code: function fieldAssign(elem, field, val) {
          try {
              $(exports).triggerHandler('fieldAssign', [elem, field, val]);
          } catch (err) {
              console.error(err);
          }
          var isCustomField = (elem[field] && elem[field].__read);
          var oldVal;
          if (isCustomField) {
              oldVal = elem[field].__write();
          } else {
              oldVal = elem[field];
          }
          _currentOperation.ops.push({
              op: OP_FIELD_ASSIGN,
              arg: {
                  _id: elem._id,
                  f: field,
                  n: val,
                  o: oldVal
              }
          });
      }
docs: Assign value to field.
  
  @param {Element} elem
  @param {string} field
  @param {?} val