Clamping a Number

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In computer science, we use the word clamp as a way to restrict a number between two other numbers. When clamped, a number will either keep its own value if living in the range imposed by the two other values, take the lower value if initially lower than it, or the higher one if initially higher than it.

As a quick example before going any further:

$clamp: clamp(42, $min: 0, $max: 1337);    // 42
$clamp: clamp(42, $min: 1337, $max: 9000); // 1337
$clamp: clamp(42, $min: 0, $max: 1);       // 1

Coming back to CSS. There are a few ways where you might need a number to be restricted between two others. Take the opacity property for instance: it must be a float (between 0 and 1). This is typically the kind of value you would want to clamp to make sure it’s neither negative nor higher than 1.

Implementing a clamp function in Sass can be done in two ways, both strictly equivalent yet one is much more elegant than the other. Let’s start with the not so great one.

#The dirty one

This version relies on nested if function calls. Basically we say: if $number is lower than $min, then keep $min, else if $number is higher than $max, then keep $max, else keep $number.

/// Clamp `$number` between `$min` and `$max`
/// @param {Number} $number - Number to clamp
/// @param {Number} $min - Minimum value
/// @param {Number} $max - Maximum value
/// @return {Number}
@function clamp($number, $min, $max) {
  @return if($number < $min, $min, if($number > $max, $max, $number));
}

If you are not very confident with nested if calls, think of the previous statement as:

@if $number < $min {
  @return $min;
} @else if $number > $max {
  @return $max;
}

@return $number;

#The clean one

This version is much more elegant since it makes a good use of both min and maxfunctions from Sass.

/// Clamp `$number` between `$min` and `$max`
/// @param {Number} $number - Number to clamp
/// @param {Number} $min - Minimum value
/// @param {Number} $max - Maximum value
/// @return {Number}
@function clamp($number, $min, $max) {
  @return min(max($number, $min), $max);
}

Literally means keep the minimum between $max and the maximum between $number and $min.

#Example

Now, let’s create a little opacity mixin just for the sake of demonstration:

/// Opacity mixin
/// @param {Float} $value - Opacity value
/// @output `opacity`
@mixin opacity($value) {
  $clamped-value: clamp($value, 0, 1);

  @if $value != $clamped-value {
    @warn "Mixin `opacity` needs a float; #{$value} given, clamped to #{$clamped-value}.";
  }

  opacity: $clamped-value;
}
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