npm install @csstools/postcss-sign-functions --save-dev

PostCSS Sign Functions lets you use the sign and abs functions, following the CSS Values 4 specification.

.sign {
	z-index: sign(-10px);
}
.sign {
	z-index: sign(0);
}
.sign {
	z-index: sign(10px);
}
.abs {
	z-index: abs(-10px);
}
.abs {
	z-index: abs(0);
}
.abs {
	z-index: abs(10px);
}
.abs {
	z-index: abs(10%);
}
/* becomes */
.sign {
	z-index: -1;
}
.sign {
	z-index: 0;
}
.sign {
	z-index: 1;
}
.abs {
	z-index: 10px;
}
.abs {
	z-index: 0;
}
.abs {
	z-index: 10px;
}
.abs {
	z-index: max((10%), -1 * (10%));
}

Note

The utility of static fallbacks for sign and abs is limited. The most interesting values are variables and dynamic values (e.g. those containing %). It is impossible to generate static fallbacks in a build process for values that are dynamic on the client.

Usage

Add PostCSS Sign Functions to your project:

npm install postcss @csstools/postcss-sign-functions --save-dev

Use it as a PostCSS plugin:

const postcss = require('postcss');
const postcssSignFunctions = require('@csstools/postcss-sign-functions');
postcss([
	postcssSignFunctions(/* pluginOptions */)
]).process(YOUR_CSS /*, processOptions */);

PostCSS Sign Functions runs in all Node environments, with special instructions for:

⚠️ About custom properties

Given the dynamic nature of custom properties it's impossible to know what the variable value is, which means the plugin can't compute a final value for the stylesheet.

Because of that, any usage that contains a var is skipped.

Options

preserve

The preserve option determines whether the original notation is preserved. By default, it is not preserved.

postcssSignFunctions({ preserve: true })

.sign {
	z-index: sign(-10px);
}
.sign {
	z-index: sign(0);
}
.sign {
	z-index: sign(10px);
}
.abs {
	z-index: abs(-10px);
}
.abs {
	z-index: abs(0);
}
.abs {
	z-index: abs(10px);
}
.abs {
	z-index: abs(10%);
}
/* becomes */
.sign {
	z-index: -1;
	z-index: sign(-10px);
}
.sign {
	z-index: 0;
	z-index: sign(0);
}
.sign {
	z-index: 1;
	z-index: sign(10px);
}
.abs {
	z-index: 10px;
	z-index: abs(-10px);
}
.abs {
	z-index: 0;
	z-index: abs(0);
}
.abs {
	z-index: 10px;
	z-index: abs(10px);
}
.abs {
	z-index: max((10%), -1 * (10%));
	z-index: abs(10%);
}