bootstrap/scss/mixins/_box-shadow.scss

@mixin box-shadow($shadow...) {
  @if $enable-shadows {
    $result: ();
    $has-single-value: false;
    $single-value: null;

    @each $value in $shadow {
      @if $value != null {
        @if $value == none or $value == initial or $value == inherit or $value == unset {
          $has-single-value: true;
          $single-value: $value;
        } @else {
          $result: append($result, $value, "comma");
        }
      }
    }

    @if $has-single-value {
      box-shadow: $single-value;
    } @else if (length($result) > 0) {
      box-shadow: $result;
    }
  }
}