Logo of c12


Smart Configuration Loader.

c12 (pronounced as /siːtwelv/, like c-twelve) is a smart configuration loader.

✅ Features

🦴 Used by


Install package:

# ✨ Auto-detect
npx nypm install c12

# npm
npm install c12

# yarn
yarn add c12

# pnpm
pnpm install c12

# bun
bun install c12


ESM (Node.js, Bun)

import { loadConfig, watchConfig } from "c12";

CommonJS (Legacy Node.js)

const { loadConfig, watchConfig } = require("c12");

Load configuration:

// Get loaded config
const { config } = await loadConfig({});

// Get resolved config and extended layers
const { config, configFile, layers } = await loadConfig({});

Loading priority

c12 merged config sources with unjs/defu by below order:

  1. Config overrides passed by options
  2. Config file in CWD
  3. RC file in CWD
  4. Global RC file in the user's home directory
  5. Config from package.json
  6. Default config passed by options
  7. Extended config layers



Resolve configuration from this working directory. The default is process.cwd()


Configuration base name. The default is config.


Configuration file name without extension. Default is generated from name (f.e., if name is foo, the config file will be => foo.config).

Set to false to avoid loading the config file.


RC Config file name. Default is generated from name (name=foo => .foorc).

Set to false to disable loading RC config.


Load RC config from the workspace directory and the user's home directory. Only enabled when rcFile is provided. Set to false to disable this functionality.


Loads .env file if enabled. It is disabled by default.


Loads config from nearest package.json file. It is disabled by default.

If true value is passed, c12 uses name field from package.json.

You can also pass either a string or an array of strings as a value to use those fields.


Specify default configuration. It has the lowest priority and is applied after extending config.


Specify default configuration. It is applied before extending config.


Specify override configuration. It has the highest priority and is applied before extending config.


Exclude environment-specific and built-in keys start with $ in the resolved config. The default is false.


Custom unjs/jiti instance used to import configuration files.


Custom unjs/jiti options to import configuration files.


Options passed to unjs/giget when extending layer from git source.


Environment name used for environment specific configuration.

The default is process.env.NODE_ENV. You can set envName to false or an empty string to disable the feature.

Extending configuration

If resolved config contains a extends key, it will be used to extend the configuration.

Extending can be nested and each layer can extend from one base or more.

The final config is merged result of extended options and user options with unjs/defu.

Each item in extends is a string that can be either an absolute or relative path to the current config file pointing to a config file for extending or the directory containing the config file. If it starts with either github:, gitlab:, bitbucket:, or https:, c12 automatically clones it.

For custom merging strategies, you can directly access each layer with layers property.


// config.ts
export default {
  colors: {
    primary: "user_primary",
  extends: ["./theme"],
// config.dev.ts
export default {
  dev: true,
// theme/config.ts
export default {
  extends: "../base",
  colors: {
    primary: "theme_primary",
    secondary: "theme_secondary",
// base/config.ts
export default {
  colors: {
    primary: 'base_primary'
    text: 'base_text'

The loaded configuration would look like this:

  dev: true,
  colors: {
    primary: 'user_primary',
    secondary: 'theme_secondary',
    text: 'base_text'


 { config: /* theme config */, configFile: /* path/to/theme/config.ts */, cwd: /* path/to/theme */ },
 { config: /* base  config */, configFile: /* path/to/base/config.ts  */, cwd: /* path/to/base */ },
 { config: /* dev   config */, configFile: /* path/to/config.dev.ts  */, cwd: /* path/ */ },

Extending config layer from remote sources

You can also extend configuration from remote sources such as npm or github.

In the repo, there should be a config.ts (or config.{name}.ts) file to be considered as a valid config layer.

Example: Extend from a github repository

// config.ts
export default {
  extends: "gh:user/repo",

Example: Extend from a github repository with branch and subpath

// config.ts
export default {
  extends: "gh:user/repo/theme#dev",

Example: Extend a private repository and install dependencies:

// config.ts
export default {
  extends: ["gh:user/repo", { auth: process.env.GITHUB_TOKEN, install: true }],

You can pass more options to giget: {} in layer config.

Refer to unjs/giget for more information.

Environment-specific configuration

Users can define environment-specific configuration using these config keys:

  • $test: {...}
  • $development: {...}
  • $production: {...}
  • $env: { [env]: {...} }

c12 tries to match envName and override environment config if specified.

Note: Environment will be applied when extending each configuration layer. This way layers can provide environment-specific configuration.


  // Default configuration
  logLevel: 'info',

  // Environment overrides
  $test: { logLevel: 'silent' },
  $development: { logLevel: 'warning' },
  $production: { logLevel: 'error' },
  $env: {
    staging: { logLevel: 'debug' }

Watching configuration

you can use watchConfig instead of loadConfig to load config and watch for changes, add and removals in all expected configuration paths and auto reload with new config.

Lifecycle hooks

  • onWatch: This function is always called when config is updated, added, or removed before attempting to reload the config.
  • acceptHMR: By implementing this function, you can compare old and new functions and return true if a full reload is not needed.
  • onUpdate: This function is always called after the new config is updated. If acceptHMR returns true, it will be skipped.
import { watchConfig } from "c12";

const config = watchConfig({
  cwd: ".",
  // chokidarOptions: {}, // Default is { ignoreInitial: true }
  // debounce: 200 // Default is 100. You can set it to false to disable debounced watcher
  onWatch: (event) => {
    console.log("[watcher]", event.type, event.path);
  acceptHMR({ oldConfig, newConfig, getDiff }) {
    const diff = getDiff();
    if (diff.length === 0) {
      console.log("No config changed detected!");
      return true; // No changes!
  onUpdate({ oldConfig, newConfig, getDiff }) {
    const diff = getDiff();
    console.log("Config updated:\n" + diff.map((i) => i.toJSON()).join("\n"));

console.log("watching config files:", config.watchingFiles);
console.log("initial config", config.config);

// Stop watcher when not needed anymore
// await config.unwatch();


Local development
  • Clone this repository
  • Install the latest LTS version of Node.js
  • Enable Corepack using corepack enable
  • Install dependencies using pnpm install
  • Run tests using pnpm dev or pnpm test


Published under the MIT license. Made by @pi0 and community 💛