JS - Link inputs, selects, radios, and checkboxes.

link-inputs.js

$( '[data-linked]' ).on( 'change keyup', function() {
	let $this = $( this ),
		name = $this.data( 'linked' ),
		type = $this.attr( 'type' ),
		val = $this.val(),
		checkboxOrRadio = ( 'radio' === type || 'checkbox' === type ),
		$linked = $( `[name="${name}"]` );
	if ( checkboxOrRadio ) {
		$linked = $( `[name="${name}"][value="${val}"]` );
		if ( $this.prop( 'checked' ) ) {
			$linked.prop( 'checked', true );
		} else {
			$linked.prop( 'checked', false );
		}
	} else {
		$linked.val( val );
	}
});