Angular 2 routerLinkActive always active for base path

As suggested by @TomRaine in this answer, you can add the property [routerLinkActiveOptions]="{ exact: true }" to your element:

<nav>
  <a [routerLink]="["https://stackoverflow.com/"]" 
      [routerLinkActive]="['nav-active']"
      [routerLinkActiveOptions]="{ exact: true }">
    HOME
  </a>
  ...
</nav>