For some strange reason, JavaScript code folding in TextMate is limited to functions; leaving out JSON, simple statements delimited by brackets, and arrays.
In order to change this behaviour, open up the Bundle Editor (Bundles → Bundle Editor → Show Bundle Editor). From the dropdown menu in the top left corner select 'Languages', expand 'JavaScript' (click the triangle) and choose the JavaScript Language Grammar (the item with the L next to it). Find 'foldingStartMarker' and 'foldingStopMarker' in the list on the right and replace their values with:
foldingStartMarker = '\{\s*$|\[\s*$|\(\s*$';
foldingStopMarker = '^\s*\}|^\s*\]|^\s*\)';
It's amazing that this isn't stock functionality, or at least a simple option. Thank you!