Node sorting bug

index.js

// Run this with node 8
// npx node@8 ./index.js
const arr = [
  { i: 1 },
  { i: 2 },
  { i: 3 },
  { i: 4 },
  { i: undefined },
  { i: undefined },
  { i: undefined },
  { i: undefined },
  { i: undefined },
  { i: undefined },
  { i: undefined }
]

const res1 = arr
  .slice()
  .sort((a, b) => a.i - b.i)
  .map(item => item.i);

const res2 = arr
  .slice()
  .map(item => item.i)
  .sort((a, b) => a - b);

console.log(res1);
console.log(res2);

// On node 8 the results are different
// On node 10 the results are the same

this will output

npx node@8 ./index.js
[ 1,
  undefined,
  3,
  4,
  undefined,
  2,
  undefined,
  undefined,
  undefined,
  undefined,
  undefined ]
[ 1,
  2,
  3,
  4,
  undefined,
  undefined,
  undefined,
  undefined,
  undefined,
  undefined,
  undefined ]