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JavaScript Document.Write Replaces All Body Content When Using AJAX

I am creating a simple ajax call that retrieves the content of a specified url and writes it to the page. The problem I am having is that it replaces the entire body contents with this information

here is the JS:

(function(){
    var mb = window.mb = {};

    function get_ad(url, parameters){
        var result = "";
        var http_request = false;

        if (window.XMLHttpRequest) { // Mozilla, Safari,...
            http_request = new XMLHttpRequest();
            if (http_request.overrideMimeType) {
                http_request.overrideMimeType('text/html');
            }
        } else if (window.ActiveXObject) { // IE
            var avers = ["Microsoft.XmlHttp", "MSXML2.XmlHttp", "MSXML2.XmlHttp.3.0", "MSXML2.XmlHttp.4.0", "MSXML2.XmlHttp.5.0"];
            for (var i = avers.length -1; i >= 0; i--) {
                try {
                    http_request = new ActiveXObject(avers[i]);
                    if (http_request){
                        break;  
                    }
                } catch(e) {}
            }
        }
        if (!http_request) {
            alert('Cannot create XMLHTTP instance');
            return false;
        }

        http_request.onreadystatechange = function(){
                                              if (http_request.readyState == 4) {
                                                 if (http_request.status == 200) {
                                                    gen_output(http_request.responseText);
                                                 } else {
                                                    alert('Error');
                                                 }
                                              }
                                           }

        http_request.open('GET', url + parameters, true);
        http_request.send(null);
    }

    function gen_output(ad_content){
        document.write("<div id="mb_ad">");
        document.write(ad_content);
        document.write("</div>");
    }

    get_ad("http://localhost/test/test.html", "");
})();

and here is the html:

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>Untitled Document</title>
</head>

<body>
    i am text before <br/>
    <script type="text/javascript" src="mb.js"></script>
    <br />
    i am text after 
</body>
</html>

using firebug to inspect, i do not see the text before or the text after, just the <div id="mb_ad"> and the content from the test.html page. If i remove the ajax call and just do 3 document.writes the text before and the text after will display properly. jQuery is not an option, I have to do this without the help of a large library as size and speed are of the essence.

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1 Answer

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by (71.8m points)

You can't use document.write once the document has completed loading. If you do, the browser will open a new document that replaces the current.

Use the innerHTML property to put HTML code inside an element:

function gen_output(ad_content){
  document.getElementById('mb_ad').innerHTML = ad_content;
}

Put the element before the script, so that you are sure that it exists when the callback function is called:

i am text before
<div id="mb_ad"></div>
i am text after
<script type="text/javascript" src="mb.js"></script>

It doesn't matter much where you place the script, as nothing will be written to the document where it is.


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