JavaScript equivalent of Python's rsplit

String.prototype.rsplit = function(sep, maxsplit) {
    var split = this.split(sep);
    return maxsplit ? [ split.slice(0, -maxsplit).join(sep) ].concat(split.slice(-maxsplit)) : split;
}

This one functions more closely to the Python version

"blah,derp,blah,beep".rsplit(",",1) // [ 'blah,derp,blah', 'beep' ]

You can also use JS String functions split + slice

Python:

'a,b,c'.rsplit(',' -1)[0] will give you 'a,b'

Javascript:

'a,b,c'.split(',').slice(0, -1).join(',') will also give you 'a,b'

Assuming the semantics of JavaScript split are acceptable use the following

String.prototype.rsplit = function (delimiter, limit) {
  delimiter = this.split (delimiter || /s+/);
  return limit ? delimiter.splice (-limit) : delimiter;
}