React.js field validation

gistfile1.js

/*
  Simple serial "one by one" sync/async promises based validation.
*/
import validator from 'validator'
import Promise from 'bluebird'

export function focusInvalidField(reactComponent) {
  return (error) => {
    if (error instanceof ValidationError) {
      if (!error.prop) return
      const el = reactComponent.getDOMNode().querySelector(`[name=${error.prop}]`)
      if (!el) return
      el.focus()
      return
    }
    throw error
  }
}

export class ValidationError extends Error {
  constructor(message: string, prop: string) {
    this.message = message
    this.prop = prop
  }
}

export default class Validation {

  constructor(object: Object) {
    this._object = object
    this._prop = null
    this._validator = validator
    this.promise = Promise.resolve()
  }

  custom(callback: Function, {required} = {}) {
    const prop = this._prop
    const value = this._object[prop]
    const object = this._object
    this.promise = this.promise.then(() => {
      if (required && !this._isEmptyString(value)) return
      callback(value, prop, object)
    })
    return this
  }

  _isEmptyString(value) {
    return !this._validator.toString(value).trim()
  }

  prop(prop: string) {
    this._prop = prop
    return this
  }

  required() {
    return this.custom((value, prop) => {
      throw new ValidationError(
        this.getRequiredMessage(prop, value),
        prop
      )
    }, {required: true})
  }

  getRequiredMessage(prop, value) {
    return `Please fill out '${prop}' field.`
  }

  email() {
    return this.custom((value, prop) => {
      if (this._validator.isEmail(value)) return
      throw new ValidationError(
        this.getEmailMessage(prop, value),
        prop
      )
    })
  }

  getEmailMessage() {
    return `Email address is not valid.`
  }

  simplePassword() {
    return this.custom((value, prop) => {
      const minLength = 5
      if (value.length >= minLength) return
      throw new ValidationError(
        this.getSimplePasswordMessage(minLength),
        prop
      )
    })
  }

  getSimplePasswordMessage(minLength) {
    return `Password must contain at least ${minLength} characters.`
  }

}


// Example
function validateForm(params) {
  return validate(params)
    .prop('email').required().email()
    .prop('password').required().simplePassword()
    .promise
}