Как ограничить вызов функции в цикле, ровно 1 раз за итерацию?

var flagRunning = false; 
function iteration(left) { 
  if (flagRunning) { 
    console.log("Cannot run - already running."); 
    return; 
  } 
  flagRunning = true; 
  console.log("steps left: ", left); 
  // do something useful 
  if (--left > 0) { 
    setTimeout(function(aLeft) { 
      flagRunning = false; 
      iteration(aLeft); 
    }, 1500/*change this to 15000*/, left); 
  } else { 
    flagRunning = false; 
    console.log("Can run again."); 
  } 
} 
 
iteration(5);

<button onclick="iteration(5)">Click</button>