pw validation check

<input type="text" id="pwtext">
<br>
<button type="pwbutton" id="pwbutton">Convert</button>
<br>
<span id="result"></span>

<script>
document.getElementById("pwbutton").onclick = run;

function isUpper (letter) {
	return letter == letter.toUpperCase();
}

function encodeNatoLetter (letter) {
	var chr = letter.charCodeAt(0);
	if (chr >= 48 && chr <= 57) {
    var numbers = [
    	"zero", "one", "two", "three", "four",
      "five", "six", "seven", "eight", "nine"
    ];

    return numbers[chr - 48];
  }
  else {
		var phonems = [
    	"alpha", "beta", "charlie", "delta", "echo",
      "foxtrot", "golf", "hotel", "india", "juliett",
      "kilo", "lima", "mike", "november", "oscar",
      "papa", "quebec", "romeo", "sierra", "tango",
      "uniform", "victor", "whiskey", "xray", "yankee",
      "zulu"
    ];

    if (chr >= 65 && chr <= 90) {
    	return phonems[chr - 65].toUpperCase();
    }
    else if (chr >= 97 && chr <= 122) {
    	return phonems[chr - 97];
    }
  }

  return letter.charAt(0);
}

function encodeNatoWord (word) {
	var words = [];
  for (var i = 0; i < word.length; i++) {
  	words.push(encodeNatoLetter(word.charAt(i)));
  }

  return words.join(" ");
}

function run() {
	var str = document.getElementById("pwtext").value;

  document.getElementById("result").innerHTML = encodeNatoWord(str);
}
</script>