Style external links against a list of approved internal domains using only Scss

_externalLinks.scss

/// List of internal urls / not deemed external
/// @type list
$domains: (
    ('github.com'),
    ('google.com')
);

/// Build chained :not() for internal domains
/// @param {list} $list [$domains] - List of approved domains
/// @requires $domains
/// @content
/// @outputs built chained :not() selector for domains
/// @example
///     @include externallink() {
///         // Styles
///     }
/// @example
///     @include externallink($domainlist) {
///         // Styles
///     }
@mixin externallink($list: $domains) {
    $output: '[href^="http"]';

    @if (length($list) == 1) {
        $list: nth($list, 1);
    }

    @each $url in $list {
        $output: $output + ':not([href*="#{$url}"])';
    }

    @debug($output);

    &#{$output} {
        @content;
    }
}


a {
    @include externallink() {
        // External link styling
    }
}