javascript equivalent of join() and toString() in c#?

if you wish to add the functionality to a string array you could do with an extension method

public static class ArrayExtension
{

  public static string AsString(this string[] array, string seperator)
  {
    return string.Join(seperator, array);
  }
}

Then you would write:

var keyStr = keyList.AsString("_");

You can use string.Join():

string.Join("_", array);

or, for lists:

string.Join("_", list);

Converting a string array into a single string is done exactly the same way: With string.Join():

string.Join(" ", stringarray);

Dan Elliott also has a nice extension method you can use to be a little closer to JavaScript, syntax-wise.