Javascript : How to concat string to object?

I have following code where i combine some variables to create path to the another existing object and his attribute.

Problem is that i alway get only string, so i would like to "convert" it into the object.

// SET CUSTOM CONTENT FOR COLUMN IF CONTACT ATTR IS EXISTS
            if(value.concatByFields != null) {
                  preparedGridColumnItem.template = function (responseData) {
                    var nameForConcat;
                    var fieldName;
                    var objectName;
                    var pathToReturn;
                    $.each(value.concatByFields, function( index, concatField ) {
                      nameForConcat = null;
                      fieldName = null;
                      objectName = null;
                      objectName = value.field;
                      fieldName = concatField.fieldName;
                      console.log("FIELD NAME IS");
                      console.log(JSON.stringify(fieldName));
                      console.log("OBJECT NAME IS");
                      console.log(objectName);
                      nameForConcat = objectName+"."+fieldName;
                      console.log("CONCATED NAME IS");
                      console.log(nameForConcat);
                      console.log("OBJECT ADDRESS IS FOLLOWING");
                      console.log("responseData."+nameForConcat);
                      pathToReturn = "responseData."+nameForConcat;

                    });
                    //TODO : IS ALWAYS RETURNED AS STRING
                  return pathToReturn;
                };
            }

Returned value should be value of another and global existing json object. But now is it always string.

It means: responseData.SomeObject.surname

How can i solve it please?

Many thanks for any help.