postcss-remtorem

A plugin for PostCSS that adjusts rem units to a match a non-standard base unit size.

Based on the postcss-pxtorem plugin by cuth.

Install

$ npm install postcss postcss-remtorem --save-dev

Why Does This Exist / Usage Scenario

The default root em size used by most browers is 1rem = 16px, but if you are working within a project using a non-standard root em size that you cannot change (e.g. 1rem = 10px), then this plugin might be useful for adjusting 3rd party stylesheets/frameworks to have appropriate element sizing out of the box.

Usage Example

"use strict";

var fs = require('fs');
var postcss = require('../node_modules/postcss');
var remtorem = require('../');
var css = fs.readFileSync('main.css', 'utf8');
var processedCss = postcss(remtorem()).process(css).css;

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

Result

// Input (main.css)
h1 {
    margin: 0 0 1rem;
    font-size: 2rem;
    line-height: 1.2;
    letter-spacing: 1px;
}

// Output (main-rem.css)
h1 {
    margin: 0 0 1.6rem;
    font-size: 3.2rem;
    line-height: 1.2;
    letter-spacing: 1px;
}

Options

Type: Object | null Default:

{
    rootValue: 10,
    unitPrecision: 5,
    propList: ['*'],
    selectorBlackList: [],
    mediaQuery: false,
    minRemValue: 0,
    exclude: null
}

• rootValue (Number) The root element font size of your project. E.g. If set to 10, rem values will be increased by 1.6x (16/10)
• unitPrecision (Number) The decimal numbers to allow the REM units to grow to.
• propList (Array) The properties for which rem values will be adjusted
  • 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
• 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-remtorem\exclude\path
  • If value is regexp, it checks to see if file path matches the regexp.
    • /exclude/i will match \project\postcss-remtorem\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; }

Use with gulp-postcss and autoprefixer

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

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

    var processors = [
        autoprefixer({
            browsers: 'last 1 version'
        }),
        remtorem({
            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.

// `rem` is converted
.convert {
    font-size: 1rem; // converted to 1.6rem
}

// `Rem` or `REM` is ignored by `postcss-remtorem` but still accepted by browsers
.ignore {
    border: 1Rem solid; // ignored
    border-width: 2REM; // ignored
}