Conflict js files

I have 3 js files with scripts. But they conflict together. Works only one script. I connect all of them inside head tag. Each of them have such code for waiting when window to load

((window, document) => {
   window.onload = () => {
      //  code
   }
})(window, document, undefined)

May it be reason of conflicts? When all code in one file it works good.

>Solution :

You can only have ONE onload. Instead use an eventListener for each of them

((window, document) => {
   window.addEventListener("load", () => {
      //  code
   })
})(window, document, undefined)