postcss-rem-to-pixel-next

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

Acknowledgements

This project is an updated version of the original postcss-rem-to-pixel plugin, which was created by jesstech. All credit for the original concept and implementation goes to them.

I have updated the plugin to utilize the new PostCSS 8 API.

Install

$ npm i postcss-rem-to-pixel-next --save-dev

Usage

Sometimes you need to include a third-party css file that uses rems. Great pracitice! Unless you can't afford to change your body font-size just for some vendor. This script converts every rem value to a px value from the properties you choose using a default font size of 16px.

Input/Output

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

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

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

Example

var fs = require("fs");
var postcss = require("postcss");
var remToPx = require("postcss-rem-to-pixel");
var css = fs.readFileSync("main.css", "utf8");
var options = {
  replace: false,
};
var processedCss = postcss(remToPx(options)).process(css).css;

fs.writeFile("main-px.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,
    minRemValue: 0
}

• rootValue (Number) The root element font size.
• unitPrecision (Number) The decimal precision px units are allowed to use, floored (rounding down on half).
• propList (Array) The properties that can change from rem to px.
  • 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 rem.
  • 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 rem to be converted in media queries.
• minRemValue (Number) Set the minimum rem value to replace.

Use with gulp-postcss and autoprefixer

var gulp = require("gulp");
var postcss = require("gulp-postcss");
var autoprefixer = require("autoprefixer");
var remToPx = require("postcss-rem-to-pixel");

gulp.task("css", function () {
  var processors = [
    autoprefixer({
      browsers: "last 1 version",
    }),
    remToPx({
      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 rem unit declaration.

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

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