How to write javascript equivalent of Convert.ToBase64String() in javascript?

I have byte array and I can convert this usin Convert.ToBase64String() method in c#. I wrote equivalent of this method in javascript like below. But the result is different.

in c#:

 byte[] data = ...
Convert.ToBase64String(data)

in js

    function GetStringFromByteArray(array) {
        var result = "";
        for (var i = 0; i < array.length; i++) {
            for (var j = 0; j < array[i].length; j++)
                result += String.fromCharCode(array[i][j]);
        }
        return result;
    }

How can I succeed this in js?