The npm install mechanisms can produce error messages and warnings that are hard to interpret. Here we suggest tools and permissions that may smooth the process.
# Permission
An install requires permission to write files. One can temporarily acquire "superuser" privilege with the sudo.
sudo npm install -g wiki
A better approach is to change the permission required to complete an install, a process explained well in the npm documentation. page 
sudo chown -R `whoami` /usr/local
# Tools
Wiki may have optional dependencies that require compiling c programs. Apple no longer bundles the necessary tools but they do offer a simple installer.
xcode-select --install