Declarative Pipeline - how to get scm variables

[nirav]

I have few problems locating scm related variables:

In scripted pipeline I used this:

            scmVars = checkout scm

            echo "scmVars: ${scmVars}"

            echo "tagName: ${env.TAG_NAME}"

            echo "branchName: ${env.BRANCH_NAME}"

            echo "changeTarget: ${env.CHANGE_TARGET}"

Since in declarative one I dont need to checkout, how do I get hang of variables?

env.TAG_NAME, env.BRANCH_NAME all coming up null.

is see 'branch' is used in conditional 'when' expression 

     when {

                branch 'master'

            }

but how many such variables are available ?