In R , is there any avaiable funcation like IFERROR formula in EXCEL

Question

In R , is there any available function like IFERROR formula in EXCEL ? I want to calculate moving average using 4 nearest figures, but if the figures less than 4 in the group then using normal average. Detail refer to below code, the IF_ERROR is just i wished function and can't work

library(tidyverse)
library(TTR)
test_data <- data.frame(category=c('a','a','a','b','b','b','b','b','b'),
             amount=c(1,2,3,4,5,6,7,8,9))

test_data %>% group_by(category) %>% mutate(avg_amount=IF_ERROR(TTR::runMedian(amount,4),
                                                                median(amount),
                                                                TTR::runMedian(amount,4))