validation.js

const isEmpty = value => value === undefined || value === null || value === "";
const join = rules => (value, data) =>
  rules.map(rule => rule(value, data)).filter(error => !!error)[0];

export function email(value) {
  if (
    !isEmpty(value) &&
    !/^[A-Z0-9._%+-]+@[A-Z0-9.-]+\.[A-Z]{2,4}$/i.test(value)
  ) {
    return "Invalid e-mail address";
  }
}

export function slug(value) {
  if (!isEmpty(value) && !/^[A-Z0-9._-]+$/i.test(value)) {
    return "Enter a valid name consisting of letters, numbers, underscores or hyphens";
  }
}

export function setRequired(value) {
  if (value.size === 0) {
    return "This field is required";
  }
}

export function required(value) {
  if (isEmpty(value)) {
    return "This field is required";
  }
}

export function minLength(min) {
  return value => {
    if (!isEmpty(value) && value.length < min) {
      return `It must be at least ${min} characters`;
    }
  };
}

export function maxLength(max) {
  return value => {
    if (!isEmpty(value) && value.length > max) {
      return `It must be at most ${max} characters`;
    }
  };
}

export function integer(value) {
  if (!Number.isInteger(Number(value))) {
    return "It must be an integer number";
  }
}

export function isNumber(value) {
  if (isNaN(value)) {
    return "It must be a valid number";
  }
}

export function oneOf(enumeration) {
  return value => {
    if (!~enumeration.indexOf(value)) {
      return `It must be one of: ${enumeration.join(", ")}`;
    }
  };
}

export function match(field) {
  return (value, data) => {
    if (data) {
      if (value !== data[field]) {
        return "It must match";
      }
    }
  };
}

export function createValidator(rules) {
  return (data = {}) => {
    const errors = {};
    Object.keys(rules).forEach(key => {
      const rule = join([].concat(rules[key])); // concat enables both functions and arrays of functions
      const error = rule(data[key], data);
      if (error) {
        errors[key] = error;
      }
    });
    return errors;
  };
}

export function isValid(errors) {
  return Object.keys(errors).find(key => errors[key] !== null) === undefined;
}

// USAGE
const v = createValidator({
  email: [required, email],
  password: [required, minLength(8)]
})

const errors = v.validate({email: "not gonna pass": "thiswillsurellypass"})