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


npm aims to implement the commonjs Packages spec. However, some adjustments have been made, which may eventually be unmade, but hopefully will be incorporated into the spec.

This document is all you need to know about what's required in your package.json file.

A lot of the behavior described in this document is affected by the config settings described in npm help config.


The most important things in your package.json are the name and version fields.

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. So, don't use characters that are annoying in those contexts, like funny UTF things or parentheses or slashes, or else it'll break.
  • 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.

Version must be semver-compliant. npm assumes that you've read the semver page, and that you comply with it. Here's how it 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 > 0.1.2-6 > 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. (This is how node's versions are generated, and has driven this design.)


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


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


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. The default is just [""] which includes the entire package folder in the tarball, but you may want to only include specific things.

If you specify modules, bins, or man pages, then those will be automatically added to the files array, even if they would not ordinarily be included.

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 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" } }


The "modules" member exposes CommonJS modules in the package. So, if you had a package named foo, and the package.json contains "modules":{"bar":"./lib/baz"}, and there was a file called ./lib/baz.js, then require("foo/bar") would include the module defined in ./lib/baz.js.

Subfolders are supported, so you can do this:

{ "name" : "foo"
, "modules" :
  { "bar/baz" : "./lib/bar/baz"
  , "quux" : "./quux"

And then, doing require("foo/bar/baz") would return the module at ./lib/bar/baz in the foo package. Doing require("foo/quux") would return the module at ./quux in the foo package.

Just like the main script, the modules linked in this fashion will have their dependencies and paths set up properly by npm. (In fact, "main" is just sugar around setting a module named "index".)


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"
, "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"
, "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"
, "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.


If you specify a "lib" directory, and do not supply a modules hash, then the lib folder will be walked and any *.js or *.node files found will be exposed as a default module hash.

Providing an explicit modules hash is encouraged over exposing the entire lib folder.


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, as well as perhaps maybe being the underpinning of some magical "track this package on git" feature someday maybe if somebody wants to write it ever.

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.

Here are some examples of Doing It Wrong:

"repository" :
  { "type" : "git"
  , "url" : "" <-- THIS IS PRIVATE!

"repository" :
  { "type" : "git"
  , "url" : "" <-- THIS IS WEBPAGE!

This is ok, but completely unnecessary:
"repository" :
  { "type" : "git"
  , "url" : ""
  , "private" : ""
  , "web" : ""


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 with has one or more space-separated descriptors, OR a range like "fromVersion - toVersion"

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 as a bundle at install time. See npm help bundle


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.


npm responds to the node and npm env-specific package.json values, which you can hang on the "overlay" key.

For example:

{ "name" : "foo"
, "version" : 7
, "description" : "generic description"
, "overlay" :
  { "node" :
    { "name" : "bar"
    , "description" : "description for node"
  , "npm" :
    { "version" : "1.0.7"
    , "description" : "description for npm"
  , "narwhal" :
    { "description" : "description for narwhal" }

In this case, this is what npm will treat it as:

{ "name" : "bar"
, "version" : "1.0.7"
, "description" : "description for npm"

This way, even if npm is not exactly the same as some other package management system, you can still use both, and it can be a happy planet.

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