NodeJS: Arithmetic series gets the wrong result for large numbers using the iteration

Code:

var sum_with_formula = function (n) {
  return n/2*(n+1);
}

var sum_with_loop = function (n) {
  var sum = 0;
  for (var i=1; i<=n; i++) {
    sum = sum + i;
  }
  return sum;
}

var n = 1000000000;
const assert = require('assert');
assert.equal(sum_with_formula(n), sum_with_loop(n));

Result:

assert.js:85
  throw new assert.AssertionError({
  ^
AssertionError: 500000000500000000 == 500000000067109000

Why is there a difference between calculations with iteration and calculations by formula?

But if n = 100000000, the result is correct.

If n = 200000000, the result is wrong:

AssertionError: 20000000100000000 == 20000000067108864

I am using NodeJS v6.9.2.

via Fakhri Izzudin