null') {
      delete fields[f];  // ignore null fields like submit
    } else {
      if (f.match(/\[\]$/)) {
        // if our key uses array syntax we can make qs.parse produce the intended result
        // by removing the trailing [] on the key
        var key = f.replace(/\[\]$/,'');
        fields[key] = fields[f].map(function(i) { return toNumber(i) });
        delete fields[f]; 
      } else {
        // for scalar values, just extract the single value
        fields[f] = toNumber(fields[f][0]);
      }
    }
  }
  
  return qs.parse(fields);
};

// later in your app...
form.parse(req, function(err, fields, files) {
  var _fields = reformatFields(fields);
  console.log(_fields);
});