bootstrap/scss/utilities/_api.scss

@if $enable-utility-classes {
  // Loop over each breakpoint
  @each $breakpoint in map-keys($grid-breakpoints) {
    // Generate media query if needed
    @include media-breakpoint-up($breakpoint) {
      $infix: breakpoint-infix($breakpoint, $grid-breakpoints);

      // Loop over each utility property
      @each $key, $utility in $utilities {
        // The utility can be disabled with `false`, thus check if the utility is a map first
        // Only proceed if responsive media queries are enabled or if it's the base media query
        @if type-of($utility) == "map" and (map-get($utility, responsive) or $infix == "") {
          @include generate-utility($utility, $infix);
        }
      }
    }
  }

  // RFS rescaling
  @media (min-width: $rfs-mq-value) {
    @each $breakpoint in map-keys($grid-breakpoints) {
      $infix: breakpoint-infix($breakpoint, $grid-breakpoints);

      @if (map-get($grid-breakpoints, $breakpoint) < $rfs-breakpoint) {
        // Loop over each utility property
        @each $key, $utility in $utilities {
          // The utility can be disabled with `false`, thus check if the utility is a map first
          // Only proceed if responsive media queries are enabled or if it's the base media query
          @if type-of($utility) == "map" and map-get($utility, rfs) and (map-get($utility, responsive) or $infix == "") {
            @include generate-utility($utility, $infix, true);
          }
        }
      }
    }
  }


  // Print utilities
  @media print {
    @each $key, $utility in $utilities {
      // The utility can be disabled with `false`, thus check if the utility is a map first
      // Then check if the utility needs print styles
      @if type-of($utility) == "map" and map-get($utility, print) == true {
        @include generate-utility($utility, "-print");
      }
    }
  }
}

// Generate utility placeholders

$utilities-map: build-utilities-map(); // stylelint-disable-line scss/dollar-variable-default

@mixin util($class) {
  @if map-has-key($utilities-map, $class) {
    $definition: map-get($utilities-map, $class);
    $breakpoint: map-get($definition, breakpoint);
    @if $breakpoint != null {
      @include media-breakpoint-up($breakpoint) {
        @each $property in map-get($definition, properties) {
          #{$property}: map-get($definition, value);
        }
      }
    }
    @else {
      @each $property in map-get($definition, properties) {
        #{$property}: map-get($definition, value);
      }
    }
  }
  @else {
    @debug "Unknown utility class " + $class;
  }
}