Custom Sass mixin for using media queries

sass-mixin-breakpoints.scss

// Define the breakpoints
$breakpoint-small: 600px;
$breakpoint-med-small: 960px;
$breakpoint-med: 1175px;

@mixin screen($size, $type: max, $pixels: $breakpoint-small) {
  @if $size == 'small' {
    @media screen and ($type + -width: $breakpoint-small) {
        @content;
    }
  }
  @else if $size == 'med-small' {
    @media screen and ($type + -width: $breakpoint-med-small) {
        @content;
    }
  }
  @else if $size == 'med' {
    @media screen and ($type + -width: $breakpoint-med) {
        @content;
    }
  }
 @else if $size == 'large' {
    @media screen and ($type + -width: $breakpoint-med) {
        @content;
    }
  }
  @else if $size == 'custom' {
    @media screen and ($type + -width: $pixels + px) {
     @content;
    }
  }
  @else {
    @content;
  }
}
// Using the mixin
.foo {
  @include screen(large) {
    width: 20%;
  }
  @include screen(med) {
    width: 40%;
  }
  @include screen(med-small) {
    width: 60%;
  }
  @include screen(small) {
    width: 80%;
  }
  @include screen(custom, max, 400) {
    width: 100%;
  }
}

Some better option:

$breakpoints: (
  'small'  : 767px,
  'medium' : 992px,
  'large'  : 1200px
);
@mixin screen($size, $type: max) {
  $width: map-get($breakpoints, $size);

  // If the key exists in the map
  @if $width == null {
    $width: $size;
  }

  @media screen and ($type + -width: $breakpoint-small) {
      @content;
  }
}