how to store array of array in localstorage ?

The local storage is based on name/value pairs, like all associative containers in Javascript (essentially all Javascript object, including arrays, are such associative arrays):
http://en.wikipedia.org/wiki/JavaScript#Dynamic[^],
http://en.wikipedia.org/wiki/Associative_arrays[^].

Please see how it works: http://diveintohtml5.info/storage.html[^].

The associated container does not have certain size: the size grows as you add data. Let's see how it works with a single array indexed by integers:

localStorage[10] = "the value @10";
// if your local storage was empty before, you have only one element (not 11 or 10 of them)

localStorage[111] = "the value @111";
// only two elements for now

alert(localStorage[10]); // will display "the value @10"

Likewise, you can have array of arrays, which you can simply interpret as the array indexed with compound index, which can be, in this case, just a pair of indices:

localStorage[1,13] = "the value @[1,13]"; 
localStorage[111] = "the value @112"; // you can even combine 1D and 2D arrays
alert(localStorage[1,13]); // will display "the value @[1,13]"

By definition, you will have more flexibility than you expected: you will get jagged arrays: http://en.wikipedia.org/wiki/Jagged_array[^].

That's all.

—SA