Textbox Lines

<html>
<head>
<script>
var box; var code;

function init(){
 box = document.getElementById('html_text');
 code = document.getElementById('html_code');
 code.onkeyup = boxCheck;
}

String.prototype.indexesOf = function(x){
 var r = [];
 for(var i=0;i<this.length;i++) if(this[i] == x) r.push(i);
 return r;
}

function boxCheck(){
    var endLines = code.value.indexesOf('\n');
    box.style.display = '';
    for(var i=0;i<2;i++){
        if(endLines[i+1] == undefined && code.value.length - endLines[i] > 46){
            box.style.display = 'none';
            break;
        }
        if(endLines[i] - endLines[i+1] > 46){
            box.style.display = 'none';
            break;
        }
    }
    if(endLines.length == 0 && code.value.length > 45)
        box.style.display = 'none';
}
</script>
</head>
<body onload="init();">
<div id="html_text"><h5 style="
margin-top: 17px;
margin-left: 400px;
position: fixed;
color: black;
z-index: 1;
font-family: Arial, Helvetica, sans-serif;
border-style: solid;
border-width: thin;
padding-left: 5px;
padding-right: 5px;
padding-top: 5px;
padding-bottom: 3px;
-webkit-touch-callout: none;
-webkit-user-select: none;
-khtml-user-select: none;
-moz-user-select: none;
-ms-user-select: none;
user-select: none;">HTML</h5></div>

<textarea rows="20" style="
margin-left: 20px;
font-family: 'Courier New', Courier, monospace;
height: 280px;
width: 460px;
resize: none;
overflow: scroll;
overflow-x: hidden;
position: absolute;
z-index: 0;" id="html_code" spellcheck="false"></textarea>
</body>
</html>