javascript adding up numbers in sequence returns incorrect number when big numbers are passed

Question

So I was trying to compare the two functions and I realized that when you pass 1000000000 as n to both functions they don't return the same value, even though they should. The sum function returns 500000000067109000 when the correct answer is 500000000500000000. Why is this happening?

function sum(n) {
  let sum = 0;

  for (let i = 1; i <= n; i++) {
    sum += i;
  }
  return sum
}

function sum2(n) {
  return n * (n + 1) / 2
}

console.log(sum(1000000000)); //returns 500000000067109000 (incorrect)
console.log(sum2(1000000000)); //returns 500000000500000000 (correct)