Sass z-index function

_function-z-index.scss

$z-index: (
    'under': -5,
    'zero': 0,
    'default': 5,
    'over': 10
);

$z-level: (    
    'footer': 20,
    'header': 30,
    'tooltip': 100,
    'dropdown': 200,
    'modal': 500
);

$z-combo: map-merge($z-index, $z-level);

@function z($index: null, $level: null) {
  @if $index and map-has-key($z-combo, $level) {
    @return map-get($z-combo, $index) + map-get($z-combo, $level);
  } 
  @else if $index == auto and $level == null {
    @return auto;
  }
  @else if $index == null and $level == null {
    @return auto;
  }
  @else if $index == null and $level {
    @return map-get($z-combo, $level);
  } 
  @else if $index == $level and $level == null map-has-key($z-combo, $index) {
    @return map-get($z-combo, $index);
  }
}

// examples

.index {
  z-index: z(over);
}

.index-level {
  z-index: z(dropdown, under);
}

.null-level {
  z-index: z(null, dropdown);
}

.level {
  z-index: (z(dropdown) + z(default)*4);
}

.auto {
  z-index: z(auto);
}

.null {
  z-index: z();
}

output.css

.index {
  z-index: 10;
}

.index-level {
  z-index: 195;
}

.null-level {
  z-index: 200;
}

.level {
  z-index: 220;
}

.auto {
  z-index: auto;
}

.null {
  z-index: auto;
}