An empty sequence object is created with a call to IFileDocument.CreateSequence method. This means that you must have a valid File Document object in order to create a sequence object.
var fdoc = new ActiveXObject("FileDocument.FileDocument");
var seq= fdoc.CreateSequence();
// work with seq object
Created object is not related to the document object, may live longer that its creator and may be used in calls to methods provided by other document objects.