/// Native `url(..)` function wrapper
/// @param {String} $base - base URL for the asset
/// @param {String} $type - asset type folder (e.g. `fonts/`)
/// @param {String} $path - asset path
/// @return {Url}
@function asset($base, $type, $path) {
  @return url($base + $type + $path);
}

/// Returns URL to an image based on its path
/// @param {String} $path - image path
/// @param {String} $base [$base-url] - base URL
/// @return {Url}
/// @require $base-url
@function image($path, $base: $base-url) {
  @return asset($base, 'images/', $path);
}

/// Returns URL to a font based on its path
/// @param {String} $path - font path
/// @param {String} $base [$base-url] - base URL
/// @return {Url}
/// @require $base-url
@function font($path, $base: $base-url) {
  @return asset($base, 'fonts/', $path);
}

/// Native `url(..)` function wrapper
///
/// @param {String} $base - base URL for the asset
/// @param {String} $type - asset type folder (e.g. `fonts/`)
/// @param {String} $path - asset path
////
/// @return {Url} A url() ready value
@function asset($base, $type, $path) {
  @return url($base + $type + $path);
}

/// Returns URL to an image based on its path
///
/// @param {String} $path - image path
/// @param {String} $base [$base-url] - base URL
///
/// @require $base-url
///
/// @return {Url} A url() ready value
@function image($path, $base: $base-url) {
  @return asset($base, 'images/', $path);
}

/// Returns URL to a font based on its path
///
/// @param {String} $path - font path
/// @param {String} $base [$base-url] - base URL
///
/// @require $base-url
///
/// @return {Url} A url() ready value
@function font($path, $base: $base-url) {
  @return asset($base, 'fonts/', $path);
}

/// Converts a pixel value into an em
///
/// @param {Number} $pixel - A value in pixels to convert
/// @param {Number} $base-size [16px] - The default conversion rate
///
/// @return {Number} The corresponding em value
@function em($pixels, $base-size: 16px) {
  @return ($pixels / $base-size) * 1em;
}

Add px to em Sass helper function
Add px to em Sass helper function