load to iframe1:
IFrames (and their associated JavaScript files) are loaded asynchronously so that they don't block the rest of the page from loading. Downside is that to control when the JavaScript library has been loaded you'll need to add some kind of callback function at the end of the library. Upside is (compared to XHR and eval) that you're not constrained by the same origin policy (as long as the iframe src points to the same domain).
You can use parent
window's JavaScript functions (or DOM) via parent
variable. To access IFrames JavaScript functions from the parent
window you can use for example frames['ifr1'].document