npm-scripts(1) -- How npm handles the "scripts" field

DESCRIPTION

npm supports the "scripts" member of the package.json script, for the following scripts:

preinstall: Run BEFORE the package is installed
install, postinstall: Run AFTER the package is installed.
preactivate: Run BEFORE the package is activated.
activate, postactivate: Run AFTER the package has been activated.
predeactivate, deactivate: Run BEFORE the package is deactivated.
postdeactivate: Run AFTER the package is deactivated.
preuninstall, uninstall: Run BEFORE the package is uninstalled.
postuninstall: Run AFTER the package is uninstalled.
preupdate: Run BEFORE the package is updated with the update command.
update, postupdate: Run AFTER the package is updated with the update command.
preupdatedependencies: Run BEFORE the package dependencies are pointed to the new version.
updatedependencies, postupdatedependencies: Run AFTER the package dependencies are pointed to the new version.

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.

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"
configuration vars: 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.
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" :
  { "install" : "scripts/install.js"
  , "postinstall" : "scripts/install.js"
  , "activate" : "scripts/install.js"
  , "uninstall" : "scripts/uninstall.js"
  }
}

then the scripts/install.js will be called for the install, post-install, and activate stages of the lifecycle, and the scripts/uninstall.js would be called when the package is uninstalled. Since scripts/install.js is running for three 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"
  }
}

EXITING

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 {root}/.npm/.hooks/{eventname}, and it'll get run for all packages when they are going through that point in the package lifecycle.

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/deactivate 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 npm help 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 environ 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.

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