register-rest-field.php

<?php

const FIELD_NAME = 'field_name';

add_action( 'rest_api_init', __NAMESPACE__ . '\\register_api_fields' );

function register_api_fields() {
 register_rest_field(
		'post',
		FIELD_NAME,
		[
			'get_callback'    => function ( $object ) {
				$post_id = $object['id'] ?? 0;
				if ( ! current_user_can( 'edit_post', $post_id ) ) {
					return '';
				}

				return 'something';
			},
			'update_callback' => function ( $value, $object, $field_name ) {
				return true;
			},
			'schema'          => array(
          'type'        => 'string',
          'arg_options' => array(
              'sanitize_callback' => function ( $value ) {
                  // Make the value safe for storage.
                  return sanitize_text_field( $value );
              },
              'validate_callback' => function ( $value ) {
                  // Valid if it contains exactly 10 English letters.
                  return (bool) preg_match( '/\A[a-z]{10}\Z/', $value );
              },
          ),
      ),
		]
	); 
}