The createForm function can also accept a validation function. It's a function that accepts the current form values and needs to return an object with the same shape, but instead of the property values it can have a string or array of strings containing the validation messages.

const { form } = createForm({
  // ...
  validate: (values) => {
    const errors = {}
    if (! || !/^[^@ \t\r\n]+@[^@ \t\r\n]+\.[^@ \t\r\n]+/.test( { = 'Must be a valid email';
    if (!values.password) errors.password = [
      'Must not be empty',
      'Must be over 8 characters',
    if (values.password && values.password.length < 8) {
      errors.password = ['Must be over 8 characters'];
    return errors;
  // ...

It shouldn't be a hard task to use a third party library, as long as you transform their result into something that Felte understands.

Note that the validate function can be asynchronous.

Felte will validate whichever field it considers as touched as you fill the form, and it will validate all fields (and set them as touched) when submitting it.

Server errors

You can add an onError function to the createForm configuration that will be called if the onSubmit function throws an error. It will be called with the value thrown from the onSubmit function. You can use the onError function to shape your server errors into the same shape that the validate function expects and return them from the same function for them to be handled.

const { form } = createForm({
  // ...
  // Assuming your server already returns them with the appropriate shape.
  // Might not be the case for most cases.
  onError: (errors) => errors,
  // ...

Error handling

createForm returns also its errors and touched fields as stores.

  import { createForm } from 'felte';

  const { form, errors, touched } = createForm({ \*...*\ });

  <!-- Prettify the errors and display them in HTML -->
  {JSON.stringify($errors, null, 2)}

  {JSON.stringify($touched, null, 2)}

You can read more above them in the stores section.

You don't need to manually handle this errors. Felte provides four official packages to handle your errors for you, either using Tippy, directly mutating the DOM, providing a Svelte component or using the browser's built-in constraint validation API. You can read more about this in the reporters section.

Multiple validations

The validate property of the configuration object can also be an array of validation functions. The resulting errors from running each validation function will be merged into a single object. If a single property was assigned two or more errors from the validation functions, the property will be an array of strings. This might not be useful for common scenarios, but it will come useful when using any of our validator packages.