defines the addEvent and executes it immediately, and after executing it once, the execution function of addEvent is determined. You are misled by the returned execution function. After addEvent initialization, it has nothing to do with the original function body. You can change the way of thinking and change the return function of addEvent to return an object, which makes it easier to understand

var obj = (function(){
      console.log('init')
      if(window.addEventListener){
        return {
          func:window.addEventListener
        }
      } else {
        return {
          func:window.attachEvent
        }
      }
    })()

    console.log(obj.func)
    console.log(obj.func)
    console.log(obj.func)

isn't this the immediate execution of the function? It will only be run once at the beginning, and then addEvent saves the result of its run, that is, the returned function