Why is this map empty after array.reduce [duplicate]

Question

Why does this map print empty? I'd expect it to be populated from the array? I'm probably missing something small.

let numbers = [1, 2, 3];

let numberMap = numbers.reduce((map, obj) => {
  return map.set(obj, obj + 1);
}, new Map());

console.log(JSON.stringify(numberMap));