You may need to transform your data in between it being set in your inputs and it being set in the data store. For example for handling numeric values on text inputs or transforming a value to a Date instance. Felte allows you to do this by using the transform property in prepareForm's configuration object (or the element's property). It accepts either a function or an array of functions. These functions will receive the values of the form before they're assigned to the data store, and returns a new object that will be assigned to the store. This can even allow you to change your values shape in Felte.

const felteForm = document.querySelector('felte-form');
felteForm.configuration = {
  transform: (values) => ({
    shouldBeNumber: parseInt(values.shouldBeNumber, 10),

These transformation functions run also when directly setting values to the data store (e.g. via bind:value or assigning to the data store directly).

NOTE: Transformations must be synchronous.


TypeScript users should keep in mind that when using a transform function, the types of Felte become looser. setFields and setData will accept any value expecting you to transform your data into the appropriate shape in your transform function. This also means that initialValues will have a type of unknown and the type of your data can no longer be inferred from it.