code with object.assign in it

oassign-code.js

import "babel-polyfill";

const backendResponse = [
  { title: '', timestamp: '1960-04-15T18:34:38+05:00' },
  { title: '', timestamp: '2007-05-15T18:34:38+05:00' }
];

const timeline = backendResponse.map(event => {
  const date = new Date(event.timestamp).toLocaleDateString('en-US');
  return Object.assign(event, { date }); // <=
});

console.log('timeline', timeline);