Javascript: 2 ways to insert new function dynamically

I personally recommend the first way:

function addFunc() ...{
  var sFunc = ""
    +"function myOnUnLoad()"
    +"{"
      +"alert('unload...');"
    +"};"
    +"window.document.body.onunload = myOnUnLoad;";

  window.eval(sFunc);
}

Second way, by this way you have to write down all your code in a external js file, in this case, let us say myjs.js file:

function addFunc()
...{
  var o = window.document.createElement("script");
  o.type="text/javascript";
  o.src="myjs.js";

  //if you use o.innerHTML="..." or o.innerText="..." here, you will get a runtime error.
  //that is why you have to use a external js file, and use o.scr="myjs.js" to insert function code

  window.document.getElementsByTagName("head")[0].appendChild(o);
}

//-- code in myjs.js file
window.document.body.onunload = function ()
...{
  alert("unload...");
}