register-list-of-strings-for-mutation-input-and-querying.php

// Add Hobbies to GraphQl
add_action('graphql_init', function () {
	$hobbies = [
		// 'type' => 'String',
		'type'        => ['list_of' => 'String'],
		'description' => __('Custom field for user mutations', 'your-textdomain'),
		'resolve'     => function ($post) {
			$hobbies = get_post_meta($post->ID, 'hobbies', true);
			return !empty($hobbies) ? $hobbies : '';
		},
	];

	register_graphql_field('Post', 'hobbies', $hobbies);
	register_graphql_field('CreatePostInput', 'hobbies', $hobbies);
	register_graphql_field('UpdatePostInput', 'hobbies', $hobbies);
});

// Update Post Meta
add_action( 'graphql_post_object_mutation_update_additional_data', 'graphql_register_edit_mutation', 10, 5 );

function graphql_register_edit_mutation( $post_id, $input, $mutation_name, $context, $info ) {

	$sanitized = ! empty( $input['hobbies'] ) ? array_map( static function( $hobby ) {
		return sanitize_text_field( $hobby );
	}, $input['hobbies'] ) : [];

	if ( ! empty( $sanitized ) ) {
		update_post_meta( $post_id, 'hobbies', $sanitized );
	}

	return [ 'value' => $input['hobbies'] ];
}