Strange JavaScript behavior in CodePen with humongous arrays

When you write code in Codepen - they actually don't execute it as-is but rather first apply some transformations to it.

They parse it into an abstract syntax tree, find loops and insert instructions explicitly to stop executing the loop if too much time has passed.

When you do:

for(let i = 0; i < 114000000; i++ ){
  arr.push(new Point());
  avg += arr[i].x / 1000;
}

Your code runs as:

for (var i = 0; i < 114000000; i++) {
    if (window.CP.shouldStopExecution(1)) { // <- injected by Codepen!!!
        break;
    }
    arr.push(new Point());
    avg += arr[i].x / 1000;
    iter++;
}

You can see this by inspecting the frame code inside CodePen itself.

They inject shouldStopLoop calls inside your code. They have a script called stopExecutionOnTimeout which does something like this (source from Codepen):

 var PenTimer {
   programNoLongerBeingMonitored:false,
   timeOfFirstCallToShouldStopLoop:0, // measure time
   _loopExits:{}, // keep track of leaving loops
   _loopTimers:{}, // time loops
   START_MONITORING_AFTER:2e3, // give the script some time to bootstrap
   STOP_ALL_MONITORING_TIMEOUT:5e3, // don't monitor after some time
   MAX_TIME_IN_LOOP_WO_EXIT:2200, // kill loops over 2200 ms
   exitedLoop:function(o) { // we exited a loop 
     this._loopExits[o] = false; // mark
   },
   shouldStopLoop:function(o) { // the important one, called in loops
      if(this.programKilledSoStopMonitoring)  return false; // already done
      if(this.programNoLongerBeingMonitored)return true;
      if(this._loopExits[o])  return false; 
      var t=this._getTime(); // get current time
      if(this.timeOfFirstCallToShouldStopLoop === false) 
        this.timeOfFirstCallToShouldStopLoop = t;
        return false;
      }
      var i= t - this.timeOfFirstCallToShouldStopLoop; // check time passed
      if(i<this.START_MONITORING_AFTER) return false; // still good   
      if(i>this.STOP_ALL_MONITORING_TIMEOUT){
        this.programNoLongerBeingMonitored = true;
        return false;
      }
      try{
        this._checkOnInfiniteLoop(o,t);
      } catch(n) {
        this._sendErrorMessageToEditor(); // send error about loop
        this.programKilledSoStopMonitoring=false;
        return true; // killed
      }
      return false; // no need
   },
   _sendErrorMessageToEditor:function(){/*... */
      throw "We found an infinite loop in your Pen. We've stopped the Pen from running. Please correct it or contact [email protected].";
};

If you want to run it yourself - JSBin has similar functionality and they have open sourced it as the loop-protect library - under 500 LoC.