How do I fix the npm UNMET PEER DEPENDENCY warning?

I'm on Windows 10, with Node 5.6.0 and npm 3.6.0. I'm trying to install angular-material and mdi into my working folder. npm install angular-material mdi errors with:

+-- [email protected]

+-- UNMET PEER DEPENDENCY angular-animate@^1.5.0

+-- UNMET PEER DEPENDENCY angular-aria@^1.5.0

+-- [email protected]

+-- UNMET PEER DEPENDENCY angular-messages@^1.5.0 `-- [email protected]

npm WARN enoent ENOENT: no such file or directory, open
'C:\Users\xxxxx\Desktop\ngClassifieds\package.json' 

npm WARN [email protected] requires a peer of
angular-animate@^1.5.0 but none was installed. 

npm WARN [email protected] requires a peer of angular-aria@^1.5.0
but none was installed. 

npm WARN [email protected] requires a peer of
angular-messages@^1.5.0 but none was installed.

How do I resolve this to get AngularJS Material and MDI installed?

npm no longer installs peer dependencies so you need to install them manually, just do an npm install on the needed deps, and then try to install the main one again.

Reply to comment:

it's right in that message, it says which deps you're missing

UNMET PEER DEPENDENCY angular-animate@^1.5.0 +-- 
UNMET PEER DEPENDENCY angular-aria@^1.5.0 +-- [email protected] +
UNMET PEER DEPENDENCY angular-messages@^1.5.0 `-- [email protected]` 

So you need to npm install angular angular-animate angular-aria angular-material angular-messages mdi

UNMET PEER DEPENDENCY error is thrown when the dependencies of one or more modules specified in the package.json file is not met. Check the warnings carefully and update the package.json file with correct versions of dependencies.

Then run

rm -rf node_modules/
npm cache clean
npm install

This will install all the required dependencies correctly.

In my case all the dependencies were already there. Please update NPM in that case as it might have been crashed. It solved my problem.

npm install -g npm

npm-install-peers worked for me.

npm install -g npm-install-peers

EDIT 2020

From npm v7.0.0, npm automatically installs peer dependencies. It is one of the reasons to upgrade to v7.

https://github.blog/2020-10-13-presenting-v7-0-0-of-the-npm-cli/

Also this page explains the rationale of peer dependencies very well. https://github.com/npm/rfcs/blob/latest/implemented/0025-install-peer-deps.md

This answer doesn’t apply all cases, but if you can’t solve the error by simply typing npm install , this steps might help.

Let`s say you got this error.

UNMET PEER DEPENDENCY [email protected]

npm WARN [email protected] requires a peer of packageA@^3.1.0 but none was installed.

This means you installed version 4.2.0 of packageA, but [email protected] needs version 3.x.x of pakageA. (explanation of ^)

So you can resolve this error by downgrading packageA to 3.x.x, but usually you don`t want to downgrade the package.
Good news is that in some cases, packageB is just not keeping up with packageA and maintainer of packageB is trying hard to raise the peer dependency of packageA to 4.x.x.
In that case, you can check if there is a higher version of packageB that requires version 4.2.0 of packageA in the npm or github.

For example, Go to release page

Oftentimes you can find breaking change about dependency like this.

packageB v4.0.0-beta.0

BREAKING CHANGE
package: requires packageA >= v4.0.0

If you don’t find anything on release page, go to issue page and search issue by keyword like peer. You may find useful information.

At this point, you have two options.

1. Upgrade to the version you want
2. Leave error for the time being, wait until stable version is released.

If you choose option1:
In many cases, the version does not have latest tag thus not stable. So you have to check what has changed in this update and make sure anything won`t break.

If you choose option2:
If upgrade of pakageA from version 3 to 4 is trivial, or if maintainer of pakageB didn’t test version 4 of pakageA yet but says it should be no problem, you may consider leaving the error.

In both case, it is best to thoroughly test if it does not break anything.

Lastly, if you wanna know why you have to manually do such a thing, this link explains well.