Simple object validation code. Loosely based on https://medium.com/javascript-inside/form-validation-as-a-higher-order-component-pt-1-83ac8fd6c1f0#.6yiftk939, writen in a more easy to understand manner

validation.js

const getErrors = (inputData, validationRules, globalOptions) => {
  const errors = {}

  const addError = (key, errorMessage) => {
    if(!(key in errors)) errors[key] = []

    errors[key].push(errorMessage)
  }

  Object.keys(inputData).forEach((key) => {
    // skip if there is no rule for the key
    if(!(key in validationRules)) return;

    const value = inputData[key]

    validationRules[key].forEach((rule) => {
      const [ validator, message, validatorOptions ] = Array.isArray(rule)
        ? rule // Array syntax
        : [rule.validator, rule.message, rule.options] // Object syntax

      // same props are used for the validator and message functions
      const props = [
        value,
        key,
        inputData,
        validationRules,
        validatorOptions,
        globalOptions
      ]

      const valid = validator(...props)

      if(!valid){
        const errorMessage = (typeof message === 'function')
          ? message(...props) // String maker
          : message // String

        addError(key, errorMessage)
      }
    })

  })

  return errors
}



// USAGE

const isGreaterThan = (size) => (value) => value > size
const isGreaterOrEqualsThan = (size) => (value) => value >= size
const hasLengthGreaterThan = (size) => (value) => isGreaterThan(size)(value.length)
const passwordMatches = (passwordInputKey) => (value, _, inputData) => value === inputData[passwordInputKey]
const hasCapitalLetter = (value) => /[A-Z]/.test(value)

const validationRules = {
  name: [
    [hasLengthGreaterThan(6), "Minimum length of 6 is required."],
    [hasCapitalLetter, "Should contain at least one uppercase letter."],
  ],
  age: [
    [isGreaterOrEqualsThan(18), "Minimum age required is 18."]
  ],
  password: [
    [hasLengthGreaterThan(6), "Minimum length of 6 is required."],
  ],
  passwordRepeat: [
    // Alternative rule syntax as an object
    {
      validator: passwordMatches('password'),
      // Message can also be a function
      message: (value, key, inputData, validationRules, validatorOptions) =>
        `Must be equals to field ${validatorOptions.fieldName}`,
      // You can pass options which will be passed as the
      // 5th argument for validator and message function
      options: {fieldName: "Password"}
    }
  ]
}

const inputData = {
  name: "phil",
  age: 17,
  password: '12345678',
  passwordRepeat: '1234567'
}

// You can pass an global options object as the 3rd
// argument which will be passed as the 6th argument
// for validator and message function
const errors = getErrors(inputData, validationRules)

console.log(errors)
// { 
//  name: [ 'Minimum length of 6 is required.', 'Should contain at least one uppercase letter.' ],
//  age: [ 'Minimum age required is 18.' ],
//  passwordRepeat: [ 'Must be equals to field Password' ]
// }