Configuration file loader with JSON schema support

Usage no npm install needed!

<script type="module">
  import servallAppConfig from 'https://cdn.skypack.dev/@servall/app-config';



App Config

Licensed under MPL 2.0 Code Coverage Build Status npm

Simple configuration loader for node. Comes with strong typing, schema validation, many supported file formats, secrets, environment variables, and more.

Why yet-another-config-package? We want strong typing and strict validation of our configurations, for confidence in development and production. We also want the flexibility of using different file formats, using environment variables, and keeping secrets safe. It should be abundantly easy and straightforward to change configuration with confidence.

What is it not useful for (as of today):

  • dynamic configuration (behaviour), since it only allows static configuration
  • system global configuration, since it assumes configuration local to your application
  • multi-tenant production systems, since it assumes only one APP_CONFIG variable
  • in-browser config loading, since it relies on filesystem libraries


  • Types - All config variables have types. Environment variables alone do not store type information.
  • Validation - Config variables are validated against a JSON schema. This ensures that configurations are valid, when the app loads instead of when you least expect it.
  • Hierarchy - Config variables can have hierarchy and structure.
  • Code Generation - Configuration schemas can be used to generate correct types in your language of choosing (typescript, swift, rust, and many more), all automatically.
  • Configuration Formats - Everyone has their own preferences for configuration format - supports JSON, JSON5, TOML, YAML out of the box.
  • Lists - Config can include lists of variables. This can be useful when a dynamic number of settings is required, and can be difficult to do with plain environment variables.


Core Functionality

  • Loads your configuration from environment variable or a file (YAML, TOML, JSON, JSON5)
  • Validates your configuration using JSON Schema
  • Generates strong types from your schema (using quicktype)
  • Includes a CLI tool for injecting configuration as a environment variables
  • Global configuration extension, allowing CI specific configuration without overriding config
  • Explicit secret config items, disallowed in non-secret configuration

Getting Started

Add the module to your project, using your preferred NPM client. Then run the init command.

yarn add @servall/app-config

npx app-config init

The CLI init command simply creates .app-config.* files. You can then run npx app-config variables at any time to see your populated configuration.

You might start with a simple configuration, like:


port = 3000

host = "localhost"
port = 5432
name = "master"


type: object
required: [server, database]
    type: object
    required: [port]
        type: number
    type: object
    required: [host, port, name]
        type: string
        type: number
        type: string

Check out the JSON Schema documentation for details.

Files and Formats

This module supports YAML, TOML, JSON, and JSON5 out of the box, for any of the files. Your schema could be YAML, and config be TOML (like the CLI defaults to), or any other way around.

  • Configuration files are .app-config.{ext} or app-config.{ext}
  • Secret configuration items
  • Schema files are .app-config.schema.{ext} or app-config.schema.{ext}
  • Meta files are .app-config.meta.{ext} or app-config.meta.{ext}
  • Meta properties are specified in the root of any file (except meta files) under the app-config key
  • The APP_CONFIG environment variable always takes precedent over files, and can be in any supported format
  • The APP_CONFIG_CI or APP_CONFIG_EXTEND environment variable is parsed and deeply merged with your main configuration, if loading from a file


This module comes with a CLI tool for common usage. Running npx app-config --help will give you a list of commands and options.

Common options are:

app-config -- {cmd-that-consumes-env-variables}

# for example:
app-config -- docker-compose up -d

# spits out all config to stdout
app-config vars

# spits out all config as a different format
app-config create --format json

# runs code-generation from your schema types
app-config generate

Of course, you can and should use app-config in your package.json scripts. Note that running commands this way (--) also get a APP_CONFIG variable in TOML.


This module uses JSON Schema to define constraints on your configuration. As shown above, the schema is loaded from the .app-config.schema.{ext} file. This cannot be specified by an environment variable, so be certain that it is included in your production environment.

Schemas have support for loading $ref properties, to reduce duplication of properties. That means that if you define a type as $ref: '../another-file.yml#/definitions/CommonItem' that resolution will be completed automatically.


Secrets are defined in your schema.

  type: string
  format: uri
  secret: true

Inserting that non-standard key will automatically prevent you from putting that property in your main .app-config.{ext} file, requiring you to put it in .app-config.secrets.{ext}. You should gitignore the secrets file to prevent ever accidentally adding secrets to VCS.

Code Generation (typescript & others)

This module supports automatic type generation thanks to quicktype. You write the schema, and a typed representation can be generated for you so that your code is correct.

In your meta file, you can define:

  - { type: 'ts', file: 'src/config.d.ts' }

You can also do so in a top-level property of your config or schema file.

After this, you can simply run app-config gen. This will write the file(s) you specified in meta configuration. It's recommended to add a call to this in your dev script, so that your code is always in sync with your schema.

Right now, we only officially support typescript, but quicktype is extensible to other languages. You might have luck simply generating a different file type.

More options are available for generation:

  • type: string changes the file type, which is inferred by the filename
  • name: string changes the default config interface's name
  • augmentModule: boolean turns on or off the automatic module augmentation of app-config's default export
  • leadingComments: string[] changes the top-of-file comment block that's generated
  • rendererOptions: { [key: string]: string } adds any quicktype supported renderer option (like just-types)

Config Generation

We use the same infrastructure as described above for "config generation". That means that when running generation, you can just specify an output file like my-other-config.yaml. And instead of static types like above, the full configuration is spit out into that file.

This is extensible (and mostly only useful for) "selection", like in the creation CLI.

  - { file: 'rustw.toml', select: '#/build/cargo-src' }

There is also the includeSecrets boolean option, which is disabled by default.


This module supports a special 'extending' syntax for deduplicating configuration.

In your main config:

extends = ["other-file.yml"]

This will do a deep merge of other-file.yml into your main configuration.

Environment Specific Config

This module supports environment specific config and will attempt to load the config based on your NODE_ENV|ENV|APP_CONFIG_ENV environment variable and fallback to the default config file if nothing matching is found.

Environment specific configuration files are named [.]app-config.{NODE_ENV|ENV|APP_CONFIG_ENV}.{ext}.

If you prefer, you can alias the development and production config filenames with the short naming convention: [.]app-config.dev.{ext} and [.]app-config.prod.{ext}.

Secret files follow the same pattern and are loaded based on your NODE_ENV|ENV|APP_CONFIG_ENV.

You can also use a special $env property within any objects. This also supports aliasing.

  logging: false
      port: 3000
      port: 8080
      port: 80

The config above would result in:

  • default: { my-server-config: { logging: false, port: 3000 } }
  • development: { my-service-config: { logging: false, port: 8080 } }
  • production: { my-server-config: { logging: false, port: 80 } }

Extends Combined with Environment Specific Config

Using $env alongside extends, you can load different files for each environment.

        - 'test-database-config.yml'
        - 'database-config.yml'
        - 'database-config.yml'

This way, only 'database-config.yml' will be included in 'production' config; Where as 'test-databases-config.yml' and 'database-config.yml' will be included in 'development' config.

Using External Variables

If you need to, you can access environment variables with the string syntax $VAR_NAME or ${VAR_NAME}. This will in-place change the value to that environment variable.

You can also use the bash fallback syntax, like ${VAR:-fallback value} or ${VAR:-${OTHER_VAR}}.

Node API

This module exports a basic stable node API to do what you would expect.

// this is an already validated config object
export default config;

export {
  // loads app config from the filesystem
  // loads app config schema from the filesystem
  // validates the config against a schema
  validate(LoadedConfig & { schema }, cwd?),
  // loads app config and validates the schema
  // creates generated files defined in the schema
} from './schema';

Alternatively, if you don't want the config to be loaded on import, you'll need to:

import { loadValidated } from '@servall/app-config/dist/exports';



  • Webpack loader
  • Child extends properties
  • Non-js language support