Run a script (like postinstall) after npm installing a single package?

Question

I'm starting to play around with Snowpack. It takes a different approach from Webpack by bundling individual packages right after they're installed.

The "issue" is, when I install a package I have to first run npm install --save my-package and then I have to manually pack it with npx snowpack. The Snowpack docs mention that I can include a prepare script that would snowpack everything after running npm install but that doesn't apply to individual packages, just on a generic npm install of all dependencies in my package.json. As far as I can tell, this is the case for all npm hooks mentioned in the npm docs.

Is there any way I can automatically run a script whenever I install an individual package? The only way I can think of would be to overwrite the install script and add something to it. Are there any examples of this on GitHub or elsewhere?

Update: For clarification, I'd like to run npx snowpack every time I install a new package with --save but preferably not with --save-dev or without --save. This will never be different for any package. This will be specific to a certain repo/project, not global on my system.

It is not sufficient to run snowpack after simply running npm install as you would get by hooking into postinstall or release. Additionally, I want to make sure developers working on my project can use npm install --save newdep as they normally would and then snowpack will run. I do not want to require devs to use a custom named script.

Answer 1

Short answer: Unfortunately, npm does not provide any built-in feature(s) to meet your requirement.

Lifecycle hooks/scripts such as postinstall are invoked only when running the generic npm install command, and not when someone runs npm install --save <pkg_name> during the projects development phase.

---

Workaround: Consider customizing the logic of the npm install --save compound command by essentially overriding the npm command at the shell level.

The following solution, albeit a Bash one, describes how this custom logic can be actualized for a specific project(s). However, this solution is dependent on the following conditions:

• Developers working on your project must have their the shell set to Bash when running the npm install --save compound command.
• Developers working on your project will need to customize their Bash startup files, namely ~/.bashrc and possibly ~/.bash_profile.
• The project directory, i.e. the project directory for which you want the custom logic to be effective, must contain a custom .bashrc file.

---

Bash Solution:

The following three steps are necessary to configure your project, and operating system(s), so that when a developer runs npm install --save <pkg_name> (or variations of it) the npx snowpack command is subsequently invoked.

Note: Points two and three (below) are the tasks developers need to carry out (once) to customize their Bash startup files.

1. The project specific .bashrc file:

   Firstly create the following "project specific" .bashrc file in the root of your project directory, i.e. save it at the same level as where your projects package.json file resides:

   /some/path/to/my-project/.bashrc

   npm() {
   
     local name_badge="\x1b[37;40mpostinstall\x1b[0m"
   
     array_includes() {
       local word=$1
       shift
       for el in "$@"; do [[ "$el" == "$word" ]] && return 0; done
     }
   
     log_warn_message() {
       local cmd_name=$1 warn_badge warn_mssg
       warn_badge="\x1b[30;43mWARN!\x1b[0m"
       warn_mssg="${cmd_name} command not found. Cannot run npx snowpack."
       echo -e "\n${name_badge} ${warn_badge} ${warn_mssg}" >&2
     }
   
     log_run_message() {
       echo -e "\n${name_badge} Running pseudo postinstall hook."
     }
   
   
     if [[ $* == "install "* || $* == "i "* ]] && array_includes --save "$@"; then
   
       # 1. Run the given `npm install --save ...` command.
       command npm "$@"
   
       # 2. Check whether the `npx` command exists globally.
       command -v npx >/dev/null 2>&1 || {
         log_warn_message npx
         return 1
       }
   
       log_run_message
   
       # 3. Run the pseudo "postinstall" command.
       command npx snowpack
   
     else
       # Run all other `npm` commands as per normal.
       command npm "$@"
     fi
   }
   

   _{Note: For a better understanding of what this file does refer to the "Explanation" section below.}

2. The ~/.bashrc file:

   To make the custom logic, i.e. the npm function in the aforementioned .bashrc file, effective, it's necessary to configure Bash to read the aforementioned "project specific" .bashrc file. To configure this, add the following line of code to ~/.bashrc:

   PROMPT_COMMAND='if [[ "$bashrc" != "$PWD" && "$PWD" != "$HOME" && -e .bashrc ]]; then bashrc="$PWD"; . .bashrc; fi'
   

   _{Note: For a better understanding of what this line of code does refer to the "Explanation" section below.}

3. The ~/.bash_profile file:

   Typically your ~/.bash_profile contains the following line of code to load the ~/.bashrc file (or some variation of it):

   if [ -f ~/.bashrc ]; then . ~/.bashrc; fi
   

   If this is not present, then it must be added to ~/.bash_profile.

---

Additional info.

Setup/Configuration helpers:

Consider your developers utilizing the following two commands to aid configuration of their Bash startup files, as per the aforementioned steps two and three.

1. For step two, run the following command:

   echo $'\n'"PROMPT_COMMAND='if [[ \"\$bashrc\" != \"\$PWD\" && \"\$PWD\" != \"\$HOME\" && -e .bashrc ]]; then bashrc=\"\$PWD\"; . .bashrc; fi'" >> ~/.bashrc
   

   This will add the PROMPT_COMMAND=... line of code to the existing ~/.bashrc file, or create a new one if it doesn't already exist:

2. For step three, run the following command to append the line of code necessary in the ~/.bash_profile for loading the ~/.bashrc file:

   echo $'\n'"if [ -f ~/.bashrc ]; then . ~/.bashrc; fi" >> ~/.bash_profile
   

Is my shell configured to Bash?

To check whether the shell is configured to Bash you can create a new session, i.e. create a new Terminal window and run:

echo $0

If it prints -bash then it's using Bash.

How do I configured my shell to Bash?

If echo $0 doesn't print -bash then you'll need to change the shell. To change it to Bash run:

chsh -s /bin/bash

Note: You'll need to create a new session for this change to become effective.

---

Explanation

1. The project specific .bashrc file:

   This .bashrc file contains a shell function named npm. The body of this function contains the logic necessary to override the default npm install|i --save command.

   • The conditions specified in the if statement, i.e, the part that reads;

     if [[ $* == "install "* || $* == "i "* ]] && array_includes --save "$@"; then
       ...
     fi
     

     essentially reads the $* special parameter to check whether the argument(s) passed to the npm function begin with either; install , or it's shorthand equivalent i , and whether the --save option/argument has been passed too.

     To check for the existence of the --save argument we pass the $@ special parameter to the array_includes function. We handle this argument differently because the position of the --save option may differ in the compound command. For instance, a user may install a package by running this;

     # Example showing `--save` option at the end
     npm install <pkg_name> --save
     

     or this (or some other variation):

     # Example showing `--save` option in the middle
     npm i --save <pkg_name>
     

   • When the conditions specified in the if statement are met, i.e. they're true, we perform the following tasks in its body:

     1. Run the given npm install --save ... command as-is via the line that reads:

        command npm "$@"
        

     2. Check whether the npx command exists globally via the part that reads:

        command -v npx >/dev/null 2>&1 || {
          log_warn_message npx
          return 1
        }
        

        If the npx command is not available (globally) we warn the user that the npx snowpack command cannot be run, and return from the function early with an exit status of 1.

        _{Note: My logic in this check assumes that you'll be installing npx globally. However if you're installing npm locally within your project then you'll need to change this logic. Perhaps by checking whether ./node_modules/.bin/npx exists instead. Or, you may be confident that npx command will always exists, therefore conclude that this check is unnecessary.}

     3. If the npx command exists globally we then run the pseudo "postinstall" command, i.e.

        command npx snowpack
        

   • When the conditions specified in the if statement are NOT met, i.e. they're false, the user is essentially running any other npm command that is not npm install --save <pkg_name>. Therefore in the else branch we run the command as-is:

     command npm "$@"
     

2. The ~/.bashrc file:

   In section 5.2 Bash Variables of the "Bash Reference Manual" the PROMPT_COMMAND variable is described as follows:

   PROMPT_COMMAND

   If set, the value is interpreted as a command to execute before the printing of each primary prompt ($PS1).

   So, this line of code (here it is again):

   PROMPT_COMMAND='if [[ "$bashrc" != "$PWD" && "$PWD" != "$HOME" && -e .bashrc ]]; then bashrc="$PWD"; . .bashrc; fi'
   

   loads the "project specific" .bashrc (if one exists), which in turn overrides the npm command with the npm function. This is what essentially provides a mechanism for overriding the npm install --save compound command for a specific project(s).

   _{See this answer by @Cyrus for further explanation.}

Answer 2

With newer versions of Snowpack (>=2) you can run snowpack dev and it will watch your npm_modules folder for new modules to build.

Answer 3

I think the best bet would be to create a new script that performs the desired action. Something along the following lines in your package.json:

{
  "scripts": {
    "snowpack-install" : "npm install --save && npx snowpack"
  }
}

Correction

You can actually use the postinstall option in package.json. The postinstall will run "AFTER the package is installed". This would look something like the following:

{
  "scripts": {
    "postinstall" : "npx snowpack"
  }
}