Recently I started a new project (random password generator) in javascript and something isn't working its not finished yet. (Im just a beginner in js)

i have 4 string, depending on which boxes are checked i want them to connect in one string and then randomly select letters from this one string, of course if you know a better way to do that please tell me, as i said before, I am a beginner in js.

```js
var uppercase_letters = ['A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K', 'L', 'M', 'N', 'O', 'P', 'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X', 'Y', 'Z'];
var lowercase_letters = ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z'];
var numbers = ['1', '2', '3', '4', '5', '6', '7', '8', '9', '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', '0'];
var symbols = ['/', '[', ']', '$', '-', ':', '_', '?', '{', '}', '~', '!', '@', '#', '%', '&', '*', '(', ')', '"', '^', '§', ',', '.', '°', '+', '-', ':'];
var length;
//--------------------
var check1;
var check2;
var check3;
var check4;
var inputnum;

function getvalues() {
  inputnum = document.getElementById("length").value;
  length = inputnum;
  check1 = document.getElementsByClassName("check")[1];
  check2 = document.getElementsByClassName("check")[2];
  check3 = document.getElementsByClassName("check")[3];
  check4 = document.getElementsByClassName("check")[4];
  generatestring();
}

function generatestring() {
  var string;

  if (check1.checked === true) {
    string = string.concat(uppercase_letters)
  };

  if (check2.checked === true) {
    string = string.concat(lowercase_letters)
  };

  if (check3.checked === true) {
    string = string.concat(numbers)
  };

  if (check4.checked === true) {
    string = string.concat(symbols)
  };

  alert("Working?");
  show(string);
}

function show(string) {
  alert(string);
}
```

```html
<div class='container'>
  <input type='checkbox' class='check' value='uppercase'>uppercase letters
  <br />
  <input type='checkbox' class='check' value='lowercase'>lowercase letters
  <br />
  <input type='checkbox' class='check' value='number'>numbers
  <br />
  <input type='checkbox' class='check' value='symbol'>symbols
  <br />
  <input type='number' id='length' value='length'>
  <br />
  <br />
  <input type='button' id='generate' value='generate' onclick="getvalues()">
</div>
```

So, the alert with the string (just for testing) isn't showing up, thats why i added the one with the message "Working?", and this one isn't showing up as well. I have no idea why... I am sure that the problem are these "if's" but i don't know how to do it in another way :( Could someone please help me out?

Probably I am doing everything completly wrong, so please give me some tipps.

Thank you!