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/// Remove the unit of a length
/// @param {Number} $number - Number to remove unit from
/// @return {Number} - Unitless number
@function strip-unit($number) {
  @if type-of($number) == 'number' and not unitless($number) {
    @return $number / ($number * 0 + 1);
  }

  @return $number;
}

//usage
$length: 42px;
$value: strip-unit($length);
// -> 42