How to get a unique identifier for a file?

[kabouter]

I'm trying to implement Onlyoffice and it "works" almost, but I need a unique identifier for my files.

I'm trying to get the ContentsHashes or modified time, but i don't know how to get this in javascript.
I tried this in Javascript but I don't get the hash.

select: function (event, elfinderInstance) {
                                console.log(elfinderInstance.getContentsHashes());  
                            }

The file name hash of the file is not good enough, because Onlyoffice doesn't want the same identifier for files that were changed in the meantime and are still in the Onlyoffice cache.

[kabouter]

I solved this with md5_file in php, but I'm still wondering how getContentsHashes() works.