readFile gives TypeError: Cannot read property 'F_OK' of undefined

I have

import { Workbook } from "exceljs";
let workbook = new Workbook();
files.forEach((file) => {
const data = workbook.csv.readFile(file.name)
})

Tried with workbook.xlsx.readFile and this gives same error.

I have version 3.8.1 installed.