Breaking Change: Strict Unary Operators

Sass has historically allowed - and + to be used in ways that make it ambiguous whether the author intended them to be a binary or unary operator. This confusing syntax is being deprecated.

How is this property compiled?

SCSS Syntax

$size: 10px;

div {
  margin: 15px -$size;
}

Sass Syntax

$size: 10px

div
  margin: 15px -$size

Some users might say "the - is attached to $size, so it should be margin: 20px -10px". Others might say "the - is between 20px and $size, so it should be margin: 5px." Sass currently agrees with the latter opinion, but the real problem is that it’s so confusing in the first place! This is a natural but unfortunate consequence of CSS’s space-separated list syntax combined with Sass’s arithmetic syntax.

That’s why we’re moving to make this an error. In the future, if you want to use a binary - or + operator (that is, one that subtracts or adds two numbers), you’ll need to put whitespace on both sides or on neither side:

  • Valid: 15px - $size
  • Valid: (15px)-$size
  • Invalid: 15px -$size

If you want to use a unary - or + operator as part of a space-separated list, you’ll (still) need to wrap it in parentheses:

  • Valid: 15px (-$size)

Transition PeriodTransition Period permalink

Compatibility:
Dart Sass
since 1.55.0
LibSass
✗
Ruby Sass
✗

We’ll make this an error in Dart Sass 2.0.0, but until then it’ll just emit a deprecation warning.

💡 Fun fact:

Remember, you can silence deprecation warnings from libraries you don’t control! If you’re using the command-line interface you can pass the --quiet-deps flag, and if you’re using the JavaScript API you can set the quietDeps option to true.

Automatic MigrationAutomatic Migration permalink

You can use the Sass migrator to automatically update your stylesheets to add a space after any - or + operators that need it, which will preserve the existing behavior of these stylesheets.

$ npm install -g sass-migrator
$ sass-migrator strict-unary **/*.scss

Can I Silence the Warnings?Can I Silence the Warnings? permalink

Sass provides a powerful suite of options for managing which deprecation warnings you see and when.

Terse and Verbose ModeTerse and Verbose Mode permalink

By default, Sass runs in terse mode, where it will only print each type of deprecation warning five times before it silences additional warnings. This helps ensure that users know when they need to be aware of an upcoming breaking change without creating an overwhelming amount of console noise.

If you run Sass in verbose mode instead, it will print every deprecation warning it encounters. This can be useful for tracking the remaining work to be done when fixing deprecations. You can enable verbose mode using the --verbose flag on the command line, or the verbose option in the JavaScript API.

⚠️ Heads up!

When running from the JS API, Sass doesn’t share any information across compilations, so by default it’ll print five warnings for each stylesheet that’s compiled. However, you can fix this by writing (or asking the author of your favorite framework’s Sass plugin to write) a custom Logger that only prints five errors per deprecation and can be shared across multiple compilations.

Silencing Deprecations in DependenciesSilencing Deprecations in Dependencies permalink

Sometimes, your dependencies have deprecation warnings that you can’t do anything about. You can silence deprecation warnings from dependencies while still printing them for your app using the --quiet-deps flag on the command line, or the quietDeps option in the JavaScript API.

For the purposes of this flag, a "dependency" is any stylesheet that’s not just a series of relative loads from the entrypoint stylesheet. This means anything that comes from a load path, and most stylesheets loaded through custom importers.

Silencing Specific DeprecationsSilencing Specific Deprecations permalink

If you know that one particular deprecation isn’t a problem for you, you can silence warnings for that specific deprecation using the --silence-deprecation flag on the command line, or the silenceDeprecations option in the JavaScript API.

⚠️ Heads up!

This option is only available in the modern JS API.