Array ting

ting.js

console.log('hello');

const data = [
  {
    id: 1,
    country: [
      { id: '5a60626f1d41c80c8d3f8a85' },
      { id: '5a6062661d41c80c8b2f0413' }
    ]
  },
  {
    id: 2,
    country: [
      { id: '5a60626f1d41c80c8d3f8a83' },
      { id: '5a60626f1d41c80c8d3f8a84' }
    ]
  }
];

const result = data.reduce(
  (arr, elem) => [...arr, ...elem.country.map(({ id }) => id)],
  []
);

console.log(result);