Source

File: wp-includes/rest-api/class-wp-rest-server.php

protected function get_json_last_error() {
		$last_error_code = json_last_error();

		if ( JSON_ERROR_NONE === $last_error_code || empty( $last_error_code ) ) {
			return false;
		}

		return json_last_error_msg();
	}