You probably need to define node in your system path variables. I'm not sure exactly which ones are needed or what the install locations are, but here is an example:

coolestguidesontheplanet.com/add-shell-path-osx

You will likely need to do some more configuration if you are using brew. The most recommended way is to download it direct from nodejs.org and install it. leave all the default configuration, don't modify anything during installation and you have your nodejs set up on your system.

you just need use

brew link node@8

and then follow the terminal message. it's mine

Warning: node@8 is keg-only and must be linked with --force
Note that doing so can interfere with building software.

If you need to have this software first in your PATH instead consider running:
  echo 'export PATH="/usr/local/opt/node@8/bin:$PATH"' >> ~/.bash_profile