index.js

import Controller from '@ember/controller';
import { toLeft, toRight } from 'ember-animated/transitions/move-over';

export default Controller.extend({
  activeTab: 1,

  rules({ firstTime, newItem, oldItem }) {
    // Only animate on change, not on the initial value.
    if (firstTime) {
      return;
    }

    // Higher-numbered tabs slide in from the right, lower from the left.
    return newItem > oldItem ? toLeft : toRight;
  }
});