postcss-pxtorem

What is postcss-pxtorem?

The postcss-pxtorem package is a PostCSS plugin that converts pixel units to rem units in CSS. This is particularly useful for responsive web design, as rem units are relative to the root element's font size, making it easier to scale elements proportionally across different screen sizes.

What are postcss-pxtorem's main functionalities?

Basic Conversion

This configuration converts all pixel values to rem values based on a root font size of 16 pixels. The `propList` option specifies which properties should be converted; in this case, all properties are included.

module.exports = {
  plugins: {
    'postcss-pxtorem': {
      rootValue: 16,
      propList: ['*']
    }
  }
};

Selective Conversion

This configuration converts only the specified properties (font, margin, and padding) from pixels to rems. This allows for more granular control over which CSS properties are affected.

module.exports = {
  plugins: {
    'postcss-pxtorem': {
      rootValue: 16,
      propList: ['font', 'margin', 'padding']
    }
  }
};

Excluding Specific Selectors

This configuration excludes specific selectors (in this case, `.ignore` and `.no-rem`) from the conversion process. This is useful when you want certain elements to retain their pixel values.

module.exports = {
  plugins: {
    'postcss-pxtorem': {
      rootValue: 16,
      propList: ['*'],
      selectorBlackList: ['.ignore', '.no-rem']
    }
  }
};

Other packages similar to postcss-pxtorem

postcss-px-to-viewport

The postcss-px-to-viewport package converts pixel units to viewport units (vw, vh). This is useful for creating responsive designs that scale based on the viewport size. Unlike postcss-pxtorem, which uses rem units relative to the root font size, postcss-px-to-viewport uses viewport units, making it more suitable for fluid layouts.

postcss-rem

The postcss-rem package converts rem units to pixel units, essentially the reverse of what postcss-pxtorem does. This can be useful in scenarios where you need to ensure pixel-perfect designs and want to avoid the scaling issues that can come with rem units.

README

postcss-pxtorem

A plugin for PostCSS that generates rem units from pixel units.

Install

$ npm install postcss postcss-pxtorem --save-dev

Usage

Pixels are the easiest unit to use (opinion). The only issue with them is that they don't let browsers change the default font size of 16. This script converts every px value to a rem from the properties you choose to allow the browser to set the font size.

Input/Output

With the default settings, only font related properties are targeted.

// input
h1 {
    margin: 0 0 20px;
    font-size: 32px;
    line-height: 1.2;
    letter-spacing: 1px;
}

// output
h1 {
    margin: 0 0 20px;
    font-size: 2rem;
    line-height: 1.2;
    letter-spacing: 0.0625rem;
}

Example

var fs = require('fs');
var postcss = require('postcss');
var pxtorem = require('postcss-pxtorem');
var css = fs.readFileSync('main.css', 'utf8');
var options = {
    replace: false
};
var processedCss = postcss(pxtorem(options)).process(css).css;

fs.writeFile('main-rem.css', processedCss, function (err) {
  if (err) {
    throw err;
  }
  console.log('Rem file written.');
});

options

Type: Object | Null
Default:

{
    rootValue: 16,
    unitPrecision: 5,
    propList: ['font', 'font-size', 'line-height', 'letter-spacing'],
    selectorBlackList: [],
    replace: true,
    mediaQuery: false,
    minPixelValue: 0,
    exclude: /node_modules/i
}

• rootValue (Number | Function) Represents the root element font size or returns the root element font size based on the input parameter
• unitPrecision (Number) The decimal numbers to allow the REM units to grow to.
• propList (Array) The properties that can change from px to rem.
  • Values need to be exact matches.
  • Use wildcard * to enable all properties. Example: ['*']
  • Use * at the start or end of a word. (['*position*'] will match background-position-y)
  • Use ! to not match a property. Example: ['*', '!letter-spacing']
  • Combine the "not" prefix with the other prefixes. Example: ['*', '!font*']
• selectorBlackList (Array) The selectors to ignore and leave as px.
  • If value is string, it checks to see if selector contains the string.
    • ['body'] will match .body-class
  • If value is regexp, it checks to see if the selector matches the regexp.
    • [/^body$/] will match body but not .body
• replace (Boolean) Replaces rules containing rems instead of adding fallbacks.
• mediaQuery (Boolean) Allow px to be converted in media queries.
• minPixelValue (Number) Set the minimum pixel value to replace.
• exclude (String, Regexp, Function) The file path to ignore and leave as px.
  • If value is string, it checks to see if file path contains the string.
    • 'exclude' will match \project\postcss-pxtorem\exclude\path
  • If value is regexp, it checks to see if file path matches the regexp.
    • /exclude/i will match \project\postcss-pxtorem\exclude\path
  • If value is function, you can use exclude function to return a true and the file will be ignored.
    • the callback will pass the file path as a parameter, it should returns a Boolean result.
    • function (file) { return file.indexOf('exclude') !== -1; }
• unit (String) Set the default unit to convert, default is px.

Use with gulp-postcss and autoprefixer

var gulp = require('gulp');
var postcss = require('gulp-postcss');
var autoprefixer = require('autoprefixer');
var pxtorem = require('postcss-pxtorem');

gulp.task('css', function () {

    var processors = [
        autoprefixer({
            browsers: 'last 1 version'
        }),
        pxtorem({
            replace: false
        })
    ];

    return gulp.src(['build/css/**/*.css'])
        .pipe(postcss(processors))
        .pipe(gulp.dest('build/css'));
});

A message about ignoring properties

Currently, the easiest way to have a single property ignored is to use a capital in the pixel unit declaration.

// `px` is converted to `rem`
.convert {
    font-size: 16px; // converted to 1rem
}

// `Px` or `PX` is ignored by `postcss-pxtorem` but still accepted by browsers
.ignore {
    border: 1Px solid; // ignored
    border-width: 2PX; // ignored
}