Example of how to utilize setTimeout and accumulation to have non blocking long lasting recurrent calculations

non_blocking_factorial.js

function _factorial(acc, n, callback){
  if(n==0){
    callback(acc);
  }else{
    var callback_wrapper = function(result){
       callback(result);
    };
    setTimeout(function(){_factorial(acc * n, n-1, callback_wrapper)}, 10);
  }
}

function factorial(n, callback){
  _factorial(1, n, callback);
}

factorial(1000, function(result){console.log(result)});