validator.js

// pick is something ala lodash pick (fetching nested values from objects), but in it's simplest case
function pick (name, obj) {
  return obj[name];
}

function validate (name, predicate, message, obj) {
  if (!predicate(pick(name, obj))) {
     return { valid: false, message, name };
  }
  return { valid: true, name };
}

function partial (fn, args) {
  return function (innerArgs) {
    return fn(...args.concat(innerArgs));
  }
}

function isEmpty (input) {
  return typeof input === "undefined" || input === '';
}

function generateValidationObject (fns, obj) {
  return fns.reduce(function (validObject, fn) {
    const { valid, name, message } = fn(obj);
    if (valid) return validObject;
    const hasArray = Array.isArray(validObject[name]);
    validObject[name] = hasArray ? validObject[name].concat(message) : [message];
    return validObject;
  }, {});
}

export default function validator (fields = []) {
  return {
    notEmpty (name) {
      const validationFunc = partial(validate, name, isEmpty, `${name} is required input`);
      return validator(fields.concat(validationFunc));
    },
    isValid (obj) {
      return generateValidationObject(fields, obj);
    }
  }
}