Selecting Javascript array elements?

Selecting Javascript array elements?

Sure that I'm doing something really dumb.  But I'm trying to add the text of a textarea form to the string that's contained at:

myarray[a][b][c]

It was easy to pull out the value of the textarea with this:

textinput = $('#mytextarea').val();

But then when I try to add the variable into the array, it doesn't work.  I'm not able (apparently) to do this.  Fingers crossed that I'm doing something dumb?

This doesn't work:

myarray[a][b][c] += newstringvariable;

Nor this:

myarray[a][b][c] = myarray[a][b][c] + textinput;

I'm guessing there is a very simple (probably syntax related!) solution to this.  Can anyone offer suggestions?  I'm open to doing this via jQuery or regular javascript.  Thanks very much!