failed `npm install` with npm@3

[okekhj]

NOTE: npm versions 1 and 2 will automatically install peerDependencies if they are not explicitly depended upon higher in the dependency tree. In the next major version of npm (npm@3), this will no longer be the case. You will receive a warning that the peerDependency is not installed instead.

[colthreepv]

Acknowledged, where is the issue?
This only means you have to install joi in your project, but 100% of projects using express-validation already satisfies this requirement.