How to retrieve all localStorage items without knowing the keys in advance?

If you modify your function to this you can list all items based on key (will list the items only):

function allStorage() {

    var values = [],
        keys = Object.keys(localStorage),
        i = keys.length;

    while ( i-- ) {
        values.push( localStorage.getItem(keys[i]) );
    }

    return values;
}

Object.keys is a new addition to JavaScript (ECMAScript 5). It lists all own keys on an object which is faster than using a for-in loop which is the option to this.

However, this will not show the keys. For that you need to return an object instead of an array (which is rather point-less IMO as this will bring you just as far as you were before with localStorage just with a different object – but for example’s sake):

function allStorage() {

    var archive = {}, // Notice change here
        keys = Object.keys(localStorage),
        i = keys.length;

    while ( i-- ) {
        archive[ keys[i] ] = localStorage.getItem( keys[i] );
    }

    return archive;
}

If you want a compact format listing then do this instead – here each item in the array will have key=item which you later can split into pairs and so forth:

function allStorage() {

    var archive = [],
        keys = Object.keys(localStorage),
        i = 0, key;

    for (; key = keys[i]; i++) {
        archive.push( key + '=' + localStorage.getItem(key));
    }

    return archive;
}