_z-index.scss

// _z-index.scss

$zIndexList: (
  Background: -50100,
  ContentFront: 50100,
  UI: 50200,
  UIPopup: 50300,
  Modal: 50400,
);

@function zIndex($value, $list: $zIndexList) {
  $_i: map-get($list, $value);
  @if $_i {
    @return $_i;
  } @else {
    @error "Function `zIndex` has taken invalid argument: #{$value}.";
  }
}

// foo.scss

@import 'z-index';

.foo {
  z-index: zIndex(UI);
}