I want to be able to insert a Java applet into a web page dynamically using a Javascript function that is called when a button is pressed. (Loading the applet on page load
document.write()
Will overwrite your entire document. If you want to keep the document, and just want an applet added, you'll need to append it.
var app = document.createElement('applet');
app.id= 'Java';
app.archive= 'Java.jar';
app.code= 'Java.class';
app.width = '400';
app.height = '10';
document.getElementsByTagName('body')[0].appendChild(app);
This code will add the applet as the last element of the body tag. Make sure this is run after the DOM has processed or you will get an error. Body OnLoad, or jQuery ready recommended.