The operation is the same on both Bash and zsh, with the caveat that to persist them you need to use .bashrc and .zshrc, respectively.

Setting them in the shell is the same:

$ export VARIABLE=something

To make sure it was set, type

$ $VARIABLE

If you edit a dot file, to apply the changes to the current shell use source .dotfile.

This works for Bash and Zsh.

With Fish you prepend env:

env API_KEY=123123 node app.js