JQuery :: Textarea Form - Selecting Array Elements?

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. I'm open to doing this via jQuery or regular javascript.