All Shoelace components make use of a [shadow DOM](https://developer.mozilla.org/en-US/docs/Web/Web_Components/Using_shadow_DOM) to encapsulate markup, styles, and behavior. One caveat of this approach is that native `<form>` elements will not recognize Shoelace form controls.
This component solves that problem by serializing _both_ Shoelace form controls and native form controls when the form is submitted. The resulting form data is exposed in the `sl-submit` event as a [`FormData`](https://developer.mozilla.org/en-US/docs/Web/API/FormData) object in `event.detail.formData`. You can also find an array of form controls in `event.detail.formControls`.
Shoelace forms don't make use of `action` and `method` attributes and they don't submit the same way as native forms. To handle submission, you need to listen for the `sl-submit` event as shown in the example below and make an XHR request with the resulting form data.
On submit, a [`FormData`](https://developer.mozilla.org/en-US/docs/Web/API/FormData) object will be attached to `event.detail.formData`. You can use this along with [`fetch()`](https://developer.mozilla.org/en-US/docs/Web/API/fetch) to pass data to the server.
It's sometimes useful to have form values in a plain object or a JSON string. You can convert the submitted `FormData` object to JSON by iterating and placing the name/value pairs in an object.
Client-side validation can be enabled through the browser's [Constraint Validation API](https://developer.mozilla.org/en-US/docs/Web/Guide/HTML/HTML5/Constraint_validation) for many form controls. You can enable it using props such as `required`, `pattern`, `minlength`, and `maxlength`. As the user interacts with the form control, the `invalid` attribute will reflect its validity based on its current value and the constraints that have been defined.
When a form control is invalid, the containing form will not be submitted. Instead, the browser will show the user a relevant error message. If you don't want to use client-side validation, you can suppress this behavior by adding `novalidate` to the `<sl-form>` element.
All form controls support validation, but not all validation props are available for every component. Refer to a component's documentation to see which validation props it supports.
!> Client-side validation can be used to improve the UX of forms, but it is not a replacement for server-side validation. **You should always validate and sanitize user input on the server!**
### Required Fields
To make a field required, use the `required` prop. The form will not be submitted if a required form control is empty.
To restrict a value to a specific [pattern](https://developer.mozilla.org/en-US/docs/Web/HTML/Attributes/pattern), use the `pattern` attribute. This example only allows the letters A-Z, so the form will not submit if a number or symbol is entered. This only works with `<sl-input>` elements.
To create a custom validation error, use the `setCustomValidity` method. The form will not be submitted when this method is called with anything other than an empty string, and its message will be shown by the browser as the validation error. To make the input valid again, call the method a second time with an empty string as the argument.
The `invalid` attribute reflects the form control's validity, so you can style invalid fields using the `[invalid]` selector. The example below demonstrates how you can give erroneous fields a different appearance. Type something other than "shoelace" to demonstrate this.
To opt out of the browser's built-in validation and use your own, add the `novalidate` attribute to the form. This will ignore all constraints and prevent the browser from showing its own warnings when form controls are invalid.
Remember that the `invalid` attribute on form controls reflects validity as defined by the [Constraint Validation API](https://developer.mozilla.org/en-US/docs/Web/Guide/HTML/HTML5/Constraint_validation). You can set it initially, but the `invalid` attribute will update as the user interacts with the form control. As such, you should not rely on it to set invalid styles using a custom validation library.
