デバッグコンソールで　front.js:8 Uncaught ReferenceError: browser is not defined　出る。

Javasript 初心者です。

今、htmlで下記のようにjavascriptを読み込ませているのですが、
デバッグコンソールに、「front.js:8 Uncaught ReferenceError: browser is not defined
at HTMLDocument.<anonymous>」と出ます。
ソースHtmlは、下記になります。

<!DOCTYPE html>
<html>
<head><title>岡村ちゃんフアン交流チャット</title>
<LINK href="./css_files/default.css" type=text/css rel=stylesheet>
<script type="text/javascript"	language="JavaScript" src="rpg4.js" ></script>

</head>

<body>

<div class="main2">
<iframe name="main"  id="main"
	src="/11/rpg8.cgi?iframe=main&name=nana&entry=1529784667&file=char0.png"
	scrolling="no"  height="400px" width="100%"  frameborder="0" 
	>
</iframe>
</div>
<div class="main2">
<iframe name="input" id="input"
	src="/11/rpg8.cgi?iframe=input&name=nana&entry=1529784667"
	scrolling="no" height="220px" width="100%"  frameborder="0" 
	>
</iframe>
</div>
</body>
</html>

javascript

function DomToolkit()
{
	if(document.all) {
		// IE
		Layer.prototype.setVisibility = function(f) {
			this.div.style.visibility = (f ? "visible" : "hidden");
		};
		Layer.prototype.getX = function() {
			return this.div.style.pixelLeft;
		};
		Layer.prototype.getY = function() {
			return this.div.style.pixelTop;
		};
		Layer.prototype.moveTo = function(x, y) {
			this.div.style.pixelLeft = x;
			this.div.style.pixelTop  = y;
		};
		Layer.prototype.moveBy = function(dx, dy) {
			this.div.style.pixelLeft += dx;
			this.div.style.pixelTop  += dy;
		};
		
		this.getLayer = function(id) {
			return new Layer(document.all[id]);
		};
	}
	else if(document.getElementById) {
		// Mozilla
		Layer.prototype.setVisibility = function(f) {
			this.div.style.visibility = (f ? "visible" : "hidden");
		};
		Layer.prototype.getX = function() {
			return parseInt(this.div.style.left);
		};
		Layer.prototype.getY = function() {
			return parseInt(this.div.style.top);
		};
		Layer.prototype.moveTo = function(x, y) {
			this.div.style.left = x;
			this.div.style.top  = y;
		};
		Layer.prototype.moveBy = function(dx, dy) {
			this.div.style.left = parseInt(this.div.style.left) + dx;
			this.div.style.top  = parseInt(this.div.style.top) + dy;

		};
		
		this.getLayer = function(id) {
			return new Layer(document.getElementById(id));
		};
	}
	else {
		// NN4
		Layer.prototype.setVisibility = function(f) {
			this.div.visibility = (f ? "show" : "hide");
		};
		Layer.prototype.getX = function() {
			return this.div.left;
		};
		Layer.prototype.getY = function() {
			return this.div.top;
		};
		Layer.prototype.moveTo = function(x, y) {
			this.div.moveTo(x, y);
		};
		Layer.prototype.moveBy = function(dx, dy) {
			this.div.moveBy(dx, dy);
		};
		
		this.getLayer = function(id) {
			var	i = arguments.length - 1;
			var	layer = document.layers[arguments[i--]];
			while(i >= 0 && layer) {
				layer = layer.layers[arguments[i--]];
			}
			return new Layer(layer);
		};
	}
}

function Layer(div)
{
	this.div = div;
}


var domtool;		// DOM依存のツール

var left, top, width, height, cell;
var mv = 3;
var pathinfo;
var reloading = false;

var ctrlbreak = false;
var movingID = null;


function init(x, y, w, h, c, t)
{
	domtool = new DomToolkit();
	
	left = x;
	top = 0;
	width = w;
	height = 288;
	cell = c;
	
	pathinfo = location.search.replace(/&px=\d*|&py=\d*|&ctrl=\d*|&?msg=[^&]*/ig, "");
	setTimeout("reloadFrame()", t * 1000);
	
	setInterval("breakControl()", 60);
	
	setTimeout("initEventHandler()", 100);
}

function initEventHandler()
{
	// 入力フレームの生成待ち
	if(!parent.input
	|| !parent.input.document
	|| !parent.input.document.cursor
	|| !parent.input.document.cursor.down) {
		setTimeout("initEventHandler()", 100);
		return;
	}
	
	if(document.all) {
		// IE
		document.onkeypress = handleKeypress;
		
		with(parent.input) {
			document.cursor.left.onmousedown = handleMouseDown;
			document.cursor.right.onmousedown = handleMouseDown;
			document.cursor.up.onmousedown = handleMouseDown;
			document.cursor.down.onmousedown = handleMouseDown;
			document.onmouseup = handleMouseUp;
		}
	}
	else if(document.getElementById) {
		// Mozilla
		document.addEventListener("keypress", handleKeypress, true);
		
		with(parent.input) {
			document.cursor.left.addEventListener("mousedown", handleMouseDown, true);
			document.cursor.right.addEventListener("mousedown", handleMouseDown, true);
			document.cursor.up.addEventListener("mousedown", handleMouseDown, true);
			document.cursor.down.addEventListener("mousedown", handleMouseDown, true);
			document.addEventListener("mouseup", handleMouseUp, false);
		}
	}
	else {
		// NN4
		document.onkeypress = handleKeypress;
		document.captureEvents(Event.KEYPRESS);
		
		with(parent.input) {
			document.cursor.left.onmousedown = handleMouseDown;
			document.cursor.right.onmousedown = handleMouseDown;
			document.cursor.up.onmousedown = handleMouseDown;
			document.cursor.down.onmousedown = handleMouseDown;
			document.onmouseup = handleMouseUp;
			document.captureEvents(Event.MOUSEUP);
		}
	}
}

function handleKeypress(event)
{
	if(ctrlbreak) {
		return false;
	}
	ctrlbreak = true;
	
	if(document.all) {
		key = window.event.keyCode;
	}
	else {
		key = event.which;
	}
	moveChar(String.fromCharCode(key));
}

function breakControl()
{
	ctrlbreak = false;
}


function handleMouseDown(event) {
	if(movingID != null) {
		clearInterval(movingID);
		movingID = null;
	}
	
	if(document.all) {
		button = parent.input.event.srcElement.name;
	}
	else {
		button = event.target.name;
	}
	
	switch(button) {
	case 'left':
		keych = '4'; break;
	case 'right':
		keych = '6'; break;
	case 'up':
		keych = '8'; break;
	case 'down':
		keych = '2'; break;
	default:
		keych = '5';
	}
	movingID = setInterval("moveChar('" + keych + "')", 60);
}

function handleMouseUp(event) {
	if(movingID != null) {
		clearInterval(movingID);
		movingID = null;
	}
}

function reloadFrame()
{
	if(reloading) {
		return;
	}
	reloading = true;
	
	layer = domtool.getLayer("char0");
	x = layer.getX();
	y = layer.getY();
	location.search = pathinfo + "&px=" + x + "&py=" + y;
}

function moveChar(key)
{
	if(reloading) {
		return;
	}
	
	layer = domtool.getLayer("char0");
	x = layer.getX();
	y = layer.getY();
	
	dx = 0;
	dy = 0;
	switch(key) {
	case '4':		// left
		dx = -mv;
		if(x + dx < left) {
			location.search = pathinfo + "&px=" + x + "&py=" + y + "&ctrl=4";
			reloading = true;
		}
		break;
	
	case '6':		// right
		dx = mv;
		if(x + dx + cell > left + width) {
			location.search = pathinfo + "&px=" + x + "&py=" + y + "&ctrl=6";
			reloading = true;
		}
		break;
	
	case '8':		// up
		dy = -mv;
//		if(y + dy < top) {
		if(y + dy < 0) {
			location.search = pathinfo + "&px=" + x + "&py=" + y + "&ctrl=8";
			reloading = true;
		}
		break;
	
	case '2':		// down
		dy = mv;
//		if(y + dy + cell > top + height) {
		if(y + dy + cell > 288 ) {
			location.search = pathinfo + "&px=" + x + "&py=" + y + "&ctrl=2";
			reloading = true;
		}
		break;
	}
	
	if(!reloading) {
		layer.moveBy(dx, dy);
		
		layer2 = domtool.getLayer("balloon0");
		layer3 = domtool.getLayer("text0");
		
		if(layer2.div && layer3.div) {
			layer2.moveBy(dx, dy);
			layer3.moveBy(dx, dy);
		}
	}
}

function getCharLeft()
{
	return domtool.getLayer("char0").getX();
}

function getCharTop()
{
	return domtool.getLayer("char0").getY();
}

エラーが、
front.js:8 Uncaught ReferenceError: browser is not defined
at HTMLDocument.<anonymous> (front.js:8)
(anonymous) @ front.js:8
と、出ます。

申し訳ございませんが、アドバイスの程、宜しくおねがいします。