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0004-npm-scripts.md

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Support running npm scripts

Proposal

Add support for users to provide npm scripts to be executed after installing modules. Users will provide them via a build-time environment variable and the execution will handled by a node-run-script buildpack.

Motivation

There are cases where users like the buildpack to run npm scripts in addition to installing dependencies. Many frontend web application frameworks like Angular, React, Vue.js etc. use node package managers like npm and yarn to generate static files like html, js and css. The application workflow using these frameworks involves generating a package.json which typically contains a script under build (or another custom event name) that is run to generate these static assets.

For e.g., a user building a simple Angular app would use the ng new command to generate a package.json that looks like follows:

 "scripts": {
    "ng": "ng",
    "start": "ng serve",
    "build": "ng build",
    "test": "ng test",
    "lint": "ng lint",
    "e2e": "ng e2e"
  },

Here the command to build static resources are in the build script event. The user then runs the command npm run-script build (or its alias npm run build) to invoke this script which generates the static files in the <app>/dist directory. This is a pattern we see in other frameworks as well. Here are a few app examples:

As of now, the only way for the user to have these executed with buildpacks is by editing the package.json and copying the command from the custom event to a preinstall or postinstall script event that npm and yarn recognizes as part of the npm install. Many users do not wish to make changes to their app source code, and there has been requests to support execution of these commands by the buildpacks (e.g.).

Implementation

This RFC proposes a new node-run-script buildpack that runs the lifecycle events provided by the user via build-time environment variable $BP_NODE_RUN_SCRIPTS. The value of the variable should be a comma separated list of events listed in the app's package.json.

e.g.

$BP_NODE_RUN_SCRIPTS="build,custom-script"

From the contents of the app directory and the buildpack plan entries, the buildpack should pick the correct package manager (npm, yarn) to execute the script.

The buildpack will figure in the Node.js language family buildpack order as follows:

[[order]]
  [[order.group]]
    id = "paketo-buildpacks/node-engine"

  [[order.group]]
    id = "paketo-buildpacks/yarn"

  [[order.group]]
    id = "paketo-buildpacks/yarn-install"

  [[order.group]]
    id = "paketo-buildpacks/node-run-script"
    optional = true

  [[order.group]]
    id = "paketo-buildpacks/yarn-start"
  ...

[[order]]
  [[order.group]]
    id = "paketo-buildpacks/node-engine"

  [[order.group]]
    id = "paketo-buildpacks/npm-install"

  [[order.group]]
    id = "paketo-buildpacks/node-run-script"
    optional = true

  [[order.group]]
    id = "paketo-buildpacks/npm-start"

Open questions

  1. Web framework examples mostly demonstrate cases where a postinstall event would be useful. What are some real world cases (if any) where preinstall would be used? Just support running scripts after installing modules.

  2. Do NPM and Yarn need separate env variables or should they read the same set of env vars (e.g. $BP_NODE_{PRE,POST}INSTALL)? Consolidate.

  3. Should the implementation be moved away from the {npm,yarn}-install buildpacks to separate npm run-script buildpack(s)? Yes.

Logistics

  • A new buildpack repository named node-run-script should be created in the paketo-buildpacks organization under the Node.js subteam.