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@modular-css/processor NPM Version NPM License NPM Downloads

The core functionality of modular-css exposed as a JS API.


$ npm i @modular-css/processor


Instantiate a new Processor instance, call it's .file(<path>) or .string(<name>, <contents>) methods, and then use the returned Promise to get access to the results/output.

const Processor = require("@modular-css/processor");
const processor = new Processor({
    // See "API Options" for valid options to pass to the Processor constructor

// Add entries, either from disk using .file() or as strings with .string()
const result = await processor.file("./entry.css");

// result contains
//  .exports - Scoped selector mappings
//  .files - metadata about the file hierarchy

await processor.string("./fake-file.css", ".class { color: red; }");

// Once all files are added, use .output() to get at the rewritten CSS
const results = await processor.output();

// Output CSS lives on the .css property

// Source map (if requested) lives on the .map property


.string(file, css)

Returns a promise. Add file to the Processor instance with css contents.


Returns a promise. Add file to the Processor instance, reads contents from disk using fs.

.root(file, Root)

Returns a promise. Add file to the Processor instance, re-uses a Postcss Root object, avoiding unnecessarily parsing an AST again.

.output({ args })

Returns a promise. Finalize processing of all added CSS and create combined CSS output file.

Passing files as part of args will result in getting back combined output CSS just for the listed files and their dependencies.

Passing to as part of args will be passed along to teh after and done hooks for proper path adjustment in maps & any plugins that use it.

WARNING: Calling .output() before any preceeding .file(...)/.string(...) calls have resolved their returned promises will return a rejected promise. See usage for an example of correct usage.

Includes the following keys that you probably care about:

  • .css, the generated CSS representing the files being output
  • .map, the (optional) source map for the files being output
  • .compositions, the selector hierarchies for all the files being output


Remove files from the Processor instance. Accepts a single file or array of files.


Mark a file (and any files that depend on it) as invalid. If any of those files are then re-added via either .string() or .file() they will be replaced with the new values instead of using the cached results from the previous run.


Returns an array of file paths. Accepts a single file argument to get the dependencies for, will return entire dependency graph in order if argument is omitted.



Specify an array of PostCSS plugins to be run against each file before it is processed.

new Processor({
    before : [ require("postcss-import") ]


Specify an array of PostCSS plugins to be run after files are processed, but before they are combined. Plugin will be passed a to and from option.

By default postcss-url is used in after mode.

new Processor({
    after : [ require("postcss-someplugin") ]


Specify an array of PostCSS plugins to be run against the complete combined CSS.

new Processor({
    done : [ require("cssnano")()]


Enable source map generation. Can also be passed to .output().

Default: false

new Processor({
    map : true


Specify the current working directory for this Processor instance, used when resolving composes/@value rules that reference other files.

Default: process.cwd()

new Processor({
    cwd : path.join(process.cwd(), "/sub/dir")


Specify a function (that takes filename & selector as arguments to produce scoped selectors.

Default: Function that returns "mc" + unique-slug(<file>) + "_" + selector

new Processor({
    namer : function(file, selector) {
        return file.replace(/[:\/\\ .]/g, "") + "_" + selector;


Specify an object that contains any of the PostCSS .process() Options. Note that from and to will usally be overwritten to match the files being processed. This feature allows for the use of custom parser, stringifier, and syntax settings.


If you want to provide your own file resolution logic you can pass an array of resolver functions. Each resolver function receives three arguments:

  • src, the file that included file
  • file, the file path being included by src
  • resolve, the default resolver function

Resolver functions should either return an absolute path or a falsey value. They must also be synchronous.

Default: See /processor/lib/resolve.js for the default implementation.

new Processor({
    resolvers : [
        (src, file, resolve) => ...,


Enable exporting :global identifiers.

Default: true

new Processor({
    exportDefaults: false
/* exportGlobals: true */
.a {}
:global(.b) {}

/* Outputs
    "a" : "mc12345_a",
    "b" : "b"

/* exportGlobals: false */
.a {}
:global(.b) {}

/* Outputs
    "a" : "mc12345_a"



Returns an object keyed by absolute file paths of all known files in the Processor instance.


Returns the options object passed to the Processor augmented with the defaults.


npm i @modular-css/processor



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