Properties in Doctype

Frappe Framework Customization
1

I have
Doctype A

Field A1
Field A2
Field A3

Doctype B

Field B1
Field B2 (link) A1
Field B3

From doctypeb.js I want to access the A3 field of the doctype A

As I do?

2

you mean Link of DocType A

i.e. field B2 - DocType A link

If yes, then you can access all values of DocType A using field B2 (i.e link of A)

e.g.frappe.db.get_value("DocType A", "docname(field B2)", "field A3" )

OR

Simply use,

cur_frm.add_fetch("field B2", "field to fetch from A", "target field of B")