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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.


The url to the project homepage.

"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 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.


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.

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.


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!


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.


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
  • * Matches any version
  • "" (just an empty string) Same as *
  • version1 - version2 Same as >=version1 <=version2.

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"


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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