For your specific case the easiest fix (in my opinion) is to change the double quotes for the attributes in object to single ones (else it ends the document.write string). This should work

<script type="text/javascript">
    var stream = location.pathname;
    switch (stream) {
       case "/defatank":
          document.write("<object type='application/x-shockwave-flash' height='378' width='620' id='live_embed_player_flash' data='http://www.twitch.tv/widgets/live_embed_player.swf?channel=defatank' bgcolor='#000000'><param name='allowFullScreen' value='true' /><param name='allowScriptAccess' value='always' /><param name='allowNetworking' value='all' /><param name='movie' value='http://www.twitch.tv/widgets/live_embed_player.swf' /><param name='flashvars' value='hostname=www.twitch.tv&channel=defatank&auto_play=true&start_volume=25' /></object>");
          break;
       case "/seeingblue":
          document.write("SeeingBlue's Live Stream Page");
          break;
       case "/shiroshii":
          document.write("Shiroshii's Live Stream Page");
          break;
       case "/theend66":
          document.write("TheEnd66's Live Stream Page");
          break;
       case "/wakawaka647":
          document.write("WakaWaka647's Live Stream Page");
          break;
       case "/xtheguythatplays":
          document.write("XTheGuyThatPlays' Live Stream Page");
          break;
    }
</script>

The JavaScript interpreter needs to know where a string begins and ends and the quote marks are used to do that. You can use single or double quotes for JavaScript strings. If you want to use the same quote type in a string that you used to deliminate the string, then that quote characters must be escaped - otherwise the interpreter gets confused and thinks the string has ended early.

Some examples

1. "Good string"
1. "Broken "hi" string"
1. "Good 'hi' string"
1. "Good \"hi\" string"

The 2nd string will not work because the interpreter thinks it ended just before the word hi - as indicated by the double quotes.

The 3rd string is fine because it uses single quotes for hi instead.

The 4th string is also fine because it "escapes" the double quote characters to let the interpreter know the following character should be treated literally and not as JavaScript code.

There's a little more details on JavaScript strings here: http://www.w3schools.com/js/js_obj_string.asp

So in summary, as the HTML spec allows for single, double, or not quoted attributes, changing the double quotes to single resolves the issue.

You need to escape the quotation marks inside the string if you use quotation marks as delimiters. Usually I use apostropes to delimit a string that has quotation marks.

(Note however that the & characters in the attributes should be HTML encoded as & to make it correct HTML code.)

document.write('<object type="application/x-shockwave-flash" height="378" width="620" id="live_embed_player_flash" data="http://www.twitch.tv/widgets/live_embed_player.swf?channel=defatank" bgcolor="#000000"><param name="allowFullScreen" value="true" /><param name="allowScriptAccess" value="always" /><param name="allowNetworking" value="all" /><param name="movie" value="http://www.twitch.tv/widgets/live_embed_player.swf" /><param name="flashvars" value="hostname=www.twitch.tv&amp;channel=defatank&amp;auto_play=true&amp;start_volume=25" /></object>');

根据http://www.w3schools.com/jsref/met_doc_write.asp，

The write() method writes HTML expressions or JavaScript code to a document.

这意味着您不能将 DOMObject 或常规 Object 传递到 document.write。如果您列出的那个“对象”实际上是一个字符串，那么您可以将它传递给 document.write。如果您尝试将 DOMObject 添加到页面，请尝试使用document.body.addChild(object);

你肯定不想用document.write。相反，使用 DOM 方法来创建和附加元素：

var obj = document.createElement('object'),
    param = document.createElement('param');
object.setAttribute('type', 'application/x-shockwave-flash');
param.setAttribute('name', 'allowFullScreen');
obj.appendChild(param);
// rest of params and attributes
document.body.appendChild(obj);