Take care... If you use document.write() after the page has loaded it will completely rewrite (i.e. obliterate) the page, not append to it...
To "...update the body text after it has loaded...", do as follows:
Say in your html you have <div id="blah">some text</div>
document.getElementById("blah").innerHTML = "Different Text";
Also, you can add child nodes (e.g. divs or whatever tag you want) with createElement() and appendChild()
e.g. say you have <div id="table_holder"> </div>
Then you can do:
var table_div = document.getElementById("table_holder");
var table_to_insert = document.createElement("table");
Similarly, you can add tr's as children of the table_to_insert and td's as their children... or whatever tags you want to add.
Hope that makes sense