Non-blocking execution of expensive function

non-blocking.js

var data = [1, 2, 3];
var REFRESH_RATE = 16.67;

function veryExpensive(item) {
  // ...
}

function forEachExpensive(data, callback) {
  var index = 0;
  var dataLength = data.length;
  return new Promise(function(success) {
    var callRecursively = function() {
      callback(data[index]);
      index++;
      window.setTimeout(function() {
        if (index < dataLength) {
          callRecursively();
        } else {
          success();
        }
      }, REFRESH_RATE);
    };
    callRecursively();
  });
}

forEachExpensive(data, veryExpensive).then(() => console.log('done'));
forEachExpensive(data, veryExpensive).then(() => console.log('done'));