Compass mixin retira display

input.scss

@mixin background-image-retina($file, $type) {
    $fileName : $file + '.' + $type;
    $retinaFileName : $file + '@2x.' + $type;
    
    $width : image-width($fileName);
    $height : image-height($fileName);

    width: $width;
    height: $height;
    
    background-image: url($file + '.' + $type);

    @media (-webkit-min-device-pixel-ratio: 2), (-moz-min-device-pixel-ratio: 2) {
        & {
            background-image: url($retinaFileName);
            -webkit-background-size: $width $height;
        }
    }
}

// Example
#foo {
  @include background-image-retina('foobar', 'png', 10px, 20px);
  background: repeat;
}

output.css

#foo {
  width : $width;
  height: $height;
  background-image: url("foobar.png");
  background: repeat; }
  @media (-webkit-min-device-pixel-ratio: 2) {
    #foo {
      background-image: url("foobar@2x.png");
      -webkit-background-size: 10px 20px; } }

i tried of compressing a application build with d3js and jquery.using gulp.i got node_modules folder as a result.how to know that build is success.and how to use the compressed gulp file