# File lib/new_relic/agent/javascript_instrumentor.rb, line 89
      def browser_timing_header #THREAD_LOCAL_ACCESS
        return '' unless js_enabled_and_ready? # fast exit

        state = NewRelic::Agent::TransactionState.tl_get

        return '' unless insert_js?(state) # slower exit

        bt_config = browser_timing_config(state)
        return '' if bt_config.empty?

        bt_config + browser_timing_loader
      rescue => e
        ::NewRelic::Agent.logger.debug "Failure during RUM browser_timing_header construction", e
        ''
      end