What is the right way to add an onload function?

It seems my onload is replaced by wordpres’s.

I’m adding javascript in the header (not a link) from a plugin.

I tried the following no success:

function addLoadEvent(func) {
  var oldonload = window.onload;
  if (typeof window.onload != 'function') {
    window.onload = func;
  } else {
    window.onload = function() {
      if (oldonload) {
        oldonload();
      }
      func();
    }
  }
}
addLoadEvent(function() {
  /* my code here */
});

Is there an official way to do it?