If the output from fromJSON is not a character, then you could use one of the other variants. There is map_dbl for class double, map_int for integers, map_lgl for logicals, map_dfr for dataframes/tibbles that you want binded by rows (i.e. binded like bind_rows or rbind) and map_dfc for dataframes/tibbles you want binded by column (i.e. binded by bind_cols or cbind).
If you are not sure of the output, you could simply use map in place of map_chr and it will return a list instead of a vector.
Ex: Say one of my files has 13 rows. And the one after also has 13. After I finish running my code, I should have 26 rows. My data is data from a JSON file, and is a dataframe, if that helps.