SOURCE

var a = function(){
			console.log(1)
		}
		var _a = a;

		a = function(){
			_a()
			console.log(2)
		}
		a()

		window.onload = function(){
			console.log(3)
		}
		var _onload = window.onload || function(){}

		window.onload = function(){
			_onload()
			console.log(4)
		}