How do I add a line of text to an existing file in ionic 2?

I thought it was a stupid question, but ...

I tried try writeExistingFile(path, fileName, text)

but replaced the file.

I tried writeFile(path, fileName, text, true)

the same

I tried writeFile(path, fileName, text, false)

and I got PATH_EXIST_ERROR

I can read a file, add a new line, write a file ... but it seems a little crazy! Is someone helping?

R.