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appendChild method is not defined when using with a div element
<!doctype html>
<html>
    <head>
    </head>

    <body>
        <p>This is paragraph 1.</p>
        <p>This is paragraph 2.</p>
        <p>This is paragraph 3.</p>
        <p>This is paragraph 4.</p>
        <div>
            <p id="foo">This is paragraph 5.</p>
        </div>
        <script type="text/javascript" src="script.js"></script>
    </body>

</html>

(function(){
    var divElement = document.getElementsByTagName("div");
        el = document.createElement("p");
        content = document.createTextNode("This is text");
    el.appendChild(content);
    document.divElement.appendChild(el);
}());

var divElement = document.getElementsByTagName("div")[0];

document.divElement.appendChild(el);

divElement.appendChild(el);

(function() {
    var divElements = document.getElementsByTagName("div");
    var el = document.createElement("p");
    var content = document.createTextNode("This is text");

    el.appendChild(content);

    for (var i = 0; i < divElements.length; i++) {
        divElements[i].appendChild(el);
    }
}());