TopMenu.vue

<template>
<div>
  <b-navbar toggleable="lg" type="dark" variant="info">
    <b-navbar-brand href="#" class="d-none d-md-block">Group Membership Service</b-navbar-brand>

    <b-navbar-toggle target="nav-collapse"></b-navbar-toggle>

    <b-collapse id="nav-collapse" is-nav>

      <!-- Right aligned nav items -->
      <b-navbar-nav class="ml-auto">
        <b-nav-form>
          <b-form-input size="sm" class="mr-sm-2" placeholder="Search"></b-form-input>
          <b-button size="sm" class="my-2 my-sm-0" type="submit">Search</b-button>
        </b-nav-form>

        <b-nav-item-dropdown right v-if="user">
          <template slot="button-content"><em>{{user}}</em></template>
          <b-dropdown-item href="#">Preferences</b-dropdown-item>
        </b-nav-item-dropdown>
      </b-navbar-nav>
    </b-collapse>
  </b-navbar>
</div>
</template>

<script>
export default {
  name: 'TopMenu',
  props: {
    user: String
  }
}
</script>