This is macOS-centric.
brew install deno
In ~/.zprofile, in addition to HTTP_PROXY, HTTPS_PROXY, NO_PROXY :
DENO_TLS_CS_STORE=system
mkdir ~/.oh-my-zsh/custom/plugins/deno
deno completions zsh > ~/.oh-my-zsh/custom/plugins/deno/_denothen add deno to plugins section of .zshrc