woocommerce/assets/js/frontend/woocommerce.js

jQuery( function( $ ) {

	// Orderby
	$( '.woocommerce-ordering' ).on( 'change', 'select.orderby', function() {
		$( this ).closest( 'form' ).submit();
	});

	// Quantity buttons
	$( 'div.quantity:not(.buttons_added), td.quantity:not(.buttons_added)' ).addClass( 'buttons_added' ).append( '<input type="button" value="+" class="plus" />' ).prepend( '<input type="button" value="-" class="minus" />' );

	// Target quantity inputs on product pages
	$( 'input.qty:not(.product-quantity input.qty)' ).each( function() {
		var min = parseFloat( $( this ).attr( 'min' ) );

		if ( min && min > 0 && parseFloat( $( this ).val() ) < min ) {
			$( this ).val( min );
		}
	});

	$( document ).on( 'click', '.plus, .minus', function() {

		// Get values
		var $qty		= $( this ).closest( '.quantity' ).find( '.qty' ),
			currentVal	= parseFloat( $qty.val() ),
			max			= parseFloat( $qty.attr( 'max' ) ),
			min			= parseFloat( $qty.attr( 'min' ) ),
			step		= $qty.attr( 'step' );

		// Format values
		if ( ! currentVal || currentVal === '' || currentVal === 'NaN' ) currentVal = 0;
		if ( max === '' || max === 'NaN' ) max = '';
		if ( min === '' || min === 'NaN' ) min = 0;
		if ( step === 'any' || step === '' || step === undefined || parseFloat( step ) === 'NaN' ) step = 1;

		// Change the value
		if ( $( this ).is( '.plus' ) ) {

			if ( max && ( max == currentVal || currentVal > max ) ) {
				$qty.val( max );
			} else {
				$qty.val( currentVal + parseFloat( step ) );
			}

		} else {

			if ( min && ( min == currentVal || currentVal < min ) ) {
				$qty.val( min );
			} else if ( currentVal > 0 ) {
				$qty.val( currentVal - parseFloat( step ) );
			}

		}

		// Trigger change event
		$qty.trigger( 'change' );
	});
});