SharePoint Online JavaScript creates folder instead of document set

warrenbarton 

This issue seems to be due to some changes in SharePoint Online after 12th January.

To fix this issue: use the list.contentTypeID instead of web.ContentTypeID.

Change your code like this:

var docSetContentTypeID = '<The list DocumentSet contentTypeID>';
currentDate = new Date().getFullYear();
ctx.load(list);
folderUrl = "dummy" + currentDate;
var folder = web.getFolderByServerRelativeUrl(folderUrl);
ctx.load(folder); 
//use list.get_contentTypes()
var docsetContentType = list.get_contentTypes().getById(docSetContentTypeID);
ctx.load(docsetContentType);

Check more information in below related threads:

1. SharePoint Online JavaScript creates folder instead of document set 
2. Document Set created programmatically not working anymore since 12 Jan 2021 

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