customers = [];
$scope.accounts = {customers};

$scope.addAccount = function () {
    $scope.accounts.customers.push($scope.account);
    $scope.save();
};

$scope.save = function () {
    document.addEventListener("deviceready", function() {
    window.requestFileSystem(LocalFileSystem.PERSISTENT, 0, fsReceived, errorHandler);
        }, false);
    };

function fsReceived() {
var data = angular.toJson($scope.accounts);
        $cordovaFile.writeExistingFile(cordova.file.externalRootDirectory, "accounts.json", data).then(function (success) {
            alert('Success');
        }, errorHandler);
    };

{"customers" = [{a:"a", b:"b"}, {c:"c", d:"d"}]}

{"customers" = [{a:"a", b:"b"]}, "customers" = [{c:"c", d:"d"}]