code: function checkDuration (val, name, vnode) {
    if (typeof val !== 'number') {
      warn(
        "<transition> explicit " + name + " duration is not a valid number - " +
        "got " + (JSON.stringify(val)) + ".",
        vnode.context
      );
    } else if (isNaN(val)) {
      warn(
        "<transition> explicit " + name + " duration is NaN - " +
        'the duration expression might be incorrect.',
        vnode.context
      );
    }
  }
docs: only used in dev mode