SP.Taxonomy.TermSetItem.customSortOrder property (sp.taxonomy)
Specifies the custom sort order for the child items of the current item.
Applies to: apps for SharePoint | SharePoint Foundation 2013 | SharePoint Server 2013
var value = SP.Taxonomy.TermSetItem.get_customSortOrder()
SP.Taxonomy.TermSetItem.set_customSortOrder(a)