simple-functional-validation.js

implementation-example.js

const isEven = n => (n % 2 === 0)
const gte = min => n => n >= min
const isNil = x => x === undefined || x === null

const validatorA = {
  pred: isEven,
  message: n => `"${n}" should be an even number.`,
  accessor: obj => obj.num1,
}

const validatorB = {
  pred: gte(20),
  message: n => `"${n}" should be greater than or equal to 20`,
  accessor: obj => obj.num2,
}

console.log(makeValidate([validatorA, validatorB])({value: {name: 'Jeff', num1: 43, num2: 15}}))
// {
//   value: { name: 'Jeff', num1: 43, num2: 15 },
//   messages: [
//     '"43" should be an even number.',
//     '"15" should be greater than or equal to 20',
//   ]
// }

const validatorC = {
  pred: a => !isNil(a),
  message: _ => 'Please provide a value.',
}

console.log(makeValidate([validatorC])({value: undefined}))
// { value: undefined, messages: [ 'Please provide a value.' ] }

validate.js

const makeValidate = validators =>
  ({value, messages = []}) =>
    validators.reduce((result, validator) => {
      const {pred, accessor = x => x, message} = validator
      return pred(accessor(value))
        ? result
        : {value, messages: result.messages.concat(message(accessor(value)))}
    }, {value, messages})