D3 data selection

dataSelection.js

// html:
// <ul>
//   <li></li>
//   <li></li>
// </ul>

const users = [{
  id: 1,
  name: 'Nancy',
  role: 'person'
}, {
  id: 2,
  name: 'Bart',
  role: 'cartoon'
}, {
  id: 3,
  name: 'Maggie',
  role: 'cartoon'
}];

const selection = d3.select('ul.app-user-list')
  .selectAll('li.app-user')
  .data(users);

// prints "2"
// Nancy and Bart datums were matched to the two "li" elements in the document.
console.log(selection.size())