What are peerDependencies in package.json?

Imagine a phone case sold for a specific phone model. The case does not ship with a phone; it assumes you already own one. A peer dependency says the same: "I work with express 4, but you must bring your own express."

If a plugin listed express as a normal dependency, npm could install a second copy inside the plugin's folder. Two copies of the same library can cause strange bugs, especially for things that must be a single instance, like React or a framework's internal registry.

{
  "name": "express-fest-logger",
  "version": "1.0.0",
  "peerDependencies": {
    "express": "^4.18.0"
  }
}

Behaviour:

• npm 7 and later install missing peer dependencies automatically, and warn or fail if the versions conflict.
• npm 6 only printed a warning, so older tutorials say you must install them yourself.
• Add the same package under devDependencies in the library too, so you can test it locally.

Apps rarely need peerDependencies themselves. It matters mainly when you publish a package for others.

• Putting a framework like express in dependencies of a plugin and ending up with duplicate copies.
• Ignoring ERESOLVE peer conflict errors by always passing --force.