Convert timestamp to date string in JavaScript

convert_timestamp.js

function timestampToDateStr(timestamp) {
  var format = 'Y/M/D h:m:s';
  var formatArr = ['Y','M','D','h','m','s'];
  var returnArr = [];

  var formatNumber = function(n) {
    n = n.toString();
    return n[1] ? n : '0' + n;
  };
  var date = new Date(timestamp * 1000);
  returnArr.push(date.getFullYear());
  returnArr.push(formatNumber(date.getMonth() + 1));
  returnArr.push(formatNumber(date.getDate()));

  returnArr.push(formatNumber(date.getHours()));
  returnArr.push(formatNumber(date.getMinutes()));
  returnArr.push(formatNumber(date.getSeconds()));

  for (var i in returnArr) {
    format = format.replace(formatArr[i], returnArr[i]);
  }
  return format;
}