I'm just starting to learn node.js (literally my second task) and I need to write two modules (one for prime numbers under 100 and one for even numbers under 50 - this is one separate requirement). The next requirement is that I import these modules to a node.js file and write the outcomes to 'nums.txt' file. If I write straight forward strings to files and run tests on them it executes perfectly, but when I try to write Array.toString() to the file I get

TypeError [ERR_INVALID_ARG_TYPE]: The "data" argument must be of type string or an instance of Buffer, TypedArray, or DataView. Received type function ([Function (anonymous)])

What am I missing? I've read through most questions on here, gone through the documentation and tried everything from changing my module return statements and even trying to add toString in the filehandler.

These are the modules:

exports.primes = () => {
  const isPrime = () => {
    let sieve = [],
      i,
      j,
      n = 100,
      primeArr = [];

    for (i = 2; i < n; ++i) {
      if (!sieve[i]) {
        primeArr.push(i);
        for (j = i << 1; j <= n; j += i) {
          sieve[j] = true;
        }
      }
    }
    console.log(primeArr);
    return primeArr.toString();
  };
  isPrime();
};

exports.evenNums = () => {
  const isEven = () => {
    let n = 50;
    let evenArr = [];
    if (n === 1 || n === 0) {
      return false;
    }
    for (let i = 2; i <= n; ++i) {
      if (i % 2 === 0) {
        //console.log(i);
        evenArr.push(i);
      }
    }
    return evenArr.toString();
  };
  isEven();
};

My Node.js file

const primeN = require("./primes");
const evenN = require("./evenNums");
const fileHandler = require("fs");
const http = require("http");

const prime = primeN.primes();
const even = evenN.evenNums();
fileHandler.writeFile("nums.txt", prime, (err) => {
  if (err) {
    console.log("File not written\n");
  } else {
    console.log("File written successfully\n");
    console.log("The file has the following contents:");
    console.log(fileHandler.readFileSync("nums.txt"));
  }
  fileHandler.appendFile("nums.txt", even, (err) => {
    if (err) {
      console.log("File not written\n");
    } else {
      console.log("File written successfully\n");
      console.log("The file has the following contents:");
      console.log(fileHandler.readFileSync("nums.txt"));
    }
  });
});

http
  .createServer(function(req, res) {
    res.write("nums.txt");
    res.end();
  })
  .listen(8000);

I just know it is something simple that I'm missing but I can't seem to figure it out. Any tips on debugging Node.js would also be greatly appreciated.