The package.json File

This document is all you need to know about what's required in your package.json file. It must be actual JSON, not just a JavaScript object literal.

A lot of the behavior described in this document is affected by the config settings described in npm-config(7).

name

The most important things in your package.json are the name and version fields. Those are actually required, and your package won't install without them. The name and version together form an identifier that is assumed to be completely unique. Changes to the package should come along with changes to the version.

The name is what your thing is called.

Some rules:

  • The name must be less than or equal to 214 characters. This includes the scope for scoped packages.
  • The name can't start with a dot or an underscore.
  • New packages must not have uppercase letters in the name.
  • The name ends up being part of a URL, an argument on the command line, and a folder name. Therefore, the name can't contain any non-URL-safe characters.

Some tips:

  • Don't use the same name as a core Node module.
  • Don't put "js" or "node" in the name. It's assumed that it's js, since you're writing a package.json file, and you can specify the engine using the "engines" field. (See below.)
  • The name will probably be passed as an argument to require(), so it should be something short, but also reasonably descriptive.
  • You may want to check the npm registry to see if there's something by that name already, before you get too attached to it. https://www.npmjs.com/

A name can be optionally prefixed by a scope, e.g. @myorg/mypackage. See npm-scope(7) for more detail.

version

The most important things in your package.json are the name and version fields. Those are actually required, and your package won't install without them. The name and version together form an identifier that is assumed to be completely unique. Changes to the package should come along with changes to the version.

Version must be parseable by node-semver, which is bundled with npm as a dependency. (npm install semver to use it yourself.)

More on version numbers and ranges at semver(7).

description

Put a description in it. It's a string. This helps people discover your package, as it's listed in npm search.

keywords

Put keywords in it. It's an array of strings. This helps people discover your package as it's listed in npm search.

homepage

The url to the project homepage.

bugs

The url to your project's issue tracker and / or the email address to which issues should be reported. These are helpful for people who encounter issues with your package.

It should look like this:

{ "url" : "https://github.com/owner/project/issues"
, "email" : "project@hostname.com"
}

You can specify either one or both values. If you want to provide only a url, you can specify the value for "bugs" as a simple string instead of an object.

If a url is provided, it will be used by the npm bugs command.

license

You should specify a license for your package so that people know how they are permitted to use it, and any restrictions you're placing on it.

If you're using a common license such as BSD-2-Clause or MIT, add a current SPDX license identifier for the license you're using, like this:

{ "license" : "BSD-3-Clause" }

You can check the full list of SPDX license IDs. Ideally you should pick one that is OSI approved.

If your package is licensed under multiple common licenses, use an SPDX license expression syntax version 2.0 string, like this:

{ "license" : "(ISC OR GPL-3.0)" }

If you are using a license that hasn't been assigned an SPDX identifier, or if you are using a custom license, use a string value like this one:

{ "license" : "SEE LICENSE IN <filename>" }

Then include a file named <filename> at the top level of the package.

Some old packages used license objects or a "licenses" property containing an array of license objects:

// Not valid metadata
{ "license" :
  { "type" : "ISC"
  , "url" : "http://opensource.org/licenses/ISC"
  }
}

// Not valid metadata
{ "licenses" :
  [
    { "type": "MIT"
    , "url": "http://www.opensource.org/licenses/mit-license.php"
    }
  , { "type": "Apache-2.0"
    , "url": "http://opensource.org/licenses/apache2.0.php"
    }
  ]
}

Those styles are now deprecated. Instead, use SPDX expressions, like this:

{ "license": "ISC" }

{ "license": "(MIT OR Apache-2.0)" }

Finally, if you do not wish to grant others the right to use a private or unpublished package under any terms:

{ "license": "UNLICENSED"}

Consider also setting "private": true to prevent accidental publication.

people fields: author, contributors

The "author" is one person. "contributors" is an array of people. A "person" is an object with a "name" field and optionally "url" and "email", like this:

{ "name" : "Barney Rubble"
, "email" : "b@rubble.com"
, "url" : "http://barnyrubble.tumblr.com/"
}

Or you can shorten that all into a single string, and npm will parse it for you:

"Barney Rubble <b@rubble.com> (http://barnyrubble.tumblr.com/)"

Both email and url are optional either way.

npm also sets a top-level "maintainers" field with your npm user info.

files

The "files" field is an array of files to include in your project. If you name a folder in the array, then it will also include the files inside that folder. (Unless they would be ignored by another rule.)

You can also provide a ".npmignore" file in the root of your package or in subdirectories, which will keep files from being included, even if they would be picked up by the files array. The .npmignore file works just like a .gitignore.

Certain files are always included, regardless of settings:

  • package.json
  • README
  • CHANGES / CHANGELOG / HISTORY
  • LICENSE / LICENCE
  • The file in the "main" field

README, CHANGES & LICENSE can have any case and extension.

Conversely, some files are always ignored:

  • .git
  • CVS
  • .svn
  • .hg
  • .lock-wscript
  • .wafpickle-N
  • .*.swp
  • .DS_Store
  • ._*
  • npm-debug.log
  • .npmrc
  • node_modules

main

The main field is a module ID that is the primary entry point to your program. That is, if your package is named foo, and a user installs it, and then does require("foo"), then your main module's exports object will be returned.

This should be a module ID relative to the root of your package folder.

For most modules, it makes the most sense to have a main script and often not much else.

bin

A lot of packages have one or more executable files that they'd like to install into the PATH. npm makes this pretty easy (in fact, it uses this feature to install the "npm" executable.)

To use this, supply a bin field in your package.json which is a map of command name to local file name. On install, npm will symlink that file into prefix/bin for global installs, or ./node_modules/.bin/ for local installs.

For example, myapp could have this:

{ "bin" : { "myapp" : "./cli.js" } }

So, when you install myapp, it'll create a symlink from the cli.js script to /usr/local/bin/myapp.

If you have a single executable, and its name should be the name of the package, then you can just supply it as a string. For example:

{ "name": "my-program"
, "version": "1.2.5"
, "bin": "./path/to/program" }

would be the same as this:

{ "name": "my-program"
, "version": "1.2.5"
, "bin" : { "my-program" : "./path/to/program" } }

man

Specify either a single file or an array of filenames to put in place for the man program to find.

If only a single file is provided, then it's installed such that it is the result from man <pkgname>, regardless of its actual filename. For example:

{ "name" : "foo"
, "version" : "1.2.3"
, "description" : "A packaged foo fooer for fooing foos"
, "main" : "foo.js"
, "man" : "./man/doc.1"
}

would link the ./man/doc.1 file in such that it is the target for man foo

If the filename doesn't start with the package name, then it's prefixed. So, this:

{ "name" : "foo"
, "version" : "1.2.3"
, "description" : "A packaged foo fooer for fooing foos"
, "main" : "foo.js"
, "man" : [ "./man/foo.1", "./man/bar.1" ]
}

will create files to do man foo and man foo-bar.

Man files must end with a number, and optionally a .gz suffix if they are compressed. The number dictates which man section the file is installed into.

{ "name" : "foo"
, "version" : "1.2.3"
, "description" : "A packaged foo fooer for fooing foos"
, "main" : "foo.js"
, "man" : [ "./man/foo.1", "./man/foo.2" ]
}

will create entries for man foo and man 2 foo

directories

The CommonJS Packages spec details a few ways that you can indicate the structure of your package using a directories object. If you look at npm's package.json, you'll see that it has directories for doc, lib, and man.

In the future, this information may be used in other creative ways.

directories.lib

Tell people where the bulk of your library is. Nothing special is done with the lib folder in any way, but it's useful meta info.

directories.bin

If you specify a bin directory in directories.bin, all the files in that folder will be added.

Because of the way the bin directive works, specifying both a bin path and setting directories.bin is an error. If you want to specify individual files, use bin, and for all the files in an existing bin directory, use directories.bin.

directories.man

A folder that is full of man pages. Sugar to generate a "man" array by walking the folder.

directories.doc

Put markdown files in here. Eventually, these will be displayed nicely, maybe, someday.

directories.example

Put example scripts in here. Someday, it might be exposed in some clever way.

directories.test

Put your tests in here. It is currently not exposed, but it might be in the future.

repository

Specify the place where your code lives. This is helpful for people who want to contribute. If the git repo is on GitHub, then the npm docs command will be able to find you.

Do it like this:

"repository" :
  { "type" : "git"
  , "url" : "https://github.com/npm/npm.git"
  }

"repository" :
  { "type" : "svn"
  , "url" : "https://v8.googlecode.com/svn/trunk/"
  }

The URL should be a publicly available (perhaps read-only) url that can be handed directly to a VCS program without any modification. It should not be a url to an html project page that you put in your browser. It's for computers.

For GitHub, GitHub gist, Bitbucket, or GitLab repositories you can use the same shortcut syntax you use for npm install:

"repository": "npm/npm"

"repository": "gist:11081aaa281"

"repository": "bitbucket:example/repo"

"repository": "gitlab:another/repo"

scripts

The "scripts" property is a dictionary containing script commands that are run at various times in the lifecycle of your package. The key is the lifecycle event, and the value is the command to run at that point.

See npm-scripts(7) to find out more about writing package scripts.

config

A "config" object can be used to set configuration parameters used in package scripts that persist across upgrades. For instance, if a package had the following:

{ "name" : "foo"
, "config" : { "port" : "8080" } }

and then had a "start" command that then referenced the npm_package_config_port environment variable, then the user could override that by doing npm config set foo:port 8001.

See npm-config(7) and npm-scripts(7) for more on package configs.

engines

You can specify the version of node that your stuff works on:

{ "engines" : { "node" : ">=0.10.3 <0.12" } }

And, like with dependencies, if you don't specify the version (or if you specify "*" as the version), then any version of node will do.

If you specify an "engines" field, then npm will require that "node" be somewhere on that list. If "engines" is omitted, then npm will just assume that it works on node.

You can also use the "engines" field to specify which versions of npm are capable of properly installing your program. For example:

{ "engines" : { "npm" : "~1.0.20" } }

Unless the user has set the engine-strict config flag, this field is advisory only will produce warnings when your package is installed as a dependency.

engineStrict

This feature was deprecated with npm 3.0.0

Prior to npm 3.0.0, this feature was used to treat this package as if the user had set engine-strict.

os

You can specify which operating systems your module will run on:

"os" : [ "darwin", "linux" ]

You can also blacklist instead of whitelist operating systems, just prepend the blacklisted os with a '!':

"os" : [ "!win32" ]

The host operating system is determined by process.platform

It is allowed to both blacklist, and whitelist, although there isn't any good reason to do this.

cpu

If your code only runs on certain cpu architectures, you can specify which ones.

"cpu" : [ "x64", "ia32" ]

Like the os option, you can also blacklist architectures:

"cpu" : [ "!arm", "!mips" ]

The host architecture is determined by process.arch

preferGlobal

If your package is primarily a command-line application that should be installed globally, then set this value to true to provide a warning if it is installed locally.

It doesn't actually prevent users from installing it locally, but it does help prevent some confusion if it doesn't work as expected.

private

If you set "private": true in your package.json, then npm will refuse to publish it.

This is a way to prevent accidental publication of private repositories. If you would like to ensure that a given package is only ever published to a specific registry (for example, an internal registry), then use the publishConfig dictionary described below to override the registry config param at publish-time.

publishConfig

This is a set of config values that will be used at publish-time. It's especially handy if you want to set the tag, registry or access, so that you can ensure that a given package is not tagged with "latest", published to the global public registry or that a scoped module is private by default.

Any config values can be overridden, but of course only "tag", "registry" and "access" probably matter for the purposes of publishing.

See npm-config(7) to see the list of config options that can be overridden.

DEFAULT VALUES

npm will default some values based on package contents.

  • "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.

  • "contributors": [...]

    If there is an AUTHORS file in the root of your package, npm will treat each line as a Name <email> (url) format, where email and url are optional. Lines which start with a # or are blank, will be ignored.