When installing packages with Yarn, what does “incorrect peer dependency” mean?

Question

I'm just cloned a repo, which recommends the use of Yarn to install dependencies. When I run yarn install, it seems to be okay, but it provides this warning:

yarn install v0.20.3
[1/4] 🔍  Resolving packages...
[2/4] 🚚  Fetching packages...
[3/4] 🔗  Linking dependencies...
warning "sass-loader@4.0.2" has incorrect peer dependency "node-sass@^3.4.2".
[4/4] 📃  Building fresh packages...
✨  Done in 77.59s.

I've looked online to find out exactly what "has incorrect peer dependency" means. But all I can find are reported issues on other repositories or questions about how to fix the problem.

Can someone explain what this means and why it is only a warning, and not an error?

Also, is it something that I should try to address or report to the community behind the repo I have just cloned?

Answer 1

It is only a warning as it won't actually stop your code from running, It's just there to give you a heads up that there's something wrong with your dependencies.

Effectively, peer dependencies are a way for packages to specify, "to use me, you should also have x version of y package installed".

You should upgrade to the latest versions, see this link for more details on sass-loader dependencies