Get & Set SharePoint PeoplePicker login user info via REST api.

SetLoginUserInfo.js

$(document).ready(function () {
  SP.SOD.executeFunc('sp.js', 'SP.ClientContext', SetCurrentUserInfo);
});

function SetCurrentUserInfo() {
  $.ajax({
    url: _spPageContextInfo.webAbsoluteUrl + "/_api/SP.UserProfiles.PeopleManager/GetMyProperties",
    contentType: "application/json;odata=verbose",
    headers: { "accept": "application/json;odata=verbose" },
    success: onSuccess,
    error: onError
  });
}

function onSuccess(data, request) {
  // User Account
  var loginName = data.d.AccountName;
  // Display Name
  var dispName = data.d.DisplayName;
  // Department
  var arr = data.d.UserProfileProperties.results;
  var dept;
  for (var i = 0, len = arr.length; i < len; i++) {
    if (arr[i].Key == "Department") {
      dept = arr[i].Value;
    }
  }
  //Set PeoplePicker 
  var PeoplePickerId = $("div[title='Author']").attr('id');
  var PeoplePickerInput = $("input[title='Author']");
  PeoplePickerInput.val(dispName);
  var PeoplePicker = SPClientPeoplePicker.SPClientPeoplePickerDict[PeoplePickerId];
  PeoplePicker.AddUnresolvedUserFromEditor(true);

  //Set Department to text field.
  $("input[title='Department']").val(dept);

}

function onError(error) {
  console.log(error);
}