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npm-json(1) -- Specifics of npm's package.json handling


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 help config.


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-waf clean || true; node-waf configure build"}

    If there is a wscript file in the root of your package, npm will default the preinstall command to compile using node-waf.

  • "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.


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 tips:

  • 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 ends up being part of a URL, an argument on the command line, and a folder name. Any name with non-url-safe characters will be rejected. Also, it can't start with a dot or an underscore.
  • 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.


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.)

Here's how npm's semver implementation deviates from what's on

  • Versions can start with "v"
  • A numeric item separated from the main three-number version by a hyphen will be interpreted as a "build" number, and will increase the version. But, if the tag is not a number separated by a hyphen, then it's treated as a pre-release tag, and is less than the version without a tag. So, 0.1.2-7 > 0.1.2-7-beta > 0.1.2-6 > 0.1.2 > 0.1.2beta

This is a little bit confusing to explain, but matches what you see in practice when people create tags in git like "v1.2.3" and then do "git describe" to generate a patch version.


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.

NOTE: This is not the same as "url". If you put a "url" field, then the registry will think it's a redirection to your package that has been published somewhere else, and spit at you.

Literally. Spit. I'm so not kidding.

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" : ""
, "url" : ""

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

"Barney Rubble <> (

Both email and url are optional either way.

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


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, 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".


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.


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 link that file into place right next to wherever node is installed. (Presumably, this is in your PATH, and defaults to /usr/local/bin.) On activation, the versioned file will get linked to the main filename (just like how the main.js stuff works, but with an executable in the PATH.)

For example, npm has this:

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

So, when you install npm, it'll create a symlink from the cli.js script to /usr/local/bin/npm-version. Then, when you activate that version, it'll create a symlink from /usr/local/bin/npm-version to /usr/local/bin/npm.

Notice that if the executable file is interpreted by node (i.e., specifying node in the shebang line), npm actually installs a shim instead of symlinking it, which causes expressions require.main === module and === "." evaluate to false within the file. This seems unable to be resolved until node provides a "flexible require()".

Shortcut: If you have a single executable, and its name is already what you want it to be, then you can just supply it as a string. For example:

{ "bin" : "./path/to/program" }

would be the same as this:

{ "bin" : { "program" : "./path/to/program" } }


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


The CommonJS Packages spec details a few ways that you can indicate the structure of your package using a directories hash. 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.


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.


If you specify a "bin" directory, then all the files in that folder will be used as the "bin" hash.

If you have a "bin" hash already, then this has no effect.

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


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


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


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" : ""

"repository" :
  { "type" : "svn"
  , "url" : ""

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.


The "scripts" member is an object hash of 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 help scripts to find out more about writing package scripts.


A "config" hash 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 help config and npm help scripts for more on package configs.


Dependencies are specified with a simple hash of package name to version range. The version range is EITHER a string which has one or more space-separated descriptors, OR a range like "fromVersion - toVersion"

Please do not put test harnesses in your dependencies hash. See devDependencies, below.

Version range descriptors may be any of the following styles, where "version" is a semver compatible version identifier.

  • version Must match version exactly
  • =version Same as just version
  • >version Must be greater than version
  • >=version etc
  • <version
  • <=version
  • ~version See 'Tilde Version Ranges' below
  • 1.2.x See 'X Version Ranges' below
  • http://... See 'URLs as Dependencies' below
  • * 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.

For example, these are all valid:

{ "dependencies" :
  { "foo" : "1.0.0 - 2.9999.9999"
  , "bar" : ">=1.0.2 <2.1.2"
  , "baz" : ">1.0.2 <=2.3.4"
  , "boo" : "2.0.1"
  , "qux" : "<1.0.0 || >=2.3.1 <2.4.5 || >=2.5.2 <3.0.0"
  , "asd" : ""
  , "til" : "~1.2"
  , "elf" : "~1.2.3"
  , "two" : "2.x"
  , "thr" : "3.3.x"

Tilde Version Ranges

A range specifier starting with a tilde ~ character is matched against a version in the following fashion.

  • The version must be at least as high as the range.
  • The version must be less than the next major revision above the range.

For example, the following are equivalent:

  • "~1.2.3" = ">=1.2.3 <1.3.0"
  • "~1.2" = ">=1.2.0 <2.0.0"
  • "~1" = ">=1.0.0 <2.0.0"

X Version Ranges

An "x" in a version range specifies that the version number must start with the supplied digits, but any digit may be used in place of the x.

The following are equivalent:

  • "1.2.x" = ">=1.2.0 <1.3.0"
  • "1.x.x" = ">=1.0.0 <2.0.0"
  • "1.2" = "1.2.x"
  • "1.x" = "1.x.x"
  • "1" = "1.x.x"

You may not supply a comparator with a version containing an x. Any digits after the first "x" are ignored.

URLs as Dependencies

Starting with npm version 0.2.14, 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.


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 list these additional items in a devDependencies hash.

These things will be installed whenever the --dev configuration flag is set. This flag is set automatically when doing npm link, and can be managed like any other npm configuration param. See npm help config for more on the topic.


Array of package names that will be bundled when publishing the package.


Packages/1.0 says that you can have an "engines" field with an array of engine names. However, it has no provision for specifying which version of the engine your stuff runs on.

With npm, you can use either of the following styles to specify the version of node that your stuff works on:

{ "engines" : [ "node >=0.1.27 <0.1.30" ] }


{ "engines" : { "node" : ">=0.1.27 <0.1.30" } }

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.


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.


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 speciic registry (for example, an internal registry), then use the publishConfig hash 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 or registry, so that you can ensure that a given package is not tagged with "latest" or published to the global public registry by default.

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

See npm help config to see the list of config options that can be overridden.

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