> What happens when that singleton has dependencies? And they have dependencies?

Everyone exports/imports their deps the same way as everything else.

> Not unrealistic once you have a config block that sets stuff like language direction ltr/rtl or API endpoint at runtime based on test/prod env.

Having different environment config is standard feature in most JS FX Apps, if it changes at runtime it's maintained as App state with any changes typically propagated reactively.

I've yet to run into any issue that I needed an IOC for in a client JS App (despite always using them for backend C# Servers) but they're unnecessary in JS, our Angular templates are the only ones that use them, all other premier JS FX's happily get by without them.