question

Hi @09508315 ,
First we need to break role inheritance for the folder.

 function breakRoleInheritanceOfFile() { 
      $.ajax({
             url: siteurl + String.format("/_api/web/GetFolderByServerRelativeUrl('DocumentTest/test999')/ListItemAllFields/breakroleinheritance(true)"),
             type: "POST",
             contentType: "application/json;odata=verbose",
             headers: {
                 "Accept": "application/json;odata=verbose",
                 "X-RequestDigest": $("#__REQUESTDIGEST").val()
             },
             success: onQuerySucceeded,
             error: onQueryFailed
      });   
 }

Remove the current role assignment for the folder

 function remove(){
     $.ajax({
             url: siteurl + String.format("/_api/web/GetFolderByServerRelativeUrl('DocumentTest/test999')/ListItemAllFields/roleassignments/getbyprincipalid(11)"),
             method: "POST",
             headers: {
                    Authorization: "Bearer " + accessToken,
                    "accept": "application/json;odata=verbose",
                    "content-type": "application/json;odata=verbose",
                    "X-HTTP-Method": "DELETE"
             },
             success: onQuerySucceeded,
             error: onQueryFailed
     }); 
 }

Add the new role assignment for the folder

 function add(){
     $.ajax({
             url: siteurl + String.format("/_api/web/GetFolderByServerRelativeUrl('DocumentTest/test999')/ListItemAllFields/roleassignments/addroleassignment(principalid=11,roledefid=1073741829)"),
             method: "POST",
             headers: {
                    Authorization: "Bearer " + accessToken,
                    "accept": "application/json;odata=verbose",
                    "content-type": "application/json;odata=verbose"
             },
             success: onQuerySucceeded,
             error: onQueryFailed
     });     
 }

In order to use this API we need 'FullControl' permissions,So we need get Access Token by app

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