As charlietfl said, in the future posts you must provide examples of code, that you’ve written to solve your problem.
Here’s the solution, it’s pretty simple:
const initialObject = {
  "items": [
    {
      "_id": "admin",
      "authorities": [
        {
          "name": "ROLE_ADMIN"
        }
      ]
    }, 
    {
      "_id": "user",
      "authorities": [
        {
          "name": "ROLE_USER"
        }
      ]
    }
  ]
}
const processedObject = _
  .map(initialObject.items, item => {
    return {
      _id: item._id,
      roles: _.map(item.authorities, a => a.name)
    }
  })
solved Transform an array to another array