Sass, z-index mixin for smart z-index management

_z-index.scss

// ----
// z-index mixin to help organize z-indexes
// ----

// Start with a map
$z-layers: (
    "cog":           50,
    "overlay":       40,
    "toc":           30,
    "iframe-loader": 25,
    "tooltip":       20
) !default;

@function z($layer) {
    @if not map-has-key($z-layers, $layer) {
        @warn "No layer found for `#{$layer}` in $z-layers map. Property omitted.";
    }
    @return map-get($z-layers, $layer);
}