---
title: 'Breaking Change: Strict Function Units'
introduction: >
  Various built-in functions will become stricter in which units they allow
  and will handle those units more consistently. This makes Sass more compatible
  with the CSS spec and helps catch errors more quickly.
---

{% markdown %}
  ## Hue

  {% compatibility 'dart: "1.32.0"', 'libsass: false', 'ruby: false' %}{% endcompatibility %}

  When specifying a color's hue, CSS allows any [angle unit][] (`deg`, `grad`,
  `rad`, or `turn`). It also allows a unitless number, which is interpreted as
  `deg`. Historically, Sass has allowed *any* unit, and interpreted it as `deg`.
  This is particularly problematic because it meant that the valid CSS
  expression `hsl(0.5turn, 100%, 50%)` would be allowed by Sass but interpreted
  entirely wrong.

  [angle unit]: https://drafts.csswg.org/css-values-4/#angles

  To fix this issue and bring Sass in line with the CSS spec, we're making
  changes in multiple phases:

  ### Phase 1{#hue-phase-1}

  {% compatibility 'dart: "1.32.0"', 'libsass: false', 'ruby: false' %}{% endcompatibility %}

  At first, Sass just emitted a deprecation warning if you passed a number with
  a unit other than `deg` as a hue to any function. Passing a unitless number is
  still allowed.

  ### Phase 2{#hue-phase-2}

  {% compatibility 'dart: "1.52.1"', 'libsass: false', 'ruby: false' %}{% endcompatibility %}

  Next, we changed the way angle units are handled for hue parameters to match
  the CSS spec. This means that numbers with `grad`, `rad`, or `turn` units will
  be converted to `deg`: `0.5turn` will be converted to `180deg`, `100grad` will
  be converted to `90deg`, and so on.

  Because this change is necessary to preserve CSS compatibility, according to
  the [Dart Sass compatibility policy] it was made with only a minor version
  bump. However, it changes as little behavior as possible to ensure that Sass
  interprets all valid CSS according to the CSS spec.

  [Dart Sass compatibility policy]: https://github.com/sass/dart-sass#compatibility-policy

  ### Phase 3{#hue-phase-3}

  {% compatibility 'dart: false', 'libsass: false', 'ruby: false' %}{% endcompatibility %}

  Finally, in Dart Sass 2.0.0 color functions will throw errors if they're
  passed a hue parameter with a non-angle unit. Unitless hues will still be
  allowed.

  ## Saturation and Lightness

  When specifying an HSL color's saturation and lightness, CSS only allows `%`
  units. Even unitless numbers aren't allowed (unlike for the hue).
  Historically, Sass has allowed *any* unit, and interpreted it as `%`. You
  could even write `hsl(0, 100px, 50s)` and Sass would return the color `red`.

  To fix this issue and bring Sass in line with the CSS spec, we're making
  changes in two phases:

  ### Phase 1{#saturation-lightness-phase-1}

  {% compatibility 'dart: "1.32.0"', 'libsass: false', 'ruby: false' %}{% endcompatibility %}

  Currently, Sass just emits a deprecation warning if you pass a number with no
  unit or a unit other than `%` as a lightness or saturation to any function.

  ### Phase 2{#saturation-lightness-phase-2}

  {% compatibility 'dart: false', 'libsass: false', 'ruby: false' %}{% endcompatibility %}

  In Dart Sass 2.0.0 color functions will throw errors if they're passed a
  saturation or lightness parameter with no unit or a non-`%` unit.

  ## Alpha

  When specifying a color's alpha value, CSS (as of [Colors Level 4]) allows
  either unitless values between 0 and 1 or `%` values between `0%` and `100%`.
  In most cases Sass follows this behavior, but the functions `color.adjust()`
  and `color.change()` have historically allowed *any* unit, and interpreted it
  as unitless. You could even write `color.change(red, $alpha: 1%)` and Sass
  would return the opaque color `black`.

  [Colors Level 4]: https://www.w3.org/TR/css-color-4/#typedef-alpha-value

  To fix this issue and bring Sass in line with the CSS spec, we're making
  changes in three phases:

  ### Phase 1{#alpha-phase-1}

  {% compatibility 'dart: "1.56.0"', 'libsass: false', 'ruby: false' %}{% endcompatibility %}

  Currently, Sass just emits a deprecation warning if you pass a number with any
  unit, including `%`, as an alpha value to `color.change()` or
  `color.adjust()`.

  ### Phase 2{#alpha-phase-2}

  {% compatibility 'dart: false', 'libsass: false', 'ruby: false' %}{% endcompatibility %}

  Next, we'll change the way `%` units are handled for the alpha argument to
  `color.change()` and `color.adjust()`. Alphas with unit `%` will be divided by
  `100%`, converting them to unitless numbers between 0 and 1.

  Because this change is a bug fix that improves consistency with other Sass
  functions, it will be made with only a minor version bump. It will be changed
  at minimum three months after Phase 1 is released, to give users time to
  adjust their code and avoid the bug.

  [Dart Sass compatibility policy]: https://github.com/sass/dart-sass#compatibility-policy

  ### Phase 3{#alpha-phase-3}

  {% compatibility 'dart: false', 'libsass: false', 'ruby: false' %}{% endcompatibility %}

  Finally, in Dart Sass 2.0.0 `color.change()` and `color.adjust()` will throw
  errors if they're passed an alpha parameter with a non-`%` unit. Unitless
  alphas will still be allowed.

  ## `math.random()`

  [The `math.random()` function] has historically ignored units in `$limit` and
  returned a unitless value. For example `math.random(100px)` would drop "px"
  and return a value like `42`.

  A future version of Sass will stop ignoring units for the `$limit` argument
  and return a random integer with the same units.

  [The `math.random()` function]: /documentation/modules/math#random
{% endmarkdown %}

{% codeExample 'function-units', false %}
  // Future Sass, doesn't work yet!
  @debug math.random(100px); // 42px
  ===
  // Future Sass, doesn't work yet!
  @debug math.random(100px)  // 42px
{% endcodeExample %}

{% markdown %}
  ### Phase 1{#math-random-phase-1}

  {% compatibility 'dart: "1.54.5"', 'libsass: false', 'ruby: false' %}{% endcompatibility %}

  Currently, Sass emits a deprecation warning if you pass a `$limit` with units
  to `math.random()`.

  ### Phase 2{#math-random-phase-2}

  {% compatibility 'dart: false', 'libsass: false', 'ruby: false' %}{% endcompatibility %}

  In Dart Sass 2.0.0, passing a `$limit` number with units will be an error.

  ### Phase 3{#math-random-phase-3}

  {% compatibility 'dart: false', 'libsass: false', 'ruby: false' %}{% endcompatibility %}

  In a minor release after Dart Sass 2.0.0, passing a `$limit` number with units
  to the `math.random()` function will be allowed again. It will return a random
  integer the same units as `$limit`, instead of a unitless number.

  ## Weight

  The [`color.mix()` function] and [`color.invert()` function] have both
  historically ignored units in their `$weight` arguments, despite that argument
  conceptually representing a percentage. A future version of Sass will require
  the unit `%`.

  [`color.mix()` function]: /documentation/modules/color#mix
  [`color.invert()` function]: /documentation/modules/color#invert

  ### Phase 1{#weight-phase-1}

  {% compatibility 'dart: "1.56.0"', 'libsass: false', 'ruby: false' %}{% endcompatibility %}

  Currently, Sass emits a deprecation warning if you pass a `$weight` with no
  units or with units other than `%` to `color.mix()` or `color.invert()`.

  ### Phase 2{#weight-phase-2}

  {% compatibility 'dart: false', 'libsass: false', 'ruby: false' %}{% endcompatibility %}

  In Dart Sass 2.0.0, `color.mix()` and `color.invert()` will throw errors if
  they're passed a `$weight` with no unit or a non-`%` unit.

  ## Index

  The [`list.nth()` function] and [`list.set-nth()` function] have both
  historically ignored units in their `$n` arguments. A future version of Sass
  will forbid any units.

  [`list.nth()` function]: /documentation/modules/list#nth
  [`list.set-nth()` function]: /documentation/modules/list#set-nth

  ### Phase 1{#index-phase-1}

  {% compatibility 'dart: "1.56.0"', 'libsass: false', 'ruby: false' %}{% endcompatibility %}

  Currently, Sass emits a deprecation warning if you pass a `$weight` with no
  units or with units other than `%` to `color.mix()` or `color.invert()`.

  ### Phase 2{#index-phase-2}

  {% compatibility 'dart: false', 'libsass: false', 'ruby: false' %}{% endcompatibility %}

  In Dart Sass 2.0.0, `list.nth()` and `list.set-nth()` will throw errors if
  they're passed an index `$n` with a unit.
{% endmarkdown %}