import { applyPatch } from 'minimal-json-patch';

const originalDocument = {
  letters: ['a', 'b', 'd'],
};

const modifiedDocument = applyPatch(originalDocument, [
  { op: 'add', path: '/letters/2', value: 'c' },
]);

console.log(modifiedDocument.letters);
// [ 'a', 'b', 'c', 'd' ]