Just passed my first Juniper cert, it was pretty tough to be honest. I just barely passed, a lot of the exam was very specific to Juniper. Not so much on the networking concepts. Cisco tests have more concepts questions which I find a better way to test someone then the actually syntax.
Udemy's course on the JNCIA-Junos (very good but not enough to pass without any hands on)
Junos Routing book by O'Reilly (maybe the first two chapters)
A lot of hands on experience as I use Junos everyday ( All Juniper shop)
Also something to note that if you are doing a exam from your home or office you can't use any paper or a pens which I found ridiculous so I had to subnet and counting bits in the air which wasn't very fun as I'm used to being able to write it down. I'm not sure if you can use paper and a pen at a Pearson Vue office but I wanted to mention it as something that threw me off.
on to JNCIS-DevOps (before Oct 1 as Juniper changed the prerequisite)