Generated by SassMeister.com.

SassMeister-input.scss

// ----
// libsass (v3.3.2)
// ----

// Full range of available breakpoints
$global-breakpoints: (
	'XS': 300px,
	'S':  490px,
	'M':  740px,
	'L':  980px,
	'XL': 1220px
);

// Range of breakpoints available due to any limitation
$available-breakpoints: ('XS': 1, 'XL': 3);

@function nearest-breakpoint($available-breakpoints, $target-name) {
	$available-keys: map-keys($available-breakpoints);
	$all-keys: map-keys($global-breakpoints);

	$nearest: 1;
	$target-index: index($all-keys, $target-name);
	$difference: abs($target-index - $nearest);

	@each $key in $available-keys {
	  $i: index($available-keys, $key);
		$relative-index: index($all-keys, $key);
		$new-difference: abs($target-index - $relative-index);

		@if ($new-difference < $difference) {
			$difference: $new-difference;
			$nearest: $i;
		}
	}

	@return nth($available-keys, $nearest);
}

.the-nearest-available-breakpoint {
  to-S: nearest-breakpoint($available-breakpoints, 'S' );
  to-M: nearest-breakpoint($available-breakpoints, 'M' );
  to-L: nearest-breakpoint($available-breakpoints, 'L' );
}

SassMeister-output.css

.the-nearest-available-breakpoint {
  to-S: "XS";
  to-M: "XS";
  to-L: "XL";
}