AppendChild() is not a function javascript

Your div variable is a string, not a DOM element object:

var div = '<div>top div</div>';

Strings don’t have an appendChild method. Instead of creating a raw HTML string, create the div as a DOM element and append a text node, then append the input element:

var div = document.createElement('div');
div.appendChild(document.createTextNode('top div'));

div.appendChild(element);