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caution do not use package.json for the folder name if you want to clone this project to your machine - it will break yarn (An unexpected error occurred: "EISDIR: illegal operation on a directory, read".).

Original version of this document copied from yarnpkg.

See also npm documentation, std-pkg, clean-publish, package-json-validator, cosmiconfig, rc (as an opponent approach to cosmiconfig).

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Essentials

The two most important fields in your package.json are name and version, without them your package won't be able to install. The name and version fields are used together to create a unique id.

name

{
  "name": "my-awesome-package"
}

This is the name of your package. It gets used in URLs, as an argument on the command line, and as the directory name inside node_modules.

yarn add [name]

node_modules/[name]

https://registry.npmjs.org/[name]/-/[name]-[version].tgz

Rules

Tips

version

{
  "version": "1.0.0"
}

The current version of your package.

Info

description

{
  "description": "My short description of my awesome package"
}

The description is just a string that helps people understand the purpose of the package. It can be used when searching for packages in a package manager as well.

keywords

{
  "keywords": ["short", "relevant", "keywords", "for", "searching"]
}

Keywords are an array of strings that are useful when searching for packages in a package manager.

license

{
  "license": "MIT",
  "license": "(MIT or GPL-3.0)",
  "license": "SEE LICENSE IN LICENSE_FILENAME.txt",
  "license": "UNLICENSED"
}

All packages should specify a license so that users know how they are permitted to use it and any restrictions that you are placing on it.

You are encouraged to use an Open Source (OSI-approved) license unless you have a specific reason not to. If you built your package as part of your job it's likely best to check with your company before deciding on a license.

Must be one of the following:

Links

Various links to documentation, places to file issues and where your package code actually lives.

homepage

{
  "homepage": "https://your-package.org"
}

The homepage is the URL to the landing page or documentation for your package.

Also used by Create React App

bugs

{
  "bugs": "https://github.com/user/repo/issues"
}

The URL to your project's issue tracker. This can also be something like an email address as well. It provides users a way to find out where to send issues with your package.

repository

{
  "repository": { "type": "git", "url": "https://github.com/user/repo.git" },
  "repository": "github:user/repo",
  "repository": "gitlab:user/repo",
  "repository": "bitbucket:user/repo",
  "repository": "gist:a1b2c3d4e5f"
}

The repository is the location where the actual code for your package lives.

Maintainers

The maintainers of your project.

author

{
  "author": { "name": "Your Name", "email": "you@example.com", "url": "http://your-website.com" },
  "author": "Your Name <you@example.com> (http://your-website.com)"
}

Package author information. An author is one person.

contributors

{
  "contributors": [
    { "name": "Your Friend", "email": "friend@example.com", "url": "http://friends-website.com" }
    { "name": "Other Friend", "email": "other@example.com", "url": "http://other-website.com" }
  ],
  "contributors": [
    "Your Friend <friend@example.com> (http://friends-website.com)",
    "Other Friend <other@example.com> (http://other-website.com)"
  ]
}

Those that have contributed to your package. Contributors are an array of people.

Files

You can specify files that will be included in your project, along with the main entry point for your project.

files

{
  "files": [
    "filename.js",
    "directory/",
    "glob/*.{js,json}"
  ]
}

These are files that are included in your project. You can specify single files, whole directories or use wildcards to include files that meet a certain criteria.

main

{
  "main": "filename.js"
}

This is the primary entry point for the functionality for your project.

bin

{
  "bin": "bin.js",
  "bin": {
    "command-name": "bin/command-name.js",
    "other-command": "bin/other-command"
  }
}

Executable files included with your project that will be installed.

man

{
  "man": "./man/doc.1",
  "man": ["./man/doc.1", "./man/doc.2"]
}

If you have man pages associated with your project, add them here.

directories

{
  "directories": {
    "lib": "path/to/lib/",
    "bin": "path/to/bin/",
    "man": "path/to/man/",
    "doc": "path/to/doc/",
    "example": "path/to/example/"
  }
}

When installing your package, you can specify exact locations to put binary files, man pages, documentation, examples, etc.

Tasks

Your package can include runnable scripts or other configuration.

scripts

{
  "scripts": {
    "build-project": "node build-project.js"
  }
}

Scripts are a great way of automating tasks related to your package, such as simple build processes or development tools. Using the "scripts" field, you can define various scripts to be run as yarn run <script>. For example, the build-project script above can be invoked with yarn run build-project and will run node build-project.js.

Certain script names are special. If defined, the preinstall script is called by yarn before your package is installed. For compatibility reasons, scripts called install, postinstall, and prepublish will all be called after your package has finished installing.

The start script value defaults to node server.js.

"scripts": {"start": "node server.js"}. If there is a server.js file in the root of your package, then npm will default the start command to node server.js.

"scripts":{"preinstall": "node-gyp rebuild"}. If there is a binding.gyp file in the root of your package, npm will default the preinstall command to compile using node-gyp.

-- npm docs

npm specific scripts

npm supports the scripts property of the package.json file, for the following scripts:

Source: npm docs.

config

{
  "config": {
    "port": "8080"
  }
}

Configuration options or parameters used in your scripts.

Dependencies

Your package will very likely depend on other packages. You can specify those dependencies in your package.json file.

dependencies

{
  "dependencies": {
    "package-1": "^3.1.4"
  }
}

These are dependencies that are required in both development and production for your package.

You can specify an exact version, a minimum version (e.g., >=) or a range of versions (e.g. >= ... <).

devDependencies

{
  "devDependencies": {
    "package-2": "^0.4.2"
  }
}

These are packages that are only required when developing your package but will not be installed in production.

peerDependencies

{
  "peerDependencies": {
    "package-3": "^2.7.18"
  }
}

Peer dependencies allow you to state compatibility of your package with versions of other packages.

optionalDependencies

{
  "optionalDependencies": {
    "package-5": "^1.6.1"
  }
}

Optional dependencies can be used with your package, but are not required. If the optional package is not found, installation still continues.

bundledDependencies

{
  "bundledDependencies": [
    "package-4"
  ]
}

Bundled dependencies are an array of package names that will be bundled together when publishing your package.

System

You can provide system-level information associated with your package, such as operating system compatibility, etc.

engines

{
  "engines": {
    "node": ">=4.4.7 <7.0.0",
    "zlib": "^1.2.8",
    "yarn": "^0.14.0"
  }
}

The engines specify versions of clients that must be used with your package. This checks against process.versions as well as the current version of yarn.

os

{
  "os": ["darwin", "linux"],
  "os": ["!win32"]
}

This specifies operating system compatibility for your package. It checks against process.platform.

cpu

{
  "cpu": ["x64", "ia32"],
  "cpu": ["!arm", "!mips"]
}

Use this to specify your package will only run on certain CPU architectures. This checks against process.arch.

libc

{
  "libc": ["glibc"],
  "libc": ["musl"]
}

Use this to specify your package only runs on a specific flavor of libc. This checks against the result from detect-libc.

Publishing

private

{
  "private": true
}

If you do not want your package published in a package manager, set this to true.

publishConfig

{
  "publishConfig": {
    ...
  }
}

These configuration values will be used when publishing your package. You can tag your package, for example.

Yarn

flat

{
  "flat": true
}

If your package only allows one version of a given dependency, and you'd like to enforce the same behavior as yarn install --flat on the command line, set this to true.

Note that if your package.json contains "flat": true and other packages depend on yours (e.g. you are building a library rather than an application), those other packages will also need "flat": true in their package.json or be installed with yarn install --flat on the command line.

resolutions

{
  "resolutions": {
    "transitive-package-1": "0.0.29",
    "transitive-package-2": "file:./local-forks/transitive-package-2",
    "dependencies-package-1/transitive-package-3": "^2.1.1"
  }
}

Allows you to override a version of a particular nested dependency. See the Selective Versions Resolutions RFC for the full spec.

Note that installing dependencies via [yarn install --flat] will automatically add a resolutions block to your package.json file.

Lerna + Yarn

workspaces

If --use-workspaces is true then packages will be overridden by the value from package.json/workspaces.

Source: --use-workspaces.

Bolt

bolt

See Configuration

{
  "bolt": {
    "workspaces": [
      "utils/*",
      "apps/*"
    ]
  }
}

unpkg

unpkg

If you omit the file path (i.e. use a "bare" URL), unpkg will serve the file specified by the unpkg field in package.json, or fall back to main (source).

TypeScript

types

If your package has a main .js file, you will need to indicate the main declaration file in your package.json file as well. Set the types property to point to your bundled declaration file. For example:

{
  "main": "./lib/main.js",
  "types": "./lib/main.d.ts"
}

Note that the typings field is synonymous with types, and could be used as well.

Also note that if your main declaration file is named index.d.ts and lives at the root of the package (next to index.js) you do not need to mark the types property, though it is advisable to do so.

Source: TypeScript documentation

Note: in Flow they use different approach

Flow

flow:main

Not officially supported. Proposal is here.

browserslist

browserslist

💖 Library to share target browsers between different front-end tools. It is used in:

All tools that rely on Browserslist will find its config automatically, when you add the following to package.json:

{
  "browserslist": [
    "> 1%",
    "last 2 versions"
  ]
}

Source: browserslist.

See also: Create React App Support.

Package bundlers

See "Setting up multi-platform npm packages" for an introduction.

module

pkg.module will point to a module that has ES2015 module syntax but otherwise only syntax features that the target environments support. Full description is here.

Supported by: rollup, webpack

browser

The browser field is provided by a module author as a hint to javascript bundlers or component tools when packaging modules for client side use. Proposal is here.

Supported by: rollup, webpack, browserify

Support requested: babel-plugin-module-resolver

esnext

Full proposal is here. Short explanation:

{
  "main": "main.js",
  "esnext": {
    "main": "main-esnext.js",
    "browser": "browser-specific-main-esnext.js"
  }
}

See also: Delivering untranspiled source code via npm

es2015

Untranspiled ES6 code. Source: Angular Package Format (APF) v5.0

esm

Proposal is here: adjusted proposal: ES module "esm": true package.json flag

See also:

module-browser

See this issue

Also referred as moduleBrowser, jsnext:browser.

modules.root

Mentioned in In Defense of .js.

There is also modules.resolver.

jsnext:main

DEPRECATED

jsnext:main has been superseded by pkg.module, which indicates the location of a file with import/export declarations. Original proposal is here.

Supported by: rollup.

metro

react-native

Works similar to browser, but for react-native specific modules. Source.

webpack

sideEffects

Indicates that the package's modules have no side effects (on evaluation) and only expose exports. This allows tools like webpack to optimize re-exports.

See also: sideEffects example, proposal for marking functions as pure, eslint-plugin-tree-shaking.

microbundle

source, umd:main

See Specifying builds in package.json.

Parcel

source

See parcel-bundler/parcel#1652.

@std/esm

@std/esm

Developers have strong opinions on just about everything. To accommodate, @std/esm allows unlocking extra features with the "@std/esm" or "@std":{"esm":{}} field in your package.json.

Source: @std/esm Unlockables

jspm

jspm

You can write all package properties at the base of the package.json, or if you don't want to change existing properties that you'd like to use specifically for npm, you can write your jspm-specific configuration inside the jspm property of package.json, and jspm will use these options over the root level configuration options.

For example:

{
  "name": "my-package",
  "jspm": {
    "main": "jspm-main"
  }
}

See full specification.

ignore

If there are certain specific files or folders to ignore, they can be listed in an array.

format

Options are esm, amd, cjs and global.

When loading modules from npm, the module format is treated as cjs by default and no automatic detection is run. To load modules of another format you will need to override this property manually.

Module format esm (ECMAScript Module) currently isn't used in package.json.

registry

jspm understands dependencies in the context of a registry.

When loading packages from npm, jspm will set the default registry to npm, and treat the dependencies accordingly.

When loading packages from GitHub, the dependencies property is ignored without a registry property being present, as jspm has no way of knowing what the dependencies mean for existing GitHub repos.

Setting the registry property also determines how jspm interprets the package. For example, a GitHub package with registry: "npm" will, along with getting its dependencies from npm, be interpreted as CommonJS and support features like directory and JSON requires, exactly as if it had been installed from the npm endpoint to begin with.

A package on GitHub with its registry property set to registry: "jspm" will have its dependencies treated as jspm-style dependencies.

shim

Packages written as globals need a shim configuration to work properly in a modular environment. To shim a file some/global.js within the package, we can write:

{
  "shim": {
    "some/global": {
      "deps": ["jquery"],
      "exports": "globalExportName"
    }
  }
}

Both deps and exports are optional.

exports is detected automatically by the SystemJS loader as any globals written by the script. In most cases this detection will work out correctly.

The shortcut form of "some/global": ["jquery"] is also supported if there are no exports.

map

Map configuration will rewrite internal requires to point to different local or external modules.

Consider a package which includes its own dependency, dep located at third_party/dep. It could have a require statement internally something like:

  require('dep');

In order to use the local version, we can write:

{
  "map": {
    "dep": "./third_party/dep"
  }
}

It can also be useful to reference a package by its own name within submodules:

{
  "map": {
    "thispackage": "."
  }
}

We can then have internal requires to import 'thispackage/module' resolve correctly.

Map configuration can also reference dependency submodules.

We can also exclude modules entirely by mapping them to the empty module:

{
  "map": {
    "jquery": "@empty"
  }
}

The value returned will then be a Module object with no exports.

browserify

browserify.transform

Documentation is here

Create React App

proxy

People often serve the front-end React app from the same host and port as their backend implementation.

Source: Proxying API Requests in Development

homepage

Source: Building for Relative Paths

babel

babel

See this issue.

eslint

eslintConfig

jest

jest

{
  "jest": {
    "verbose": true
  }
}

Source: jest docs

stylelint

stylelint

See: New configuration loader

size-limit

size-limit

If you're using this library you can define its config in package.json:

{
  "size-limit": [
    {
      "limit": "9 KB",
      "path": "index.js"
    }
  ]
}

Source: size-limit

PWMetrics

pwmetrics

You cen specify options in package.json:

{
  "pwmetrics": {
    "url": "http://example.com/",
    "expectations": {
      "ttfcp": {
        "warn": ">=1500",
        "error": ">=2000"
      }
    }
  }
}

All available options are here

Source: pwmetrics

AVA

ava

Example:

"ava": {
  "require": [ "@std/esm" ]
}

Source: ava

nyc

nyc

Example:

"nyc": {
  "extension": [".js", ".mjs"],
  "require": ["@std/esm"]
}

Source: nyc

Other

preferGlobal

DEPRECATED

This option used to trigger an npm warning, but it will no longer warn. It is purely there for informational purposes. It is now recommended that you install any binaries as local devDependencies wherever possible.

style

The style attribute in package.json is useful for importing CSS packages. Proposal is here.

Supported by: parcelify, npm-less, rework-npm, npm-css.

See also: istf-spec.

less

Same as style but for less.

Supported by: npm-less.

CommonJS Packages

Reserved Properties

The following fields are reserved for future expansion: build, default, email, external, files, imports, maintainer, paths, platform, require, summary, test, using, downloads, uid.

The following fields are reserved for package registries to use at their discretion: id, type.

All properties beginning with _ or $ are also reserved for package registries to use that their discretion.

Source: CommonJS wiki

Standard JS

standard

Standard JS is a javaScript style guide, linter, and formatter, you can add some property to package.json, like parser, ignore, globals, plugins.

Example:

"standard": {
  "parser": "babel-eslint",
  "ignore": [
    "**/out/",
    "/lib/select2/",
    "/lib/ckeditor/",
    "tmp.js"
  ]
}

See also: Standard JS