https://wpdocs.osdn.jp/%E3%83%86%E3%83%B3%E3%83%97%E3%83%AC%E3%83%BC%E3%83%88%E3%82%BF%E3%82%B0/wp_dropdown_categories#JavaScript_.E3.82.92.E7.94.A8.E3.81.84.E3.81.9F.E9.80.81.E4.BF.A1.E3.83.9C.E3.82.BF.E3.83.B3.E3.81.AA.E3.81.97.E3.83.89.E3.83.AD.E3.83.83.E3.83.97.E3.83.80.E3.82.A6.E3.83.B3.E3.83.A1.E3.83.8B.E3.83.A5.E3.83.BC_.282.29
<form id="category-select" class="category-select" action="<?php echo esc_url( home_url( '/' ) ); ?>" method="get">
<?php
$args = array(
'show_option_none' => __( 'Select category' ),
'show_count' => 1,
'orderby' => 'name',
'echo' => 0,
);
?>
<?php $select = wp_dropdown_categories( $args ); ?>
<?php $replace = "<select$1 onchange='return this.form.submit()'>"; ?>
<?php $select = preg_replace( '#<select([^>]*)>#', $replace, $select ); ?>
<?php echo $select; ?>
<noscript>
<input type="submit" value="View" />
</noscript>
</form>
