Insert content into iFrame

Question

You really don't need jQuery for that:

var doc = document.getElementById('iframe').contentWindow.document;
doc.open();
doc.write('Test');
doc.close();

jsFiddle Demo

If you absolutely have to use jQuery, you should use contents():

var $iframe = $('#iframe');
$iframe.ready(function() {
    $iframe.contents().find("body").append('Test');
});

jsFiddle Demo

Please don't forget that if you're using jQuery, you'll need to hook into the DOMReady function as follows:

$(function() {  
    var $iframe = $('#iframe');
    $iframe.ready(function() {
        $iframe.contents().find("body").append('Test');
    });
});

This should do what you want:

$("#iframe").ready(function() {
    var body = $("#iframe").contents().find("body");
    body.append('Test');
});

Check this JSFiddle for working demo.

Edit: You can of course do it one line style:

$("#iframe").contents().find("body").append('Test');

You can enter (for example) text from div into iFrame:

var $iframe = $('#iframe');
$iframe.ready(function() {
    $iframe.contents().find("body").append($('#mytext'));
});

and divs:

<iframe id="iframe"></iframe>
<div id="mytext">Hello!</div>

and JSFiddle demo: link

Wait, are you really needing to render it using javascript?

Be aware that in HTML5 there is srcdoc, which can do that for you! (The drawback is that IE/EDGE does not support it yet https://caniuse.com/#feat=iframe-srcdoc)

See here [srcdoc]: https://www.w3schools.com/tags/att_iframe_srcdoc.asp

Another thing to note is that if you want to avoid the interference of the js code inside and outside you should consider using the sandbox mode.

See here [sandbox]: https://www.w3schools.com/tags/att_iframe_sandbox.asp