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If your custom element is missing from new FormData(form), make it a form-associated custom element and explicitly provide its submission value. Set static formAssociated = true, call attachInternals() once, give the element a name, and call setFormValue() whenever its value changes. Without that wiring, an autonomous custom element is not a native form control just because it sits inside a form.
Why your custom element is missing from FormData
Native form submission collects values from associated form controls. An ordinary autonomous custom element does not automatically become one: its visible UI or JavaScript state is not enough. The HTML Standard provides a form-associated custom element mechanism through ElementInternals. To contribute a named entry, the component must opt in, expose its internals, have a name, and set a form value. See the WHATWG HTML Standard.
Make the component submit a value
This minimal example wires a string value into native form submission. It demonstrates the essential API setup, not a complete production control: the component still needs to connect its UI to the value and decide how to handle disabled state, reset, and validation.
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class XRating extends HTMLElement {
static formAssociated = true;
#internals;
#value = "";
constructor() {
super();
this.#internals = this.attachInternals();
}
set value(value) {
this.#value = String(value);
this.#internals.setFormValue(this.#value);
}
get value() {
return this.#value;
}
}
customElements.define("x-rating", XRating);<form id="profile">
<x-rating name="rating"></x-rating>
</form>In this example, static formAssociated = true opts the autonomous element into the browser’s form-control model. The constructor calls attachInternals() once and retains the returned ElementInternals object. The name attribute supplies the submitted key, and setFormValue() supplies the value. The setter is where a real component should synchronize its current UI state with the form API.
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Check the common causes
- Confirm form association is enabled. The class must declare
static formAssociated = truebefore the custom element is registered. - Attach internals once. Call
this.attachInternals()in the constructor and keep the result. Calling it a second time for the same element raisesNotSupportedError; see MDN’sattachInternals()reference. - Give the element a name. The
nameattribute is the key used for submission and in the form’s elements collection. - Set the current submission value. Call
internals.setFormValue(value)when component state changes. The value can be a string, aFile, or aFormDataobject. Passingnullmeans the element does not participate in submission. - Check which form owns it.
ElementInternals.formexposes the form owner. Association can happen through normal containment or the element’sformattribute.
Inspect the form’s current entries
Use FormData to examine what the browser currently collects. For example, new FormData(form) constructs form data from associated controls and their names and values. Inspect entries() or use getAll("rating") to look for the named value:
const data = new FormData(document.querySelector("#profile"));
console.log([...data.entries()]);
console.log(data.getAll("rating"));If the value is absent, check the element’s form association, name, and the most recent value passed to setFormValue(). If the component intentionally passes null, its omission is expected.
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Decide which native control behaviors to implement
Form association connects a custom element to more than submission. The HTML Standard defines related behavior for form ownership, labels, disabled state, reset, and constraint validation. Implement the parts that match the component’s intended behavior rather than assuming its UI will acquire them automatically.
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- Validation: Use
setValidity()to report constraint state, and usecheckValidity()orreportValidity()when appropriate. An empty flags object indicates that the element satisfies its constraints. - Reset and state restoration: Make the component’s UI respond to the form lifecycle and any restored state, keeping its displayed value and submitted value in sync. The optional second argument to
setFormValue(value, state)can preserve state separately from the submitted value. - Labels and form ownership: Use the association behavior when the component needs to work with labels or be connected to a form outside its normal containment.
Check support for your target browsers
MDN marks attachInternals() as Baseline and widely available, with cross-browser availability since March 2023. That status is a general signal, not a guarantee that every method or behavior your component needs works in every browser version you support. Verify the relevant APIs against your application’s browser matrix. See MDN’s browser compatibility information.
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