Scarlets have already defeated Leinster this term, but did so while the Irish province had key players absent on international duty.
So John knows they are in for a far sterner examination of their credentials this time round in Dublin.
"Leinster is not an easy place to go to," he said.
"They'll have all their internationals back and will be a completely different side to the one we faced a few weeks ago but those are boys you want to play against. They'll be targeting a home win for themselves.
"We set ourselves a target to be top four by the time the Rugby World Cup players came back and so to be top of the league - we're much happier.
"We're taking it one game at a time. We want to stay top of the league but we're expecting a big battle up front this weekend. We'll need to perform to keep our place."
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