How to run tests in R?

I have written a few functions in R, and I want to test whether the outputs of the functions are correct. I simply want to use some kind of method, that will assert or test whether the expected result is correct or not. I see there are a few packages in CRAN, but does R has this support by default without using any external package? If not, which package is the easiest to use and most simple? As I said I only need something like assert(input, output, equal).