JSFiddle - React, Tailwind, and code Playground

JavaScript

function BadUseOfForLoop(){
  var arr = [1,2,3,4];
    for (key in arr){
      console.log(arr[key]);
    }  
}
BadUseOfForLoop();
console.log("Extend Array...");
Array.prototype.sortBy = function(){
   return "Doesnt matter...";
};
BadUseOfForLoop();