WoW API: CheckInteractDistance

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Returns 1 if you are in range of the specified unit to perform the action specified by distIndex (returns nil otherwise).

(String unit, Int distIndex)

unit

The unit to query (e.g. "target", "party1", "pet", "player")

distIndex

A value 1-4:

1 = Inspect, 5.55 yards (as of Patch 1.12, this will double)

2 = Trade, 11.11 yards

3 = Duel, 10 yards

4 = Follow, ~28 yards

canInteract: If in range to perform the action, 1 (true). Otherwise, nil (false).

If "unit" is a hostile unit, the return values are the same. But you obviously won't be able to do things like Trade.

There may also be more values for distIndex that we don't know about.

if ( CheckInteractDistance("target", 4) ) then
  FollowUnit("target");
else
  -- we're too far away to follow the target
end