userController.js

const express = require("express")
const app = express()

app.get("/users/getCSV", function (req, res) {
  // retrieving the user with a query...

  const users = [
    { name: "Patricia", surname: "Smith", age: null },
    { name: "John", surname: null, age: 56 },
    { name: "Maria", surname: "Brown", age: 37 },
  ]

  // initializing the CSV string content with the headers
  let csvData = ["name", "surname", "age"].join(",") + "\r\n"

  users.forEach((user) => {
    // populating the CSV content
    // and converting the null fields to ""
    csvData += [user.name, user.surname, user.age].join(",") + "\r\n"
  })

  // returning the CSV content via the "users.csv" file
  res
    .set({
      "Content-Type": "text/csv",
      "Content-Disposition": `attachment; filename="users.csv"`,
    })
    .send(csvData)
})