validation.js

var _ = require('underscore'); // es6 can import what we need

// will return a function
function required(field, value) {
  return value || 'Value is required';
}

function minLen(value) {
  return value.length >= 6 ? value : 'Value is too short';
}

function maxLen(value) {
  return value.length <= 25 ? value : 'Value is too long';
}

function isBool(value) {
  return typeof value === 'boolean';
}

var validate = _.compose(
  minLen,
  maxLen,
  required
);

console.log(validate('firstName', 'ericat'));

var validationRules = {
  firstName: [required, minLen, MaxLen],
  billingAddress: [required, isBool],
  password: [required, minLen, MaxLen],
  confirmPass: [required, mustMatch]
};

// Apply rules, pseudo code
export const validateFields = (field, value, ...validations) => {
  for (let v  of validations) {
    let errMess = v(field, value);

    if (errMess) {
      return {
        [field]: errMessage(field)
      };
    }
  }
  return field;
};