COUNTHIDDENSHEETS

COUNTHIDDENSHEETS()
Returns the number of hidden worksheets in the active workbook

REMARKS
You can use the built-in worksheet function SHEETS() in Excel 2013 to return the number of sheets in the active workbook.

Public Function COUNTHIDDENSHEETS() As Integer 

End Function 

await Excel.run(async (context) => { 
    const sheets = context.workbook.worksheets; 
    sheets.load("items/visibility"); 
    await context.sync(); 

    let hiddenCount = 0; 
    let veryHiddenCount = 0; 

    sheets.items.forEach(sheet => { 
        if (sheet.visibility === Excel.SheetVisibility.hidden) { 
            hiddenCount++; 
        } 
        if (sheet.visibility === Excel.SheetVisibility.veryHidden) { 
            veryHiddenCount++; 
        } 
    }); 

    console.log("Hidden:", hiddenCount); 
    console.log("Very Hidden:", veryHiddenCount); 
});