F-Validator

Functional JSON validator

Features

• Minimal, ~200 LoC, no dependencies.
• Simple, validators are just functions, no special DSL to learn
• Powerful, compose basic/custom validators to build complex ones

Installation

npm install --save f-validator

Get started

  const { string, number, optional, or, objectOf, arrayOf, error } = require('f-validator')
  const equal = require('assert').deepStrictEqual


  // Every validator is just a function
  const validator1 = string
  equal(validator1('I am a string'), null) // passed check -> null


  // Some validator can use other validators
  // e.g. `or`, `optional`, `objectOf`, `arrayOf`
  const validator2 = or(string, number)


  // To validate object, `objectOf` takes a schema where each field is a validator
  const validator3 = objectOf({
    a: or(string, number),
    b: objectOf({
      c: optional(string)
    })
  })


  // To validate array, `arrayOf` takes a validator which will be used to check every element
  const validator4 = arrayOf(validator3)

  const good = {
    a: 5,
    b: {
      c: 'I am also a string'
    }
  }

  const bad = {
    a: 5,
    b: {
      c: 42
    }
  }

  equal(validator4([good, good]), null)
  equal(validator4([good, bad]), { // an object describing the diff
    path: [1, 'b', 'c'], // key-path pointing to the exact field to blame
    expected: 'or(null or undefined, string)',
    received: 42,
    message: 'Path:\'1.b.c\', Expected: or(null or undefined, string), Received: \'42\''
  })


  // Sometimes, you'll need your own validator
  // any function can be used as a 'Validator' if it has this signature:

  /**
   * @param any subject, subject to check
   * @param []string path, a key-path pointing to the field
   * @returns
   *   null, if it passes the check
   *   Object error, an error object created with `error` utility
   */

  // Example 1: a humble even number validator
  const evenNumber = (subject, path = []) => {
    if (typeof subject == 'number' && (subject % 2 === 0)) {
      return null
    } else {
      return error(path, 'an even number', subject) // keypath, expected, received
    }
  }

  // now it can be used by other validator
  const arrayOfEvenNumber = arrayOf(evenNumber)
  equal(arrayOfEvenNumber([2, 4, 6, 9]), {
    path: [3], // the 4th element
    expected: 'an even number',
    received: 9,
    message: 'Path:\'3\', Expected: an even number, Received: \'9\''
  })

  // Example 2:  a stringified-json-object-conforming-to-a-given-validator validator
  const jsonString = validator =>
    (subject, path = []) => {
      let parsed
      try {
        parsed = JSON.parse(subject)
      } catch (e) {
        return error(path, `json string of (${validator.name})`, subject)
      }
      return validator(parsed, path) // note: `path` should be passed to it
    }

API

1. Error constructor:

error(path:[]string, expected:string, received:any, message:?string ) - create an error object describing the diff, use it when creating your own validator

2. Basic type validators (most are tested using Object.prototype.toString.call(value))

object

array

string

number

boolean

date

regexp

Null - note the initial

Undefined - note the initial

empty - valid if value is null or undefined

regex(re:regexp) - valid if value matches regex re

3. Composite type validators:

objectOf(schema:object) - valid if value is an object where for every key of schema, value[key] is validated by schema[key]

arrayOf(v:Validator) - valid if value is an array where every element is validated by v

4. Logical validators:

not(v:validator) - valid if value is invalidated by v

any - always valid

and(...vs:[]validator) - valid if value is validated by every validator of vs

or(...vs:[]validator) - valid if value is validated by every validator of vs

optional(v:validator) - valid if value is either validated by v or null/undefined

is(ref:any) - valid if value is strictly deeply equal to ref

oneOf(...refs:[]any) - valid if value is strictly deeply equal to any element of refs

5. Advanced validator:

like(ref:any) - valid if value has the same type/structure as ref (recursively)

Example:

const reference = {
  a: 42,
  b: 'I am string',
  c: {
    d: ['apple']
  },
  e: optional(string)
}

const value = {
  a: 0,
  b: 'I am another string',
  c: {
    d: ['pear', 'microsoft']
  },
  e: 'yep',  // functions in ref are taken as validators
  extra: 'I am an extra field, I will be fine'
}

like(reference)(value) // null (is valid)