Translate jQuery function to pure JavaScript

Question

It's a fairly simple jQuery function but I'm dealing with an iframe which will be displayed on clients' websites who may not be using jQuery. How would this look as pure JS?

$("#pgft-gvway-3xs-iframe").load(function() {
    $(this).height($(this).contents().find("html").height() );
});

UPDATE:

Thanks to everyone who answered. Using a combination of answers I used the following. It did what I needed it to:

 window.addEventListener('load', function(e) {
     var elem= document.getElementById("pgft-gvway-3xs-iframe");
     var insideheight= elem.contentWindow.document.height;
     elem.style.height= insideheight + "px";
 }, false);

Answer 1

As suggested by lix there are already answers for your questions in stack overflow.

For Pure JavaScript on Ready here For Pure JavaScript on height here

Answer 2

var elem= document.getElementById("pgft-gvway-3xs-iframe");
var insideheight= elem.contentWindow.document.height;