xlsx read from a starting point till last of the filled sheet

I am using xlsx node module.I am trying to read rows at certain breaks and convert it to json. I want know how can I read data for a certain range where starting point is some value but the ending point should be last filled value.

const df = XLSX.utils.sheet_to_json(workbook.Sheets[sheetNames[sheetIndex-1]], {
    raw: true,
    range: "A4:till last",
});

Also what is best way to read the sheet in parts like this. Is it efficient to call sheet_to_json and provide range.