core/OperationBuilder.js
function _getArray(elem, field) {
        var f = elem._id + "." + field;
        if (!_currentArray[f]) {
            _currentArray[f] = _.clone(elem[field]);
        }
        return _currentArray[f];
    }
function _getArray(elem, field) {
        var f = elem._id + "." + field;
        if (!_currentArray[f]) {
            _currentArray[f] = _.clone(elem[field]);
        }
        return _currentArray[f];
    }
function _getArray ( elem , field ) { var f = elem . _id + "." + field ; if ( ! _currentArray [ f ] ) { _currentArray [ f ] = _ . clone ( elem [ field ] ) ; } return _currentArray [ f ] ; }
Return a copied array of array field of a particular element.
It could be changed by `fieldInsert`, `fieldRemove`.
The reason why using this is index value maybe incorrect when performing `fieldInsert` multiple times.

@private
@param {Element} elem An element
@param {string} field Array field name
@return {number}
Return a copied array of array field of a particular element . It could be changed by fieldInsert fieldRemove . The reason why using this is index value maybe incorrect when performing fieldInsert multiple times .