[RRFC] Run preinstall / postinstall scripts on single package installation

Hi there, first of all, thanks for all of the hard work on npm and everything surrounding it! Really cool what you are doing.

Motivation ("The Why")

The behavior of npm to not run preinstall or postinstall scripts specified by the developer in the project on every npm install <pkg name> is confusing and leads to support requests such as https://github.com/npm/cli/issues/1732

Would you consider starting to run these scripts once during installation of a package?

Example

npm install node-fetch # would also run the `postinstall` script

How

Current Behaviour

preinstall and postinstall scripts are not run during npm install <pkg>

Desired Behaviour

the scripts are run

References

• Originally a Discussion in npm/feedback: https://github.com/npm/feedback/discussions/217, and @MylesBorins mentioned I should make an RFC here

My original use case was to be able to also install the corresponding @types/<pkg name> DefinitelyTyped dep (à la #328). See here for my original naïve approach (which ended up failing): https://github.com/jeffijoe/typesync/pull/65/files

Alternatives Considered

Hook Scripts

In the Open RFC Meeting on Feb 17, 2021 @darcyclarke and others mentioned Hook Scripts.

There are two downsides I've run into so far:

1. they seem to be broken in npm v7 https://github.com/npm/cli/issues/2692
2. in npm v6, a postinstall hook script, for example, runs once for every package that has been installed. So this means that with a node_modules/.hooks/postinstall script, if you run npm install with your package.json containing 20 dependencies, this script will run at least 20 times, and even more if those dependencies specify transitive dependencies...

Start by reviewing the Current Behaviour and Desired Behaviour sections, then read the linked npm/feedback discussion and related references including issues #1732, #2692, and RFC #326. Done means the proposal resolves how npm should run the project’s preinstall and postinstall scripts during single-package installation without relying on the alternatives described.