The equivalent of which.max and which.min is argmax and argmin respectively.

y = [1, 4, 3, 2]
argmax(y)
> 2

https://docs.julialang.org/en/v1/base/collections/#Base.argmax

In earlier versions there were the indmax and indmin functions, but they have been deprecated and removed in the meanwhile. It seems they were removed with Julia 0.7 and 1.0 released in August 2018.