function fileSizeToString(size) {
    var suffix = ["B", "KB", "MB", "GB"];

    var place = Math.floor(Math.log(size) / Math.log(1024));
    var fileSize = Math.round(size / Math.pow(1024, place));

    return (fileSize + suffix[place]);
};

As you can see this is a relatively simple function, and really boils down to two lines of code. The first line

var place = Math.floor(Math.log(size) / Math.log(1024));

determines which suffix will be used. While the second line

var fileSize = Math.round(size / Math.pow(1024, place));

calculates the file size, which is typically measured in units of 1024.

In my case I only needed to support up to gigabyte file sizes, but the function could easily be modified to support larger sizes.

Happy Coding!