postcss-px2unit

A plugin for PostCSS that generates rem/em/px units from pixel units.

Install

$ npm install postcss-px2unit --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 px2unit = require('postcss-px2unit');
var css = fs.readFileSync('main.css', 'utf8');
var options = {
    replace: false
};
var processedCss = postcss(px2unit(options)).process(css).css;

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

options

Type: Object | Object[] | Null
Default:

{
    unit: 'rem',
    rootValue: 16,
    unitPrecision: 5,
    propList: ['font', 'font-size', 'line-height', 'letter-spacing'],
    selectorBlackList: [],
    replace: true,
    mediaQuery: false,
    minPixelValue: 0,
    pxReplace: function(options) {},
    excludeFiles: []
}

Array options:

[
  {
    unit: 'rem',
    rootValue: 16,
    unitPrecision: 5,
    propList: ['font', 'font-size', 'line-height', 'letter-spacing'],
    selectorBlackList: [],
    replace: true,
    mediaQuery: false,
    minPixelValue: 0,
    pxReplace: function(options) {},
    excludeFiles: ['/src/']
  },
  {
    unit: 'rem',
    rootValue: 32,
    unitPrecision: 5,
    propList: ['font', 'font-size', 'line-height', 'letter-spacing'],
    selectorBlackList: [],
    replace: true,
    mediaQuery: false,
    minPixelValue: 0,
    pxReplace: function(options) {},
    excludeFiles: [/node_moduels\/vant/]
  }
]

• unit (String) Conversion unit: rem、em、px.
• rootValue (Number) The root element font size.
• 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.
• pxReplace (Function) px replace regExp Function, must be return function.

var pxRegex = /"[^"]+"|'[^']+'|url\([^\)]+\)|(\d*\.?\d+)px/g;
var pxRePlace = function (options) {
  return function (m, $1) {
    if (!$1) return m;
    var pixels = parseFloat($1);
    if (pixels < options.minPixelValue) return m;
    var fixedVal = toFixed((pixels /options.rootValue), options.unitPrecision);
    return (fixedVal === 0) ? '0' : fixedVal + options.unit;
  };
} 
// replace
value.replace(pxRegex, pxReplace());

• excludeFiles (Array<String|Regexp>) Set exclude files.

Use with gulp-postcss and autoprefixer

var gulp = require('gulp');
var postcss = require('gulp-postcss');
var autoprefixer = require('autoprefixer');
var px2unit = require('postcss-px2unit');

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

    var processors = [
        autoprefixer({
            browsers: 'last 1 version'
        }),
        px2unit([
          {
            rootValue: 16,
            excludeFiles: ['/src/']
          },
          {
            rootValue: 32,
            excludeFiles: [/node_modules\/vant/]
          }
        ])
    ];

    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-px2unit` but still accepted by browsers
.ignore {
    border: 1Px solid; // ignored
    border-width: 2PX; // ignored
}