What does npm install --legacy-peer-deps do exactly? When is it recommended / What's a potential use case?

Question

Just ran into this error:

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/react
npm ERR!   react@"17.0.1" from the root project
npm ERR! 
npm ERR! Could not resolve dependency:
npm ERR! peer react@"^16.8.0" from [email protected]
npm ERR! node_modules/react-hook-mousetrap
npm ERR!   react-hook-mousetrap@"*" from the root project
npm ERR! 

The module I am trying to install seems to have a different peer dependency from what I have installed. It seems like npm changed its behaviour in this regard and now lets the install fail.

What can I do now to fix this? I don't want to downgrade my React version for this.

I know there is a flag called --legacy-peer-deps but I am not sure what exactly this does and whether it's recommended to use it / what the potential disadvantages are? I assume there is a reason npm did let the install fail.

It's just strange because I was using yarn up until very recently and everything was fine.

Answer 1

TL;DR:

• NPM v7+ installs peerDependencies by default; this is not the case with previous versions of NPM.
• NPM modules must name specific versions of their peerDependencies
• If you already have a peerDependency installed, but not with a version named by the module, then NPM v7+ will throw an error
• Adding --legacy-peer-deps ignores this new requirement, at the risk of introducing breaking changes

--legacy-peer-deps restores peerDependency installation behavior from NPM v4 thru v6

One way of thinking of this flag is that it isn't doing something new; rather it's telling NPM not to do something new, since NPM v7 now installs peerDependencies by default.

In many cases, this is leading to version conflicts, which will break the installation process.

The --legacy-peer-deps flag was introduced with v7 as a way to bypass peerDependency auto-installation; it tells NPM to ignore peer deps and proceed with the installation anyway. This is how things used to be with NPM v4 thru v6.

If you're unclear about the difference between regular deps and peer deps, here is a bit of context:

Dependencies vs peerDependencies

Dependencies: Libraries or modules that an NPM module needs in order to work in production. (Example: I recently built a pie chart mocking library that uses Chance.js to calculate random numbers within a specified range; Chance is therefore a dependency of my module.)

peerDependencies: A peer dependency is a specific version or set of versions of a third-party software library that a module is designed to work with. They're similar in concept to the relationship between a browser extension and a browser. (Example: react-redux has two quite logical peerDependencies: react and redux.)

This issue is being driven, in part, by React v17+

Due to the large number of modules that haven't specifically added React v17 (or more recently, React 18) as a peerDependency, it's now commonplace to encounter the unable to resolve dependency tree error when running npm installs within a v17 React application.

This error will fire whenever a module (or any of its own dependencies) lists a previous major version of React as a peerDependency without specifically including React v17 as well.

(Note: Similar behavior will occur with the major-version update of any other framework or library.)

How to check peerDependencies for any given module

NPM itself doesn't list peer deps on the pages of a given module. However, there is a simple workaround to check for peer deps, either before or after install. Simply run:

npm info name-of-module peerDependencies

This command will return the name of each peerDependency along with all compatible version(s).

Answer 2

Here's how I solved this problem:

First, what's happening: react-hook-mousetrap is looking for [email protected], but it is not finding it. Instead it is finding @react17.0.1, which is a newer version. For some reason mousetrap doesn't like this newer version, and you are being notified (it is not a big deal, but they decided it was worth stopping your build).

One solution: forcibly install the specific version of react that mousetrap wants:

yarn add [email protected]

What this does is roll back your react version to a slightly older one that is compatible with mousetrap. You won't notice any difference, and in future iterations, hopefully mousetrap is updated, so this goes away.

Another solution: make a sweeping decision to not install any older version dependencies:

npm add xxxx --legacy-peer-deps

What this does is ignore old dependencies for this package. It is more comprehensive, and makes a lot of the decisions for you.

Answer 3

legacy-peer-deps:

• Default: false
• Type: Boolean

Causes npm to completely ignore peerDependencies when building a package tree, as in npm versions 3 through 6.

If a package cannot be installed because of overly strict peerDependencies that collide, it provides a way to move forward resolving the situation.

This differs from --omit=peer, in that --omit=peer will avoid unpacking peerDependencies on disk, but will still design a tree such that peerDependencies could be unpacked in a correct place.

Use of legacy-peer-deps is not recommended, as it will not enforce the peerDependencies contract that meta-dependencies may rely on.

---

If you want to continue using legacy-peer-deps without needing to add the flag to every command, you can configure it in your .npmrc (either at the project level or globally on your machine):

echo "legacy-peer-deps=true" >> .npmrc

npmrc:

npm gets its config settings from the command line, environment variables, and npmrc files.

The npm config command can be used to update and edit the contents of the user and global npmrc files.

Answer 4

I resolved (with yarn) adding the following to package.json

"resolutions": {
    "**/react": "17.0.2",
    "**/react-dom": "17.0.2"
},

Answer 5

--legacy-peer-deps jumps the installation of all the peer dependencies and gives warnings about the peer deps to notice developers install them manually. When encountering the peer deps conflicts, other than --legacy-peer-deps, another choice is use --force.
The official doc of handling peer deps conflicts is this

p.s.
Correct the top answer: --legacy-peer-deps restores peerDependency installation behavior from NPM v3 thru v6, rather than v4 thru v6.

Answer 6

If you don't want to block installing older dependencies, you can make npm neglect those warnings by forcing the script you're running. --force

Answer 7

I had a dependency error after running the command npm audit fix --force.

I used this command to fix the error npm add xxxx --legacy-peer-deps.

Answer 8

The --legacy-peer-deps flag in npm install allows installing packages even if their peer dependencies do not strictly match the versions specified in the project's package.json.

It is used in cases of compatibility issues, particularly when dealing with older projects or packages that have not been updated to support the latest npm version.

It's a temporary solution. So recommend updating the dependencies for a better and long-term solution.

Answer 9

One other way is to downgrade your npm version to version 6