operate.js

// Return operations to modify array a to b.
// e.g. var a = [{id:1}, {id:2}], b = [{id:2}]
// operations(a, b) -> [{operation: ‘remove’, item: {id:1}]
function operations(a, b) {
    var aIds = _.pluck(a, 'id');
    var bIds = _.pluck(b, 'id');

    var notInBoth = _.xor(aIds, bIds);
    return _.map(notInBoth, function(id) {
        var isInA = _.contains(aIds, id);

        return {
            operation: isInA ? 'remove' : 'add',
            item: _.find(a.concat(b), function(item) { return item.id === id })
        };
    });
}