raceCondition.js

window.globalVar = 0;
function eventHandler() { // handler
  if (window.globalVar == 0) {
    setTimeout(function() { // inside callback
      window.globalVar = window.globalVar + 1;
    });
  }
}
// Intuitively window.globalVar won't exceed 1 no matter how many times the eventHandler is triggered
// But consider the case: the eventHandler is attached to click event and two consecutive clicks happen so fast that
// following sequence of excution could happen:
//    handler 1 (if condition eval true)
// -> handler 2 (if condition eval true)
// -> inside callback 1
// -> inside callback 2
// As a result, window.globalVar becomes 2