field_custom_async.js

var form = require('express-form'),
    field = form.field;

// sync `custom`
form(
  field('email')
    .required()
    .custom(function(email) {
      if (!checkUnique(email)) {
        throw new Error('Email is not unique');
      }
    })
);

// async `custom`
form(
  field('email')
    .required()
    .custom(function(email, cb) {
      checkUnique(email, function(err) {
        if (err) {
          return cb(new Error('Email is not unique'));
        }
        cb(null);
      });
    })
);