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All npm packages contain a file, usually in the project root, called package.json – this file holds various metadata relevant to the project. This file is used to give information to npm that allows it to identify the project as well as handle the project’s dependencies. It can also contain other metadata such as a project description, the version of the project in a particular distribution, license information, even configuration data – all of which can be vital to both npm and to the end users of the package. The package.json file is normally located at the root directory of a Node.js project.

Node itself is only aware of two fields in the package.json:


“name” : “barebones”,

“version” : “0.0.0”,


The name field should explain itself: this is the name of your project. The version field is used by npm to make sure the right version of the package is being installed. Generally, it takes the form of major.minor.patch where major, minor, and patch are integers which increase after each new release.

For a more complete package.json, we can check out underscore:


“name” : “underscore”,

“description” : “JavaScript’s functional programming helper library.”,

“homepage” : “”,

“keywords” : [“util”, “functional”, “server”, “client”, “browser”],

“author” : “Jeremy Ashkenas <[email protected]>”,

“contributors” : [],

“dependencies” : [],

“repository” : {“type”: “git”, “url”: “git://”},

“main” : “underscore.js”,

“version” : “1.1.6”


As you can see, there are fields for the description and keywords of your projects. This allows people who find your project understand what it is in just a few words. The author, contributors, homepage and repository fields can all be used to credit the people who contributed to the project, show how to contact the author/maintainer, and give links for additional references.

The file listed in the main field is the main entry point for the library; when someone runs require(<library name>), require resolves this call to require(<package.json:main>).

Finally, the dependencies field is used to list all the dependencies of your project that are available on npm. When someone installs your project through npm, all the dependencies listed will be installed as well. Additionally, if someone runs npm install in the root directory of your project, it will install all the dependencies to ./node_modules.

It is also possible to add a devDependencies field to your package.json – these are dependencies not required for normal operation, but required/recommended if you want to patch or modify the project. If you built your unit tests using a testing framework, for example, it would be appropriate to put the testing framework you used in your devDependencies field. To install a project’s devDependencies, simply pass the –dev option when you use npm install.


If you plan to publish your package, the most important things in your package.json are the name and version fields as they will be required. 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. If you don’t plan to publish your package, the name and version fields are optional. 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.


If you plan to publish your package, the most important things in your package.json are the name and version fields as they will be required. 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. If you don’t plan to publish your package, the name and version fields are optional.

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


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


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


The url to the project homepage. Example: “homepage”: “”


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” : “”

, “email” : “[email protected]


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.


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)” }

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” : “[email protected]

, “url” : “”



The optional files field is an array of file patterns that describes the entries to be included when your package is installed as a dependency. File patterns follow a similar syntax to .gitignore, but reversed: including a file, directory, or glob pattern (*, **/*, and such) will make it so that file is included in the tarball when it’s packed. Omitting the field will make it default to [“*”], which means it will include all files.

Some special files and directories are also included or excluded regardless of whether they exist in the files array.

You can also provide a .npmignore file in the root of your package or in subdirectories, which will keep files from being included. At the root of your package it will not override the “files” field, but in subdirectories it will. The .npmignore file works just like a .gitignore. If there is a .gitignore file, and .npmignore is missing, .gitignore’s contents will be used instead.

Files included with the “package.json#files” field cannot be excluded through .npmignore or .gitignore.

Certain files are always included, regardless of settings:

  • json
  • The file in the “main” field

Conversely, some files are always ignored:

  • .git
  • CVS
  • .svn
  • .hg
  • .lock-wscript
  • .wafpickle-N
  • .*.swp
  • .DS_Store
  • ._*
  • npm-debug.log
  • .npmrc
  • node_modules
  • gypi
  • *.orig
  • package-lock.json (use shrinkwrap instead)


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.


If your module is meant to be used client-side the browser field should be used instead of the main field. This is helpful to hint users that it might rely on primitives that aren’t available in Node.js modules. (e.g. window)


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.


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.


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.

  • 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.
  • bin – If you specify a bin directory in directories.bin, all the files in that folder will be added.
  • man – A folder that is full of man pages. Sugar to generate a “man” array by walking the folder.
  • doc – Put markdown files in here. Eventually, these will be displayed nicely, maybe, someday.
  • example – Put example scripts in here. Someday, it might be exposed in some clever way.
  • test – Put your tests in here. It is currently not exposed, but it might be in the future.



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.


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.


Dependencies are specified in a simple object that maps a package name to a version range. The version range is a string which has one or more space-separated descriptors. Dependencies can also be identified with a tarball or git URL. Please do not put test harnesses or transpilers in your dependencies object.

  • version Must match version exactly
  • >version Must be greater than version
  • >=version etc
  • <version
  • <=version
  • ~version “Approximately equivalent to version”
  • ^version “Compatible with version”
  • 2.x 1.2.0, 1.2.1, etc., but not 1.3.0
  • http://…
  • * Matches any version
  • “” (just an empty string) Same as *
  • version1 – version2 Same as >=version1 <=version2.
  • range1 || range2 Passes if either range1 or range2 are satisfied.
  • tag A specific version tagged and published as tag

URLs as Dependencies – You may specify a tarball URL in place of a version range. This tarball will be downloaded and installed locally to your package at install time.

Local Paths – As of version 2.0.0 you can provide a path to a local directory that contains a package. Local paths can be saved using npm install -S or npm install –save, using any of these forms:





in which case they will be normalized to a relative path and added to your package.json. For example:


“name”: “baz”,

“dependencies”: {

“bar”: “file:../foo/bar”



This feature is helpful for local offline development and creating tests that require npm installing where you don’t want to hit an external server, but should not be used when publishing packages to the public registry.

devDependencies – If someone is planning on downloading and using your module in their program, then they probably don’t want or need to download and build the external test or documentation framework that you use. In this case, it’s best to map these additional items in a devDependencies object.

These things will be installed when doing npm link or npm install from the root of a package, and can be managed like any other npm configuration param. For build steps that are not platform-specific, such as compiling CoffeeScript or other languages to JavaScript, use the prepare script to do this, and make the required package a devDependency.

peerDependencies – In some cases, you want to express the compatibility of your package with a host tool or library, while not necessarily doing a require of this host. This is usually referred to as a plugin. Notably, your module may be exposing a specific interface, expected and specified by the host documentation.

bundledDependencies – This defines an array of package names that will be bundled when publishing the package. In cases where you need to preserve npm packages locally or have them available through a single file download, you can bundle the packages in a tarball file by specifying the package names in the bundledDependencies array and executing npm pack.

optionalDependencies – If a dependency can be used, but you would like npm to proceed if it cannot be found or fails to install, then you may put it in the optionalDependencies object. This is a map of package name to version or url, just like the dependencies object. The difference is that build failures do not cause installation to fail.


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 and will only produce warnings when your package is installed as a dependency.


This feature was removed in 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. It is no longer used.


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.


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


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.


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 only “tag”, “registry” and “access” probably matter for the purposes of publishing.


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”:{“install”: “node-gyp rebuild”} – If there is a binding.gyp file in the root of your package and you have not defined an install or preinstall script, npm will default the install 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.


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