Facing an error while using onkeydown event

I have a code which I want to run on a code tag every time user changes it. I know that I could use onchange event but that only takes place after user has edited the content of an element and then click anywhere out of it. But my requirement is to run the function simultaneously. I tried onkeydown event but I am a facing error with it and I cant seem to figure out the issue. The code is below, pls help me. And by the way sorry the code is pretty long and I cant cut it short as I dont really understand the code as its copied from somewhere.

synataxHighlight(document.getElementById("myDiv"));

function synataxHighlight(elmnt, mode) {
  var lang = (mode || "html");
  var elmntObj = (document.getElementById(elmnt) || elmnt);
  var elmntTxt = elmntObj.innerHTML;
  var tagcolor = "mediumblue";
  var tagnamecolor = "brown";
  var attributecolor = "red";
  var attributevaluecolor = "mediumblue";
  var commentcolor = "green";
  var cssselectorcolor = "brown";
  var csspropertycolor = "red";
  var csspropertyvaluecolor = "mediumblue";
  var cssdelimitercolor = "black";
  var cssimportantcolor = "red";  
  var jscolor = "black";
  var jskeywordcolor = "mediumblue";
  var jsstringcolor = "brown";
  var jsnumbercolor = "red";
  var jspropertycolor = "black";
  elmntObj.style.fontFamily = "Consolas,'Courier New', monospace";
  if (!lang) {lang = "html"; }
  if (lang == "html") {elmntTxt = htmlMode(elmntTxt);}
  if (lang == "css") {elmntTxt = cssMode(elmntTxt);}
  if (lang == "js") {elmntTxt = jsMode(elmntTxt);}
  elmntObj.innerHTML = elmntTxt;

  function extract(str, start, end, func, repl) {
    var s, e, d = "", a = [];
    while (str.search(start) > -1) {
      s = str.search(start);
      e = str.indexOf(end, s);
      if (e == -1) {e = str.length;}
      if (repl) {
        a.push(func(str.substring(s, e + (end.length))));      
        str = str.substring(0, s) + repl + str.substr(e + (end.length));
      } else {
        d += str.substring(0, s);
        d += func(str.substring(s, e + (end.length)));
        str = str.substr(e + (end.length));
      }
    }
    this.rest = d + str;
    this.arr = a;
  }
  function htmlMode(txt) {
    var rest = txt, done = "", php, comment, angular, startpos, endpos, note, i;
    comment = new extract(rest, "<!--", "-->", commentMode, "W3HTMLCOMMENTPOS");
    rest = comment.rest;
    while (rest.indexOf("<") > -1) {
      note = "";
      startpos = rest.indexOf("<");
      if (rest.substr(startpos, 9).toUpperCase() == "<STYLE") {note = "css";}
      if (rest.substr(startpos, 10).toUpperCase() == "<SCRIPT") {note = "javascript";}        
      endpos = rest.indexOf(">", startpos);
      if (endpos == -1) {endpos = rest.length;}
      done += rest.substring(0, startpos);
      done += tagMode(rest.substring(startpos, endpos + 4));
      rest = rest.substr(endpos + 4);
      if (note == "css") {
        endpos = rest.indexOf("</style>");
        if (endpos > -1) {
          done += cssMode(rest.substring(0, endpos));
          rest = rest.substr(endpos);
        }
      }
      if (note == "javascript") {
        endpos = rest.indexOf("</script>");
        if (endpos > -1) {
          done += jsMode(rest.substring(0, endpos));
          rest = rest.substr(endpos);
        }
      }
    }
    rest = done + rest;
    for (i = 0; i < comment.arr.length; i++) {
        rest = rest.replace("W3HTMLCOMMENTPOS", comment.arr[i]);
    }
    return rest;
  }
  function tagMode(txt) {
    var rest = txt, done = "", startpos, endpos, result;
    while (rest.search(/(s|<br>)/) > -1) {    
      startpos = rest.search(/(s|<br>)/);
      endpos = rest.indexOf("&gt;");
      if (endpos == -1) {endpos = rest.length;}
      done += rest.substring(0, startpos);
      done += attributeMode(rest.substring(startpos, endpos));
      rest = rest.substr(endpos);
    }
    result = done + rest;
    result = "<span style=color:" + tagcolor + ">&lt;</span>" + result.substring(4);
    if (result.substr(result.length - 4, 4) == "&gt;") {
      result = result.substring(0, result.length - 4) + "<span style=color:" + tagcolor + ">&gt;</span>";
    }
    return "<span style=color:" + tagnamecolor + ">" + result + "</span>";
  }
  function attributeMode(txt) {
    var rest = txt, done = "", startpos, endpos, singlefnuttpos, doublefnuttpos, spacepos;
    while (rest.indexOf("=") > -1) {
      endpos = -1;
      startpos = rest.indexOf("=");
      singlefnuttpos = rest.indexOf("'", startpos);
      doublefnuttpos = rest.indexOf('"', startpos);
      spacepos = rest.indexOf(" ", startpos + 2);
      if (spacepos > -1 && (spacepos < singlefnuttpos || singlefnuttpos == -1) && (spacepos < doublefnuttpos || doublefnuttpos == -1)) {
        endpos = rest.indexOf(" ", startpos);      
      } else if (doublefnuttpos > -1 && (doublefnuttpos < singlefnuttpos || singlefnuttpos == -1) && (doublefnuttpos < spacepos || spacepos == -1)) {
        endpos = rest.indexOf('"', rest.indexOf('"', startpos) + 1);
      } else if (singlefnuttpos > -1 && (singlefnuttpos < doublefnuttpos || doublefnuttpos == -1) && (singlefnuttpos < spacepos || spacepos == -1)) {
        endpos = rest.indexOf("'", rest.indexOf("'", startpos) + 1);
      }
      if (!endpos || endpos == -1 || endpos < startpos) {endpos = rest.length;}
      done += rest.substring(0, startpos);
      done += attributeValueMode(rest.substring(startpos, endpos + 1));
      rest = rest.substr(endpos + 1);
    }
    return "<span style=color:" + attributecolor + ">" + done + rest + "</span>";
  }
  function attributeValueMode(txt) {
    return "<span style=color:" + attributevaluecolor + ">" + txt + "</span>";
  }
  function commentMode(txt) {
    return "<span style=color:" + commentcolor + ">" + txt + "</span>";
  }
  function cssMode(txt) {
    var rest = txt, done = "", s, e, comment, i, midz, c, cc;
    comment = new extract(rest, //*/, "*/", commentMode, "W3CSSCOMMENTPOS");
    rest = comment.rest;
    while (rest.search("{") > -1) {
      s = rest.search("{");
      midz = rest.substr(s + 1);
      cc = 1;
      c = 0;
      for (i = 0; i < midz.length; i++) {
        if (midz.substr(i, 1) == "{") {cc++; c++}
        if (midz.substr(i, 1) == "}") {cc--;}
        if (cc == 0) {break;}
      }
      if (cc != 0) {c = 0;}
      e = s;
      for (i = 0; i <= c; i++) {
        e = rest.indexOf("}", e + 1);
      }
      if (e == -1) {e = rest.length;}
      done += rest.substring(0, s + 1);
      done += cssPropertyMode(rest.substring(s + 1, e));
      rest = rest.substr(e);
    }
    rest = done + rest;
    rest = rest.replace(/{/g, "<span style=color:" + cssdelimitercolor + ">{</span>");
    rest = rest.replace(/}/g, "<span style=color:" + cssdelimitercolor + ">}</span>");
    for (i = 0; i < comment.arr.length; i++) {
        rest = rest.replace("W3CSSCOMMENTPOS", comment.arr[i]);
    }
    return "<span style=color:" + cssselectorcolor + ">" + rest + "</span>";
  }
  function cssPropertyMode(txt) {
    var rest = txt, done = "", s, e, n, loop;
    if (rest.indexOf("{") > -1 ) { return cssMode(rest); }
    while (rest.search(":") > -1) {
      s = rest.search(":");
      loop = true;
      n = s;
      while (loop == true) {
        loop = false;
        e = rest.indexOf(";", n);
        if (rest.substring(e - 5, e + 1) == "&nbsp;") {
          loop = true;
          n = e + 1;
        }
      }
      if (e == -1) {e = rest.length;}
      done += rest.substring(0, s);
      done += cssPropertyValueMode(rest.substring(s, e + 1));
      rest = rest.substr(e + 1);
    }
    return "<span style=color:" + csspropertycolor + ">" + done + rest + "</span>";
  }
  function cssPropertyValueMode(txt) {
    var rest = txt, done = "", s;
    rest = "<span style=color:" + cssdelimitercolor + ">:</span>" + rest.substring(1);
    while (rest.search(/!important/i) > -1) {
      s = rest.search(/!important/i);
      done += rest.substring(0, s);
      done += cssImportantMode(rest.substring(s, s + 10));
      rest = rest.substr(s + 10);
    }
    result = done + rest;    
    if (result.substr(result.length - 1, 1) == ";" && result.substr(result.length - 6, 6) != "&nbsp;" && result.substr(result.length - 4, 4) != "&lt;" && result.substr(result.length - 4, 4) != "&gt;" && result.substr(result.length - 5, 5) != "&amp;") {
      result = result.substring(0, result.length - 1) + "<span style=color:" + cssdelimitercolor + ">;</span>";
    }
    return "<span style=color:" + csspropertyvaluecolor + ">" + result + "</span>";
  }
  function cssImportantMode(txt) {
    return "<span style=color:" + cssimportantcolor + ";font-weight:bold;>" + txt + "</span>";
  }
  function jsMode(txt) {
    var rest = txt, done = "", esc = [], i, cc, tt = "", sfnuttpos, dfnuttpos, compos, comlinepos, keywordpos, numpos, mypos, dotpos, y;
    for (i = 0; i < rest.length; i++)  {
      cc = rest.substr(i, 1);
      if (cc == "\") {
        esc.push(rest.substr(i, 2));
        cc = "W3JSESCAPE";
        i++;
      }
      tt += cc;
    }
    rest = tt;
    y = 1;
    while (y == 1) {
      sfnuttpos = getPos(rest, "'", "'", jsStringMode);
      dfnuttpos = getPos(rest, '"', '"', jsStringMode);
      compos = getPos(rest, //*/, "*/", commentMode);
      comlinepos = getPos(rest, ////, "<br>", commentMode);      
      numpos = getNumPos(rest, jsNumberMode);
      keywordpos = getKeywordPos("js", rest, jsKeywordMode);
      dotpos = getDotPos(rest, jsPropertyMode);
      if (Math.max(numpos[0], sfnuttpos[0], dfnuttpos[0], compos[0], comlinepos[0], keywordpos[0], dotpos[0]) == -1) {break;}
      mypos = getMinPos(numpos, sfnuttpos, dfnuttpos, compos, comlinepos, keywordpos, dotpos);
      if (mypos[0] == -1) {break;}
      if (mypos[0] > -1) {
        done += rest.substring(0, mypos[0]);
        done += mypos[2](rest.substring(mypos[0], mypos[1]));
        rest = rest.substr(mypos[1]);
      }
    }
    rest = done + rest;
    for (i = 0; i < esc.length; i++) {
      rest = rest.replace("W3JSESCAPE", esc[i]);
    }
    return "<span style=color:" + jscolor + ">" + rest + "</span>";
  }
  function jsStringMode(txt) {
    return "<span style=color:" + jsstringcolor + ">" + txt + "</span>";
  }
  function jsKeywordMode(txt) {
    return "<span style=color:" + jskeywordcolor + ">" + txt + "</span>";
  }
  function jsNumberMode(txt) {
    return "<span style=color:" + jsnumbercolor + ">" + txt + "</span>";
  }
  function jsPropertyMode(txt) {
    return "<span style=color:" + jspropertycolor + ">" + txt + "</span>";
  }
  function getDotPos(txt, func) {
    var x, i, j, s, e, arr = [".","<", " ", ";", "(", "+", ")", "[", "]", ",", "&", ":", "{", "}", "/" ,"-", "*", "|", "%"];
    s = txt.indexOf(".");
    if (s > -1) {
      x = txt.substr(s + 1);
      for (j = 0; j < x.length; j++) {
        cc = x[j];
        for (i = 0; i < arr.length; i++) {
          if (cc.indexOf(arr[i]) > -1) {
            e = j;
            return [s + 1, e + s + 1, func];
          }
        }
      }
    }
    return [-1, -1, func];
  }
  function getMinPos() {
    var i, arr = [];
    for (i = 0; i < arguments.length; i++) {
      if (arguments[i][0] > -1) {
        if (arr.length == 0 || arguments[i][0] < arr[0]) {arr = arguments[i];}
      }
    }
    if (arr.length == 0) {arr = arguments[i];}
    return arr;
  }
  function getKeywordPos(typ, txt, func) {
    var words, i, pos, rpos = -1, rpos2 = -1, patt;
    if (typ == "js") {
      words = ["abstract","arguments","boolean","break","byte","case","catch","char","class","const","continue","debugger","default","delete",
      "do","double","else","enum","eval","export","extends","false","final","finally","float","for","function","goto","if","implements","import",
      "in","instanceof","int","interface","let","long","NaN","native","new","null","package","private","protected","public","return","short","static",
      "super","switch","synchronized","this","throw","throws","transient","true","try","typeof","var","void","volatile","while","with","yield"];
    }
    for (i = 0; i < words.length; i++) {
      pos = txt.indexOf(words[i]);
      if (pos > -1) {
        patt = /W/g;
        if (txt.substr(pos + words[i].length,1).match(patt) && txt.substr(pos - 1,1).match(patt)) {
          if (pos > -1 && (rpos == -1 || pos < rpos)) {
            rpos = pos;
            rpos2 = rpos + words[i].length;
          }
        }
      } 
    }
    return [rpos, rpos2, func];
  }
  function getPos(txt, start, end, func) {
    var s, e;
    s = txt.search(start);
    e = txt.indexOf(end, s + (end.length));
    if (e == -1) {e = txt.length;}
    return [s, e + (end.length), func];
  }
  function getNumPos(txt, func) {
    var arr = ["<br>", " ", ";", "(", "+", ")", "[", "]", ",", "&", ":", "{", "}", "/" ,"-", "*", "|", "%", "="], i, j, c, startpos = 0, endpos, word;
    for (i = 0; i < txt.length; i++) {
      for (j = 0; j < arr.length; j++) {
        c = txt.substr(i, arr[j].length);
        if (c == arr[j]) {
          if (c == "-" && (txt.substr(i - 1, 1) == "e" || txt.substr(i - 1, 1) == "E")) {
            continue;
          }
          endpos = i;
          if (startpos < endpos) {
            word = txt.substring(startpos, endpos);
            if (!isNaN(word)) {return [startpos, endpos, func];}
          }
          i += arr[j].length;
          startpos = i;
          i -= 1;
          break;
        }
      }
    }  
    return [-1, -1, func];
  }  
}
<!DOCTYPE html>
<html>
<body>

<code id="myDiv" contenteditable onkeydown="synataxHighlight(document.getElementById('myDiv'));">
&lt;!DOCTYPE html&gt
&lt;html&gt
&lt;body&gt
&lt;h1&gt;Testing an HTML Syntax Highlighter&lt;/h2&gt
&lt;p&gt;Hello world!&lt;/p&gt
&lt;/body&gt;
&lt;/html&gt;
</code>
</body>
</html>