There is a nice VSCode extension called vscode-ruby-rubocop that you can use to automatically format a Ruby file using Format document (Shift+Alt+F). Unfortunately, that extension seems to be unmaintained for over 6 months now... 😢

But there is some good news. 🎉🎉 I have forked the repository and fixed a few things:

• Changed the Rubocop argument --auto-correct to --autocorrect because --auto-correct is deprecated.
• Added configuration option ruby.rubocop.useServer. When you enable this option, Rubocop is started with --server which dramatically increases formatting speed. Keep in mind, this option only works with Rubocop >= 1.31.

You can find the revived extension at GitHub and in the VSCode marketplace.

• Marketplace: https://marketplace.visualstudio.com/items?itemName=LoranKloeze.ruby-rubocop-revived
• GitHub: https://github.com/LoranKloeze/vscode-ruby-rubocop-revived