breakpoints.scss

$breakpoints: (
  screen: 640px
);

@mixin respond-above($breakpoint) {

  @if map-has-key($breakpoints, $breakpoint) {
    $breakpoint-value: map-get($breakpoints, $breakpoint); 
    @media (min-width: $breakpoint-value) {
      @content;
    }
  } @else {
    @warn 'Invalid breakpoint: #{$breakpoint}.';
  }
}

@mixin respond-below($breakpoint) {
  @if map-has-key($breakpoints, $breakpoint) {
    $breakpoint-value: map-get($breakpoints, $breakpoint);
    @media (max-width: ($breakpoint-value - 1)) {
      @content;
    }
  } @else {
    @warn 'Invalid breakpoint: #{$breakpoint}.';
  }
}

@mixin respond-between($lower, $upper) {
  @if map-has-key($breakpoints, $lower) and map-has-key($breakpoints, $upper) {
    $lower-breakpoint: map-get($breakpoints, $lower);
    $upper-breakpoint: map-get($breakpoints, $upper);
    @media (min-width: $lower-breakpoint) and (max-width: ($upper-breakpoint - 1)) {
      @content;
    }
  } @else {
    @if (map-has-key($breakpoints, $lower) == false) {
      @warn 'Your lower breakpoint was invalid: #{$lower}.';
    }
    @if (map-has-key($breakpoints, $upper) == false) {
      @warn 'Your upper breakpoint was invalid: #{$upper}.';
    }
  }
}

// thanks to Glenn McComb for these breakpoints SCSS mixins 
// https://glennmccomb.com/articles/useful-sass-scss-media-query-mixins-for-bootstrap/