Generated by SassMeister.com.

SassMeister-input.scss

// ----
// libsass (v3.2.5)
// ----

$spacing: (
 xs: 2.5rem,
 s: 4rem,
 m: 6rem,
 l: 8rem,
 xl: 10rem
);

@function get-prev($key, $map: $spacing) {
  $values: map-values($map);
  $keys: map-keys($map);
  $idx: index($keys, $key);
  
  @return nth($values, ($idx - 1));
}

.something {
  test: get-prev(s);
}

SassMeister-output.css

.something {
  test: 2.5rem;
}