Loading Multiple Functions Across Multiple Files?

I am familiar with the user built addEvent function used to load multiple functions in the window.onload property. However, I am curious as to the best way to do this across multiple JavaScript files. I thought I had seen somewhere code similar to the addEvent function was native to JavaScript now but I can't seem to find that anywhere.

Just to clarify, suppose I have 3 JavaScript files:

Code:
function foo() {
// do something here
}
fileB

[Code]....

note that it is not feasible to combine those functions into one file as they aren't always loaded together. What's the best way to load them all as a window.load?