WordPress Head Inline js

gf_inlinejs.php

<?php

add_action( 'wp_head', 'gf_trigger_picker' );

/**
 * Inject js code into the head, used with ACF
 * @hook wp_head
 */
function gf_trigger_picker() {
  
  // get the global post values
	global $post;
	$id = $post->ID;
	?>
	<script>
		jQuery(document).ready(function($) {
	<?php if ( get_field( 'loc_date', $id ) ): ?>

        // this function is used by the jQuery DatePicker API http://api.jqueryui.com/datepicker/
				availableDates = function(date) {

					// create the proper format for comparison (equal to the value of ACF custom field)
					var dmy = date.getFullYear() + '' + ('0' + (date.getMonth() + 1)).slice(-2) + '' + ('0' + date.getDate()).slice(-2);

					// check what date is available getting the value from the custom field
					if (dmy === '<?php echo get_field( 'loc_date', $id ); ?>') {
						return [true, "available", "Available"];
					} else {
						return [false, "", "unAvailable"];
					}
				};
        
        // inititate the datepicker
				$(".datepicker").datepicker(
						{
							// lets make sure that the dates are limited to the values of our ACF field
							beforeShowDay: availableDates
						}
				);
	<?php endif; ?>
		});
	</script>
	<?php
}