setTimeout4.js

var a = [1, 2, 3, 4]

for (var ai = 0; ai < a.length; ai++) {

  setTimeout(function(){
    // By the time this code will be executed, all the iterations of 
    // for loop would have completed and value of ai would be 4
    console.log("Value of 'ai': " + ai)
  }, 1000)
  
}

// Here, window object will have variable or property ai with value 4 
// which will be accessed by setTimeout callback function, when it will be executed
console.log(window)

/* 
  Output:
  Value of 'ai':  4
  Value of 'ai':  4
  Value of 'ai':  4
  Value of 'ai':  4
*/