construct object with key and value as list dynamically in angular js

Use Array.reduce() to convert the array of objects into a object derived from the array data.

const input = [
    {name: 'abc', country : 'US'},
    {name: 'xyz', country : 'IN'},
    {name: 'mno', country : 'US'},
    {name: 'pqr', country : 'IN'}
];

// use reduce to loop over each element and return a constructed object
const output = input.reduce(function(accumulator, element) {
  // check to see if there is an entry for this country
  if (!accumulator[element.country]) {
    // if not create a new array with just this name as the only entry
    accumulator[element.country] = [element.name];
  } else {
    // already exists, push the new value
    accumulator[element.country].push(element.name);
  }
  // return the object
  return accumulator;
}, {}); // <- initial value

The variable output results in -

{
  "US": [
    "abc",
    "mno"
  ],
  "IN": [
    "xyz",
    "pqr"
  ]
}