There's two issues, first you've not defined the content as an array, and with that code you should probably define how the array-elements should be joined:
var list = $('ul.theList li'),
result = $('div#result'),
content = [];
for( var i=0; i < list.length; i++ ){
content.push($('ul.theList li:eq(' + i + ')').text());
/* or:
content.push($('ul.theList li').eq(i).text());
*/
}
$(result).text(content.join(', '));
JS Fiddle demo using :eq() selector.
JS Fiddle demo using .eq() method.
I've used .push() to insert the text of the li (I'm assuming, here, that you want the text and not the node itself) to the content array, rather than explicitly defining the index.
References:
content=[]you are missing the initialisation