How to set root nav using getRootNavById

Ionic Framework ionic-v3
1

(getRootNav) is deprecated and will be removed in the next major release. Use getRootNavById instead.

my current code is
this.app.getRootNav().setRoot(BootScreen);

plesese tell me how to set root using getRootNavById

2

Some solutions were provided in this duplicate question

https://forum.ionicframework.com/t/getrootnav-deprecated-use-getrootnavbyid-whats-the-value-of-the-root-nav-id?source_topic_id=115443