The setFormValue() method of the ElementInternals interface sets the element's submission value and state, communicating these to the user agent.

value

A File, a string, or a FormData as the value to be submitted to the server.

state Optional

A File, a string, or a FormData representing the input made by the user. This allows the application to re-display the information that the user submitted, in the form that they submitted it, if required.

Undefined.

NotSupportedError DOMException

Thrown if the element does not have its formAssociated property set to true.

In the following example, a checkbox custom element sets on as the value to send to the server, and checked as the state.

this.internals_.setFormValue("on", "checked");