Z-index management with SCSS

// Set our z-indexes map
$z-indexes: (
  "up-btn",
  //60
    "cookies-info",
  //50
    "left-nav",
  //40
    "footer",
  //30
    "top-nav",
  //20
    "content",
  //10
);

// Z function return the z-index value
@function z($name) {
  @if index($z-indexes, $name) {
    @return ((length($z-indexes) - index($z-indexes, $name)) + 1) * 10;
  } @else {
    @warn 'There is no item "#{$name}" in this list; choose one of: #{$z-indexes}';
    @return null;
  }
}

//------

.content {
  z-index: z("content");
}
.img-1 {
  z-index: z("content") + 2;
}
.img-2 {
  z-index: z("content") + 1;
}
.top-nav {
  z-index: z("top-nav");
}
.mkt-flash {
  z-index: z("top-nav") - 1;
}
.left-nav {
  z-index: z("left-nav");
}
.footer {
  z-index: z("footer");
}
.cookies-info {
  z-index: z("cookies-info");
}
.up-btn {
  z-index: z("up-btn");
}