$weights: (
  'thin'         : 100,
  'extra-light'  : 200,
  'ultra-light'  : 200,
  'light'        : 300,
  'normal'       : 400,
  'book'         : 400,
  'regular'      : 400,
  'medium'       : 500,
  'semi-bold'    : 600,
  'demi-bold'    : 600,
  'bold'         : 700,
  'extra-bold'   : 800,
  'ultra-bold'   : 900,
  'heavy'        : 900,
  'black'        : 900,
  'ultra'        : 900
);

@function fw($weight) {

  $output: $weight;
  @if map-has-key($weights, $weight) {
    $output: map-get($weights, $weight);
  };

  @return $output;
}