df <- data.frame(x=c(1, 2, NA, NA))

df$foo1[!is.na(df$x)] <- 777

df[["foo2"]][!is.na(df$x)] <- 888

`<-`(df$foo3[!is.na(df$x)], 999)

`[[`(df, "foo4")[!is.na(df$x)] <- 0

`<-`(`[[`(df, "foo5")[!is.na(df$x)], 1)

df