NAME

Description

The "scripts" property 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 <stage>. 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 }}"


  }
}

In this example npm run compress would execute these scripts as described.

Life Cycle Scripts

There are some special life cycle scripts that happen only in certain situations. These scripts happen in addition to the pre<event>, post<event>, and <event> scripts.

prepare (since npm@4.0.0)

prepublish (DEPRECATED)

prepublishOnly

prepack

postpack

dependencies

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 ⟨https://github.com/npm/npm/issues/10074⟩. 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 ⟨https://github.com/npm/npm/issues/10074⟩ for a much lengthier justification, with further reading, for this change.

Use Cases

Use a prepare script to perform build tasks that are platform-independent and need to run before your package is used. This includes tasks such as:

Running these build tasks in the prepare script ensures they happen once, in a single place, reducing complexity and variability. Additionally, this means that:

Dependencies

The dependencies script is run any time an npm command causes changes to the node_modules directory. It is run AFTER the changes have been applied and the package.json and package-lock.json files have been updated.

Life Cycle Operation Order

npm help

npm help ci

These all run after the actual installation of modules into node_modules, in order, with no internal actions happening in between

npm help diff

npm help install

These also run when you run npm install -g <pkg-name>

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 via node-gyp rebuild

These are run from the scripts of <pkg-name>

npm help pack

npm help publish

npm help rebuild

prepare is only run if the current directory is a symlink (e.g. with linked packages)

npm help restart

If there is a restart script defined, these events are run; otherwise, stop and start are both run if present, including their pre and post iterations)

\fBnpm run <user defined>\fR ⟨/commands/npm-run⟩

npm help start

If there is a server.js file in the root of your package, then npm will default the start command to node server.js. prestart and poststart will still run in this case.

npm help stop

npm help test

npm help version

A Note on a lack of npm help uninstall scripts

While npm v6 had uninstall lifecycle scripts, npm v7 does not. Removal of a package can happen for a wide variety of reasons, and there's no clear way to currently give the script enough context to be useful.

Reasons for a package removal include:

Due to the lack of necessary context, uninstall lifecycle scripts are not implemented and will not function.

Working Directory for Scripts

Scripts are always run from the root of the package folder, regardless of what the current working directory is when npm is invoked. This means your scripts can reliably assume they are running in the package root.

If you want your script to behave differently based on the directory you were in when you ran npm, you can use the INIT_CWD environment variable, which holds the full path you were in when you ran npm run.

Historical Behavior in Older npm Versions

For npm v6 and earlier, scripts were generally run from the root of the package, but there were rare cases and bugs in older versions where this was not guaranteed. If your package must support very old npm versions, you may wish to add a safeguard in your scripts (for example, by checking process.cwd()).

For more details, see:

User

When npm is run as root, scripts are always run with the effective uid and gid of the working directory owner.

Environment

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.

path

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

npm sets the following environment variables from the package.json:

Additionally, for install scripts (preinstall, install, postinstall), npm sets these environment variables:

For example, 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.

Note: In npm 7 and later, most package.json fields are no longer provided as environment variables. Scripts that need access to other package.json fields should read the package.json file directly. The npm_package_json environment variable provides the path to the file for this purpose.

See \fBpackage.json\fR ⟨/configuring-npm/package-json⟩ for more on package configs.

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"

Examples

For example, if your package.json contains this:

{


  "scripts" : {


    "prepare" : "scripts/build.js",


    "test" : "scripts/test.js"


  }
}

then scripts/build.js will be called for the prepare stage of the lifecycle, and you can check the npm_lifecycle_event environment variable if your script needs to behave differently in different contexts.

If you want to run build commands, you can do so. This works just fine:

{


  "scripts" : {


    "prepare" : "npm run build",


    "build" : "tsc",


    "test" : "jest"


  }
}

Exiting

Scripts are run by passing the line as a script argument to /bin/sh on POSIX systems or cmd.exe on Windows. You can control which shell is used by setting the \fBscript-shell\fR ⟨/using-npm/config#script-shell⟩ configuration option.

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 Node.js or even JavaScript programs. They just have to be some kind of executable file.

Best Practices

See Also