semis.js

const Todo = (props, send)=> {
  const todo = props.todo;
  const index = props.index;

  return html`
    <li>
      <input
        type="checkbox"
        ${todo.completed ? 'checked' : ''}
        onchange=${(e)=> onChange(e, index)}
      />
      ${todo.title}
    </li>
  `;

  function onChange(e, index) {
    const updates = { completed: e.target.checked };
    send('updateTodo', { index: index, updates: updates });
  }
};