Example npm peerDependencies

This repo is an example of peerDependencies in npm.

Theory and information of the example

Official npm document about peerDependencies: https://docs.npmjs.com/cli/v10/configuring-npm/package-json#peerdependencies

More detail explain about the real-world use case that we need peerDependencies: https://nodejs.org/en/blog/npm/peer-dependencies

Summary real-world use case: When develop a plugin for a host package, we use peerDependencies to display error when user try to install the plugin that is not compatible with the host package.

Example design

The package pkg_express_3 has a peerDependencies of express with version ^3.

The main_no_express_installed package has a dependency of pkg_express_3 and does not have express installed. When we install the packages, it will automatically resolve the express dependency to ^3.

The main_installed_express_4 package has a dependency of pkg_express_3 and has express installed with version ^4. When we install the packages, it will throw an error because the express version is not compatible with the pkg_express_3 package.

Example tested with:

node: v18.10.0
npm: 8.19.2

Compatible case

Expecting the commands below run successfully

cd main_no_express_installed
npm i --install-links
node .

Not compatible case

When express version is not compatible

cd main_installed_express_4
npm i --install-links

Expected: error message

npm ERR! code ERESOLVE
npm ERR! ERESOLVE unable to resolve dependency tree
npm ERR!
npm ERR! While resolving: [email protected]
npm ERR! Found: [email protected]
npm ERR! node_modules/express
npm ERR!   express@"^4.19.2" from the root project
npm ERR!
npm ERR! Could not resolve dependency:
npm ERR! peer express@"^3" from [email protected]
npm ERR! node_modules/pkg
npm ERR!   pkg@"file:../pkg_express_3" from the root project
npm ERR!
npm ERR! Fix the upstream dependency conflict, or retry
npm ERR! this command with --force, or --legacy-peer-deps
npm ERR! to accept an incorrect (and potentially broken) dependency resolution.
npm ERR!
npm ERR! See /Users/phu.nguyen/.npm/eresolve-report.txt for a full report.

npm ERR! A complete log of this run can be found in:
npm ERR!     /Users/phu.nguyen/.npm/_logs/2024-07-21T03_49_15_052Z-debug-0.log

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