How to globally convert NaN to NA in dataframe?

Andrew,

x[] <- lapply(x, function(xx) {
    xx[is.nan(xx)] <- NA_real_
    xx
})

is different from

x <- lapply(x, function(xx) {
    xx[is.nan(xx)] <- NA_real_
    xx
})

indeed, the two are different -- but some ignorance of mine is exposed.
i wonder, can you explain why the two are different?

is it because of (or, "is the clue...") this in the "Value:" section of
: ?"[<-.data.frame"
----
     For '[<-', '[[<-' and '$<-', a data frame.
----
?

cheers, Greg