Javascript IF statement misinterpreting conditions as a function

I’ve been trying to use a modified autocomplete code yet the code for whatever reason the following code will output “Uncaught TypeError: arr[i].substr is not a function at HTMLInputElement.” instead of simply running the function as intended. What’s confusing me about this is that when using a predefined array with finite values the code works exactly as intended, yet when using an array of values that has been extracted from a MySQL database it will output the error.

<form action="Home.html" autocomplete="off">
  <div class="autocomplete" style="width:300px;">
    <input style="position: relative; top: 20px; left: 20px" class="autotext" id="myInput" type="text" name="primary_input" value="test" placeholder="placeholder">
  </div>
  <button style="position: relative; top:20px; left: 30px" class="submitButton" type="button">Submit</button>
</form>
</body>
<?php
$servername = "localhost";
$username = "username";
$password = "password";
$dbname = "database";

$conn = mysqli_connect($servername, $username, $password, $dbname);
if($conn -> connect_error){
    die("Connection failed: " . $conn -> connect_error);
}

$result = mysqli_query($conn, "SELECT field_name FROM table_name");
while($row = mysqli_fetch_assoc($result)){
        $array[] = $row;
}
?>

<script>
var mysql_array = <?php echo json_encode($array, JSON_HEX_QUOT); ?>;
console.log(mysql_array);

function autocomplete(inp, arr) {
  /*the autocomplete function takes two arguments,
  the text field element and an array of possible autocompleted values:*/
  var currentFocus;
  /*execute a function when someone writes in the text field:*/
  inp.addEventListener("input", function(e) {
      var a, b, i, val = this.value;
      /*close any already open lists of autocompleted values*/
      closeAllLists();
      if (!val) { return false;}
      currentFocus = -1;
      /*create a DIV element that will contain the items (values):*/
      a = document.createElement("DIV");
      a.setAttribute("id", this.id + "autocomplete-list");
      a.setAttribute("class", "autocomplete-items");
      /*append the DIV element as a child of the autocomplete container:*/
      this.parentNode.appendChild(a);
      /*for each item in the array...*/
      for (i = 0; i < arr.length; i++) {
        /*check if the item starts with the same letters as the text field value:*/
        if (arr[i].substr(0, val.length).toUpperCase() === val.toUpperCase(inp)) {
          /*create a DIV element for each matching element:*/
          b = document.createElement("DIV");
          /*make the matching letters bold:*/
          b.innerHTML = "<strong>" + arr[i].substr(0, val.length) + "</strong>";
          b.innerHTML += arr[i].substr(val.length);
          /*insert a input field that will hold the current array item's value:*/
          b.innerHTML += "<input type='hidden' value='" + arr[i] + "'>";
          /*execute a function when someone clicks on the item value (DIV element):*/
              b.addEventListener("click", function(e) {
              /*insert the value for the autocomplete text field:*/
              inp.value = this.getElementsByTagName("input")[0].value;
              /*close the list of autocompleted values,
              (or any other open lists of autocompleted values:*/
              closeAllLists();
          });
          a.appendChild(b);
        }
      }
  });
  /*execute a function presses a key on the keyboard:*/
  inp.addEventListener("keydown", function(e) {
      var x = document.getElementById(this.id + "autocomplete-list");
      if (x) x = x.getElementsByTagName("div");
      if (e.keyCode == 40) {
        /*If the arrow DOWN key is pressed,
        increase the currentFocus variable:*/
        currentFocus++;
        /*and and make the current item more visible:*/
        addActive(x);
      } else if (e.keyCode == 38) { //up
        /*If the arrow UP key is pressed,
        decrease the currentFocus variable:*/
        currentFocus--;
        /*and and make the current item more visible:*/
        addActive(x);
      } else if (e.keyCode == 13) {
        /*If the ENTER key is pressed, prevent the form from being submitted,*/
        e.preventDefault();
        if (currentFocus > -1) {
          /*and simulate a click on the "active" item:*/
          if (x) x[currentFocus].click();
        }
      }
  });
  function addActive(x) {
    /*a function to classify an item as "active":*/
    if (!x) return false;
    /*start by removing the "active" class on all items:*/
    removeActive(x);
    if (currentFocus >= x.length) currentFocus = 0;
    if (currentFocus < 0) currentFocus = (x.length - 1);
    /*add class "autocomplete-active":*/
    x[currentFocus].classList.add("autocomplete-active");
  }
  function removeActive(x) {
    /*a function to remove the "active" class from all autocomplete items:*/
    for (var i = 0; i < x.length; i++) {
      x[i].classList.remove("autocomplete-active");
    }
  }
  function closeAllLists(elmnt) {
    /*close all autocomplete lists in the document,
    except the one passed as an argument:*/
    var x = document.getElementsByClassName("autocomplete-items");
    for (var i = 0; i < x.length; i++) {
      if (elmnt != x[i] && elmnt != inp) {
      x[i].parentNode.removeChild(x[i]);
    }
  }
}
/*execute a function when someone clicks in the document:*/
document.addEventListener("click", function (e) {
    closeAllLists(e.target);
});
}

/*initiate the autocomplete function on the "myInput" element, and pass along the countries array as possible autocomplete values:*/
autocomplete(document.getElementById("myInput"), mysql_array);
</script>
</body>

Answer

Change this:

$result = mysqli_query($conn, "SELECT field_name FROM table_name");
while($row = mysqli_fetch_assoc($result)){
        $array[] = $row;
}

to this

$result = mysqli_query($conn, "SELECT field_name FROM table_name");
while($row = mysqli_fetch_assoc($result)){
        $array[] = $row["poke_name"];
}