Deep Copy vs Shallow Copy with Promises

What does this output?

let original = { a: 1, b: { c: 2 } }
let shallowCopy = { ...original }
let deepCopy = JSON.parse(JSON.stringify(original))

Promise.resolve().then(() => {
  shallowCopy.a = 10
  shallowCopy.b.c = 20
  deepCopy.a = 30
  deepCopy.b.c = 40
  console.log(original.a, original.b.c)
  console.log(shallowCopy.a, shallowCopy.b.c)
  console.log(deepCopy.a, deepCopy.b.c)
})

Correct Output

1 2
10 20
30 40

Why this output?

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