Question from the Node.js test

Considering the following class :

'use strict';

const fs = require('fs');

class FileManager {
  constructor(filename) {
    this.fileName = filename;
    this.fileContent = null;
  }
  open() {
    fs.readFile(this.fileName, function(err, data) {
      if (err) { throw err; }
      this.fileContent = data;
    })
  }
  getText() {
    return String(this.fileContent);
  }
}

let foobar = new FileManager('foobar.txt');
foobar.open();
console.log( foobar.getText() );

What are/is the problem(s) with this code?
(We assume that the file to read exists and that it contains text)