pewc_set_main_qty_max_from_child_stock.php

<?php
/**
 * Ensure user can't order more parent products than child product stock allows
 * Only applies to child product with 'Linked' quantities
 */
<?php
/**
 * Ensure user can't order more parent products than child product stock allows
 * Only applies to child product with 'Linked' quantities
 */
function demo_set_main_qty_max_from_child_stock() {
	?>
	<script>
	( function( $ ) {
		function pewc_update_main_qty_max_from_child_stock() {
			var lowestStock = null;

			$( '.products-quantities-linked' ).each( function() {

				// Checked inputs (checkbox/radio) with data-stock
				$( this ).find( 'input[data-stock]:checked' ).each( function() {
					var stock = parseInt( $( this ).attr( 'data-stock' ), 10 );
					if ( ! isNaN( stock ) && stock > 0 ) {
						lowestStock = ( lowestStock === null ) ? stock : Math.min( lowestStock, stock );
					}
				} );

				// Selected option with data-stock inside a select
				$( this ).find( 'select' ).each( function() {
					var $selected = $( this ).find( 'option:selected[data-stock]' );
					if ( $selected.length ) {
						var stock = parseInt( $selected.attr( 'data-stock' ), 10 );
						if ( ! isNaN( stock ) && stock > 0 ) {
							lowestStock = ( lowestStock === null ) ? stock : Math.min( lowestStock, stock );
						}
					}
				} );

			} );

			var $qtyField = $( 'form.cart .quantity input[type=number]' );

			if ( lowestStock !== null ) {
				$qtyField.attr( 'max', lowestStock );
				if ( parseInt( $qtyField.val(), 10 ) > lowestStock ) {
					$qtyField.val( lowestStock );
				}
			} else {
				$qtyField.removeAttr( 'max' );
			}
		}

		$( document ).ready( pewc_update_main_qty_max_from_child_stock );

		$( 'body' ).on(
			'change click',
			'.products-quantities-linked input[data-stock], .products-quantities-linked select',
			pewc_update_main_qty_max_from_child_stock
		);
	} )( jQuery );
	</script>
	<?php
}
add_action( 'wp_footer', 'demo_set_main_qty_max_from_child_stock' );