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Easily customizable library for validation scenarios in svelte components. Need to validate just a few simple inputs? Or need to build a huge form with dynamic steps? No problems. Svelidation was born for it!

NPM npm

Something doesn't work as expected? Any functionality is missing? Or, maybe, you have an idea/suggestion to make Svelidation better? Feel free to create an issue and describe your thoughts. Anything possible. Welcome!

Quick example

import createSvelidation from 'svelidation';

// 1. create an instance
const { createForm, createEntry } = createSvelidation();

// 2. create a validation model
const [ errorsStore, valueStore, useInput ] = createEntry({
  type: 'string',
  required: true
<!-- 3. use it in template  -->
<form use:createForm on:submit|preventDefault>
  <input type="text" use:useInput bind:value={$valueStore} />

  {#if $errorsStore.includes('required')}
    This field is required

  <button type="submit">Submit</button>

Check more examples on the demo page


npm i -S svelidation


What a dollar sign ($) means?

This is a specific Svelte reactive statements. Check here details

How can I use my own email regExp for validation?

Easy. Just update email type rule with ensureType API

import createValidation, { ensureType } from 'svelidation';
const { createEntry } = createValidation();

// extend existing type with custom rule
ensureType('email', {
  type: (value) => {
    return value.match(YOUR_EMAIL_REG_EXP);
How can I create simple email+password form without all that complicated stuff?
  const { createForm, createEntry } = createSvelidation();

  // create two validation entries
  const [ emailErrors, emailValue, emailInput ] = createEntry({
    type: 'email',
    required: true
  const [ passwordErrors, passwordValue, passwordInput ] = createEntry({
    type: 'text',
    required: true

  // create success handler
  const onSuccess = (values) => {
    // send values with api
<form use:createForm={{ onSuccess }} on:submit|preventDefault>
  <input type="email" use:emailInput bind:value={$emailValue} />
  {#if $emailErrors.length}Field is invalid{/if}
  <input type="password" use:passwordInput bind:value={$passwordValue} />
  {#if $passwordErrors.length}Field is invalid{/if}
  <button type="submit">submit</button>
How to activate validation just on form submit without any side-inputs-events?

The only thing you need to do - disable input events, because submit validation works by default

  const { createForm } = createSvelidation({ listenInputEvents: 0 });
  // ...
<form use:createForm>
  <!-- html -->
How to validate multi-steps form with different inputs on each step?

Nothing special to do here. Just create all inputs entries and bind it to inputs with use directive, and svelidation will validates only visible (created in DOM) inputs. Check DYNAMIC STEPS example

What if I don't have DOM? Can I validate pure data?

Yes. You have few options

  1. Use includeAllEntries option
const { validate, validateValueStore, createEntry } = createSvelidation({ includeAllEntries: true });
const [ errors1, value1 ] = createEntry(...);
const [ errors2, value2 ] = createEntry(...);

// validate all created entries
const allErrors = validate();

// or validate specific entry value
const firstEntryErrors = validateValueStore(value1);
  1. Use true as parameter in validate api
const { validate } = createSvelidation();

// ...create some entries

// validate all created entries
const allErrors = validate(true);
How can I clear errors manually?

Use clearErrors API

How do I know when validation fails?

There is onFail options here

  const { createForm } = createSvelidation();

  // ...

  // create fail handler
  const onFail = (errors) => {
<form use:createForm|preventDefault={{ onFail }}>

How to customize entry errors store?

Use useCustomErrorsStore option

Check CUSTOM ERRORS example

How can I get all entries values not just after success validation?

Use getValues API

How to hide warnings in the console?

Use warningsEnabled option

How can I create my own validation rule or custom type?

Use ensureRule / ensureType API

Do I have an option to avoid set required: true to every field?

Yes. Use presence: 'required' option

Is possible to avoid false positive validation of spaces-only-strings, but still to make users possible to use spaces at the start or end of their passwords (or another specific field)?

Yes. That is why trim option was created.

If you set trim: true, validator will trim all values before check. It will not change value itself, but trim it for check purpose.

Then, you can set trim: false for specific field, to avoid trim.

const { createEntry } = createSvelidation({ trim: true }); // trim all values before validation
const passwordEntry = createEntry({
  // ...options
  trim: false // don`t trim password entry before validation

And, of course, you are able to do an opposite. Leave trim options default (false), and set trim: truefor specific field

This lib can't do the things what I need

Not a problem! Create an issue and describe your needs. Will check this out

basic types

Combination of type/rules is using in here


check string length

  • { type: 'string', min: 3 }
  • { type: 'string', max: 3 }
  • { type: 'string', between: [4, 10] }


  • { type: 'email' }


check number value

  • { type: 'number', min: 3 }
  • { type: 'number', max: 3 }
  • { type: 'number', between: [4, 10] }


  • { type: 'boolean' }


check array length and specific element

  • { type: 'array', min: 3 }
  • { type: 'array', max: 3 }
  • { type: 'array', includes: 3 }

global rules


equality with value (in case of array it sorta and stringifies it), could take a function as equal value

  • { type: 'string', equal: 'my-custrom-string' }
  • { type: 'string', equal: value => (value === myFunction()) }


match textual value form by regExp

  • { type: 'number', match: '202\d' }


check value exists

  • { type: 'email', required: true }


// default values
  validateOnEvents: { change: true, input: false, blur: false },
  clearErrorsOnEvents: { reset: true, focus: false },
  listenInputEvents: 2,
  presence: 'optional',
  trim: false,
  includeAllEntries: false,
  useCustomErrorsStore: false
  warningsEnabled: true
  getValues: false
  • validateOnEvents: { [key: string]: boolean }

    • object of input events to validate input value
    • possible events: change, input, blur
  • clearErrorsOnEvents: { [key: string]: boolean }

    • object of events to clear errors
    • possible events: reset, focus
  • listenInputEvents: number

    • specific option for control input events
      • 0: dont allow input events
      • 1: allow input events always
      • 2: allow input events after first validation run
  • presence: string

    • Default inputs presence. If we set it to required - all inputs will be validated as required by default
    • This options equals optional by default, this means that all fields in validation are optional and will be validated in case of having value, or having another validation rules
  • trim: boolean

    • allow validator trim textual values before check
    • (!!!) it doesn't trim value itself, just for check purpose
  • includeAllEntries: boolean

    • By default validation works for entries that were assigned with inputs by use svelte directive. This option makes possible to validate ALL entries in validation
  • useCustomErrorsStore(errorsArray, entryParams): errorsStore

    • optional method to make possible to use custom errorsStore structure. By default this is array of strings. This method can override default store with any you want
    • errorsArray - default array of errors strings
    • entryParam - (check here)
    • check CUSTOM ERRORS example on demo page
  • warningsEnabled: boolean

    • option that makes warnings in dev mode visible. true by default
  • getValues(): values

    • optional method to build own values result
    • default getValues implementation returns Map structure Map{ [ || entryParams]: value }
    • example#1 { type: 'string' } getValues result will looks like this: Map{ { type: 'string' }: '' }
    • example#2 { type: 'string', id: 'login' } getValues result will looks like this: Map{ login: '' }
    • entries - array of entries params and values
    • result of getValues will passed to onSuccess option for form in createForm API (check here)
    • check two get values examples on the demo page

validateOnEvents, clearErrorsOnEvents, presence and trim behavior could be overrided by any input for itself in createEntry API (check here)

validation level API

import createSvelidation from 'svelidation';
const {
} = createSvelidation();


Create validation entry

const [ errorsStore, valueStore, inputFunctionForUse ] = createEntry(entryParams);

errorsStore, valueStore used for bind errors and value stores in templates. One more time. THIS IS SVELTE STORES. You might see something like errors[] below in this readme - this is just a array with strings

inputFunctionForUse function for use svelte directive on input, to assign its events to validation process

<input use:inputFunctionForUse />
<!-- or -->
<input use:inputFunctionForUse={{ clearErrorsOnEvents, validateOnEvents }} />

This is the place where clearErrorsOnEvents and validateOnEvents options could be overrided for specific input


Check list of types/rules here

// entryParams
  type, // required
  value, // initial value, required for some types of fields
  id, // optional param for getValues method

  // other rules, like min/max/required...

  trim, // works like trim option for createSvelidation function, but on input level
  optional // makes possible to override presence option of createSvelidation function

createEntries(entryParams[] | {[key: string]: entryParams})

Additional way to create a few entries at the time

const {
  first: [firstErrorsStore, firstValueStore, firstInput],
  second: [secondErrorsStore, secondValueStore, secondInput]
} = createEntries({
  first: {
    // entryParams
  second: {
    // entryParams
const [
  [firstErrorsStore, firstValueStore, firstInput],
  [secondErrorsStore, secondValueStore, secondInput]
] = createEntries([
    // entryParams
  }, {
    // entryParams


Function for form use

This function makes subscribe on submit/reset form events for validation/clearErrors by default

<form use:createForm></form>
<!-- or -->
<form use:createForm={{ onSubmit, onFail, onSuccess }}></form>

An object with callbacks could be used as param in use function

onSubmit(submitEvent, errors[]) - every form submit attempt. errors[] - array of all errors store values. Not array of stores, but array of store errors.

onFail(errors[]) - on every failed validation (when errors.length > 0)

onSuccess(values) - when there aren't any errors. Get values (check here) as result

clearErrors(includeNoFormElements: boolean)

Manually clear all errors stores

clearErrors(includeNoFormElements = false);

Only argument same as in validate

validate(includeNoFormElements: boolean): allValidationErrors[]

Manually validate stores

const allErrors = validate(includeNoFormElements = false);

Only argument makes possible to validate all created entries.

Without this param validation will check inputs assigned to nodes only (inputFunctionForUse - check createEntry API)

Return array of all errors store values

validateValueStore(valueStore): errors[]

Manually validate value store

const [ emailErrorsStore, emailValueStore ] = createEntry({ type: 'email' });
const errors = validateValueStore(emailValueStore); // array of string, not errorsStores!!!

Returns errors store value

getValues(entries): values

check here

advanced API

import {
} from 'svelidation';

ensureRule(ruleName: string, ruleFunction)

  • ruleFunction: (value, entryParams): boolean Allows to add your own global (will be available for any type) rules
import createValidation, { ensureRule } from 'svelidation';
const { createEntry, validate } = createValidation();

const myRuleFunction = (value, entryParams) => {
  // entryParams === { type: 'string'... }
  // entryParams.myRule === 'my rule value...'
  return entryParams.type === 'string';

ensureRule('myRule', myRuleFunction);

const [ stringErrors, stringValue ] = createEntry({
  type: 'string',
  myRule: 'my rule value for string'

const [ numberErrors, numberValue ] = createEntry({
  type: 'number',
  myRule: 'my rule value for email'

console.log(validate(true)); // [{ number: ['myRule'] }]

resetRule(ruleName?: string)

Remove custom rule. If calls without ruleName - it will remove all custom global rules

import createValidation, { ensureRule, resetRule } from 'svelidation';
const { createEntry, validate } = createValidation();

ensureRule('myRule', (value, { myRule, type }) => {
  // myRule === 'my rule value...'
  return type === 'string';

const [ numberErrors, numberValue ] = createEntry({
  type: 'number',
  myRule: 'my rule value for email'

console.log(validate(true)); // [{ number: ['myRule'] }]
console.log(validate(true)); // [] - there in no rule like myRule, so no validation, no errors

ensureType(typeName: string, typeRules)

  • typeRules: { [key: string]: ruleFunction }
    • ruleFunction: (value, entryParams): boolean

Extend existing/add new type to validator. In case of creating new type, there is one clause. typeRules has to have method named type. This is a basic and required method for every type and it calls everytime we validate type. And if it return false - current entry validation stops with type error

import createValidation, { ensureType } from 'svelidation';
const { createEntry, validate, validateValueStore } = createValidation();

// extend existing type with custom rule
ensureType('string', {
  between5and10: (value) => {
    return value.length >= 5 && value.length <= 10;

const [ stringErrors, stringValue ] = createEntry({
  type: 'string',
  between5and10: true,
  value: 'asdf'

console.log(validateValueStore(stringValue)); // ['between5and10']

// create new type with its own rules
ensureType('myNewType', {
  type: (value) => {
    return value === 'custom!!!';
  anotherRule: (value, { anotherRule }) => {
    return false;
  min: 'string.min' // yes, we can take some existing rules from another types

const [ myErrors, myValue ] = createEntry({
  type: 'myNewType',
  anotherRule: 'something',
  min: 10,
  value: 'custom!!!'

console.log(validateValueStore(myValue)); // ['anotherRule', 'min']

resetType(typeName?: string)

Work same as resetRule, but on the type level

omg :) I hope noone will need to use methods below

Ok, so what are you saying? Spies? O_o

Yes. Spies. Spy.

By design, this is a function to observe validation process and get into it.

Spy can observe types, global rules, specific rule of specific type. Spy can observe everything! Literally. Just create a global spy and it will observe every validation. Every rule, every type, everything will be in its hands.

If you need to prevent something, or update value somehow - special spy is the place.

You need to know just one thing. Spy has god power, but it has to give this power to the next spy. And there will always be next spy, even if its not your spy.

So, lets dive into examples

addSpy(spyFunction, spyParams?): removeSpyFunction

Method to add spy in the validation process

  • spyFunction(value, entryParams, next, abort): errors[]
    • next(nextValue, mixedParams?: entryParamsLike)
    • abort()
  • spyParams?: { ruleName?: string, type?: string }
import createValidation, { addSpy } from 'svelidation';
const { createEntry, validateValueStore } = createValidation();

const [ stringErrors, stringValue ] = createEntry({
  type: 'string',
  value: '...',
  min: 5,
  max: 10,

const removeSpy = addSpy((value, params, next, abort) => {
}, { type: 'string', ruleName: 'min' });

// LOG: >> 'spying!'

First of all, take a look at the line with next(value) call. This is VERY IMPORTANT LINE in spies paradigm. Remember? Spy has god power, but it has to give this power to next spy. This.

next(value, params) - is the function witch spy have to call with any value spy wants. This new value will be taken by next validation rules or another spies (until type validation ends). Almost same with the params. But params will be merged with original for next validators. So the value will replace the original one, params will be merged

And it can returns errors, that will be merged with validation errors

import createValidation, { addSpy, ensureType } from 'svelidation';
const { createEntry, validateValueStore } = createValidation();

const [ stringErrors, stringValue ] = createEntry({
  type: 'string',
  value: '...',
  myRule: 'hello'

const removeSpy = addSpy((value, params, next, abort) => {
  next(value, { somethingFromSpy: 'world' });
  return ['error-from-spy'];
}, { type: 'string', ruleName: 'myRule' });

ensureType('string', {
  myRule: (value, { myRule, somethingFromSpy }) => {
    console.log(myRule, somethingFromSpy); // 'hello', 'world'
    return false;

// ["error-from-spy", "myRule"]
// first is from spy, second from our rule, that always return false

What happens if we will not call next? Validation process will stop and return existing errors for the moment.

abort() method - its an emergency brake. If it calls - validation stop and return nothing.

And last, but not least spyParams?: { ruleName?: string, type?: string }. This is an optional object to describe spy's field of responsibilty.

  • { ruleName: 'min', type: 'string' } - spy for specific rule in specific type. Will be called each time, when current pair rule-type check
  • { ruleName: 'match' } - spy for specific rule. Will be called with each ruleName check
  • { type: 'array' } - spy for EVERY validation of specific type. Will be called once per type.
  • undefined - spy for everything. Will be called once before every validation.

So, for example if you create spies:

addSpy(() => {}); // 1
addSpy(() => {}, { type: 'string' }); // 2
addSpy(() => {}, { type: 'string', ruleName: 'min' }); // 3
addSpy(() => {}, { ruleName: 'min' }); // 4

// run validation for a { type: 'string', min: 5 }
// each spy will be called once

// run validation for a { type: 'string', min: 5 }, { type: 'string', max: 5 }
// 3 and 4 will be called once.
// 1 and 2 - twice, because we validate 2 entries (1st spy), and 2 strings (2nd spy)

// run validation for a { type: 'string', min: 5 }, { type: 'email', required: true }
// 1 - twice, because we validate 2 entries
// 2, 3 and 4 will be called once

Last thing about the spies.

If you create a spy that will observe type method of any type - remember, that returning an error from that spy will stop current entry validation because type method fails, so there is no point to continue validation

To remove your spy - just call removeSpyFunction that returns addSpy method.

And to remove all spies...

removeSpies(params?: { type?: string, ruleName?: string })

This is an easy one after addSpy :). It just removes all spies depends on params.

  • {type: 'string', ruleName: 'min'} - remove all spies for specific rule in specific type
  • {runeName: 'match'} - remove all spies for specific global rule
  • {type: 'string'} - remove all spies for specific type
  • undefined - remove all spies at all


  • npm run build - build demo and library files into dist
  • npm run dev - run dev server from dist folder with demo page by default
  • npm run test - run all tests on production version of build
  • npm run e2e - e2e testing of lib file from dist
  • npm run e2e:dev - dev server from e2e/dist folder with tests name in params
  • npm run unit - unit testing of spec.js files in lib
  • npm run unit:dev - dev server for unit testing
  • npm run eslint - lint source code


  • demo examples
  • complete readme
  • unit tests
  • e2e tests
  • builder demo
  • eslint
  • add comments in the code