Declarative validation library for javascript objects.
Last updated 9 months ago by dprovodnikov .
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Declarative object validation

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$ npm install @dprovodnikov/validation

Usage example

Here's a typical login form data structure:

const form = {
  login: '',
  password: '',
  repeatPassword: '',

Rules declaration

Let's say we have a set of constraints to apply to the fields of this form:

  • Login cannot be empty
  • Password should contain at least one uppercase letter
  • Password should be at least 7 chars long
  • Password should match repeatPassword

These are common rules pretty much every login form uses. Now we need to declare these rules.

import { notEmpty, hasUpperLetter, longerThen, matches } from '@dprovodnikov/validation';

const rules = {
  login: [notEmpty('Login cannot be empty')],
  password: [
    hasUpperLetter('Password should contain at least one uppercase letter'),
    longerThen(7, 'Password should be at least 7 chars long'),
  repeatPassword: [
    matches('password', 'Password should match repeatPassword'),

The library provides a couple of basic rules that are fairly common:

Rule Description
notEmpty Makes sure the given value is not empty
hasUpperLetter Makes sure the given value contains an uppercase letter
longerThen Makes sure the given value has a length of more than specified
matches Makes sure the given value matches the specified field of the form
emailFormat Makes sure the given value matches the regexp (integrated in the lib)
urlFormat Makes sure the given value matches the regexp (integrated in the lib)
shorterThan Makes sure the given value is shorter than the specified value

If you need any other rule you can easily implement one using simple api. Here's what a rule looks like:

const rule = message => (value, form) => {
  // You need to return a string.
  // This string represents the validation error message.
  // If the output is empty - the value satisfies the rule and therefore is considered valid.
  let output = '';

  // here you have access to the value this rule was assigned to.
  // the entire form object is also accessible from here for more complex and flexible validation logic

  return output;

Here's the implementation of the matches rule that is provided by the library:

export const matches = (comparisonField, message) => (value, fields) => {
  if (value !== fields[comparisonField]) {
    return message;

  return '';

As simple as that.

Rules application

In order to create a validator you need to apply the rules you declared earlier.

import { applyRules } from '@dprovodnikov/validation';

const rules = {
  field: [notEmpty('Field should not be empty')],

const validate = applyRules(rules);

const form = {
  field: '',

validate('all', form); // { field: 'Field should not be empty' }
validate('field', form); // 'Field should not be empty'

The output of the validator is a structure where keys are field names and values are error messages. If a field has multiple rules applied to it and more than one of them fail - the output will contain the first one that failed.

There is a tool to easily check if the report contains any error messages:

import { hasError } from '@dprovodnikov/validation';

const report = validate(form);

if (hasError(report)) {

There's also a method to extract the first error in the output;

import { hasError, getFirstError } from '@dprovodnikov/validation';

const report = validate(form);

if (hasError(report)) {
  const errorMessage = getFirstError(report);

The list of methods:

Method Input Output Description
applyRules rules validator function Takes in rules and returns a validator
hasError validation report boolean Takes in a validation report and returns wheather it contains an error on not
getFirstError validation report string Takes in a validation report and returns the first error


Make sure tests pass

$ npm run test

Install dependencies

$ npm install

Build from sources

$ npm run build

The output will appear in the dist folder in the project's root.



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