You can use Object.assign() to merge them into a new object:

const response = {
  lat: -51.3303,
  lng: 0.39440
}

const item = {
  id: 'qwenhee-9763ae-lenfya',
  address: '14-22 Elder St, London, E1 6BT, UK'
}

const newItem = Object.assign({}, item, { location: response });

console.log(newItem );

You can also use object spread, which is a Stage 4 proposal for ECMAScript:

const response = {
  lat: -51.3303,
  lng: 0.39440
}

const item = {
  id: 'qwenhee-9763ae-lenfya',
  address: '14-22 Elder St, London, E1 6BT, UK'
}

const newItem = { ...item, location: response }; // or { ...response } if you want to clone response as well

console.log(newItem );