How npm handles the "scripts" field

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The "scripts" property of of your package.json file supports a number of built-in scripts and their preset life cycle events as well as arbitrary scripts. These all can be executed by running npm run-script <stage> or npm run <stage> for short. Pre and post commands with matching names will be run for those as well (e.g. premyscript, myscript, postmyscript). Scripts from dependencies can be run with npm explore <pkg> -- npm run <stage>.

Pre & Post Scripts

To create "pre" or "post" scripts for any scripts defined in the "scripts" section of the package.json, simply create another script with a matching name and add "pre" or "post" to the beginning of them.

"scripts": {
"precompress": "{{ executes BEFORE the `compress` script }}",
"compress": "{{ run command to compress files }}",
"postcompress": "{{ executes AFTER `compress` script }}"

Life Cycle Scripts

There are some special life cycle scripts that happen only in certain situations. These scripts happen in addtion to the "pre" and "post" script.

  • prepare, prepublish, prepublishOnly, prepack, postpack

prepare (since npm@4.0.0)

  • Runs BEFORE the package is packed
  • Runs BEFORE the package is published
  • Runs on local npm install without any arguments
  • Run AFTER prepublish, but BEFORE prepublishOnly
  • NOTE: If a package being installed through git contains a prepare script, its dependencies and devDependencies will be installed, and the prepare script will be run, before the package is packaged and installed.

prepublish (DEPRECATED)

  • Same as prepare


  • Runs BEFORE the package is prepared and packed, ONLY on npm publish.


  • Runs BEFORE a tarball is packed (on "npm pack", "npm publish", and when installing a git dependencies).
  • NOTE: "npm run pack" is NOT the same as "npm pack". "npm run pack" is an arbitrary user defined script name, where as, "npm pack" is a CLI defined command.


  • Runs AFTER the tarball has been generated and moved to its final destination.

Prepare and Prepublish

Deprecation Note: prepublish

Since npm@1.1.71, the npm CLI has run the prepublish script for both npm publish and npm install, because it's a convenient way to prepare a package for use (some common use cases are described in the section below). It has also turned out to be, in practice, very confusing. As of npm@4.0.0, a new event has been introduced, prepare, that preserves this existing behavior. A new event, prepublishOnly has been added as a transitional strategy to allow users to avoid the confusing behavior of existing npm versions and only run on npm publish (for instance, running the tests one last time to ensure they're in good shape).

See for a much lengthier justification, with further reading, for this change.

Use Cases

If you need to perform operations on your package before it is used, in a way that is not dependent on the operating system or architecture of the target system, use a prepublish script. This includes tasks such as:

  • Compiling CoffeeScript source code into JavaScript.
  • Creating minified versions of JavaScript source code.
  • Fetching remote resources that your package will use.

The advantage of doing these things at prepublish time is that they can be done once, in a single place, thus reducing complexity and variability. Additionally, this means that:

  • You can depend on coffee-script as a devDependency, and thus your users don't need to have it installed.
  • You don't need to include minifiers in your package, reducing the size for your users.
  • You don't need to rely on your users having curl or wget or other system tools on the target machines.

Life Cycle Operation Order

npm publish

  • prepublishOnly
  • prepare
  • prepublish
  • publish
  • postpublish

npm pack

  • prepack
  • postpack

npm install

  • preinstall
  • install
  • postinstall

Also triggers

  • prepublish (when on local)
  • prepare (when on local)

npm start

npm run start has an npm start shorthand.

  • prestart
  • start
  • poststart

Default Values

npm will default some script values based on package contents.

  • "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.

  • "install": "node-gyp rebuild":

    If there is a binding.gyp file in the root of your package and you haven't defined your own install or preinstall scripts, npm will default the install command to compile using node-gyp.


If npm was invoked with root privileges, then it will change the uid to the user account or uid specified by the user config, which defaults to nobody. Set the unsafe-perm flag to run scripts with root privileges.


Package scripts run in an environment where many pieces of information are made available regarding the setup of npm and the current state of the process.


If you depend on modules that define executable scripts, like test suites, then those executables will be added to the PATH for executing the scripts. So, if your package.json has this:

"name": "foo",
"dependencies": {
"bar": "0.1.x"
"scripts": {
"start": "bar ./test"

then you could run npm start to execute the bar script, which is exported into the node_modules/.bin directory on npm install.

package.json vars

The package.json fields are tacked onto the npm_package_ prefix. So, for instance, if you had {"name":"foo", "version":"1.2.5"} in your package.json file, then your package scripts would have the npm_package_name environment variable set to "foo", and the npm_package_version set to "1.2.5". You can access these variables in your code with process.env.npm_package_name and process.env.npm_package_version, and so on for other fields.


Configuration parameters are put in the environment with the npm_config_ prefix. For instance, you can view the effective root config by checking the npm_config_root environment variable.

Special: package.json "config" object

The package.json "config" keys are overwritten in the environment if there is a config param of <name>[@<version>]:<key>. For example, if the package.json has this:

"name": "foo",
"config": {
"port": "8080"
"scripts": {
"start": "node server.js"

and the server.js is this:


then the user could change the behavior by doing:

npm config set foo:port 80

current lifecycle event

Lastly, the npm_lifecycle_event environment variable is set to whichever stage of the cycle is being executed. So, you could have a single script used for different parts of the process which switches based on what's currently happening.

Objects are flattened following this format, so if you had {"scripts":{"install":"foo.js"}} in your package.json, then you'd see this in the script:

process.env.npm_package_scripts_install === "foo.js"


For example, if your package.json contains this:

"scripts": {
"install": "scripts/install.js",
"postinstall": "scripts/install.js",
"uninstall": "scripts/uninstall.js"

then scripts/install.js will be called for the install and post-install stages of the lifecycle, and scripts/uninstall.js will be called when the package is uninstalled. Since scripts/install.js is running for two different phases, it would be wise in this case to look at the npm_lifecycle_event environment variable.

If you want to run a make command, you can do so. This works just fine:

"scripts": {
"preinstall": "./configure",
"install": "make && make install",
"test": "make test"


Scripts are run by passing the line as a script argument to sh.

If the script exits with a code other than 0, then this will abort the process.

Note that these script files don't have to be nodejs or even javascript programs. They just have to be some kind of executable file.

Hook Scripts

If you want to run a specific script at a specific lifecycle event for ALL packages, then you can use a hook script.

Place an executable file at node_modules/.hooks/{eventname}, and it'll get run for all packages when they are going through that point in the package lifecycle for any packages installed in that root.

Hook scripts are run exactly the same way as package.json scripts. That is, they are in a separate child process, with the env described above.

Best Practices

  • Don't exit with a non-zero error code unless you really mean it. Except for uninstall scripts, this will cause the npm action to fail, and potentially be rolled back. If the failure is minor or only will prevent some optional features, then it's better to just print a warning and exit successfully.
  • Try not to use scripts to do what npm can do for you. Read through package.json to see all the things that you can specify and enable by simply describing your package appropriately. In general, this will lead to a more robust and consistent state.
  • Inspect the env to determine where to put things. For instance, if the npm_config_binroot environment variable is set to /home/user/bin, then don't try to install executables into /usr/local/bin. The user probably set it up that way for a reason.
  • Don't prefix your script commands with "sudo". If root permissions are required for some reason, then it'll fail with that error, and the user will sudo the npm command in question.
  • Don't use install. Use a .gyp file for compilation, and prepublish for anything else. You should almost never have to explicitly set a preinstall or install script. If you are doing this, please consider if there is another option. The only valid use of install or preinstall scripts is for compilation which must be done on the target architecture.

See Also