function expm1 (x) {
  // http://kevin.vanzonneveld.net
  // +   original by: Brett Zamir (http://brett-zamir.me)
  // %          note 1: Precision 'n' can be adjusted as desired
  // *     example 1: expm1(1e-15);
  // *     returns 1: 1.0000000000000007e-15
  var ret = 0,
    n = 50; // degree of precision
  var factorial = function factorial(n) {
    if ((n === 0) || (n === 1)) {
      return 1;
    } else {
      var result = (n * factorial(n - 1));
      return result;
    }
  };
  for (var i = 1; i < n; i++) {
    ret += Math.pow(x, i) / factorial(i);
  }
  return ret;
}

expm1(1e-15);

1.0000000000000007e-15