Organizing z-index

z-index.scss

$z-index: (
  modal              : 200,
  navigation         : 100,
  footer             : 90,
  triangle           : 60,
  navigation-rainbow : 50,
  share-type         : 41,
  share              : 40,
);

@function z-index($key) {
  @return map-get($z-index, $key);
}

@mixin z-index($key) {
  z-index: z-index($key);
}

.navigation {
  @include z-index(navigation);
}

.modal {
  @include z-index(modal);
}