I guess "anonymous IIFE" is the part that bothers you. If someone is nesting ternary expressions in order to distinguish three or more cases, I think the switch is generally going to be clearer. Writing `foo = ...` in each case, while it might seem redundant, is not really any worse than writing `return ...` in each case, sure. But I might very well use an explicit, separately written function if there's something obvious to call it. Just for the separation of concerns: working through the cases vs. doing something with the result of the case logic.

It just looked way more complex (and it's easy to miss the () at the end of the whole expression that makes it II). And the point of the rule was to make code more readable.

Basically it's a shame that Typescript doesn't have a switch-style construct that is an expression.

And that nowadays you can't make nested ternaries look obvious with formatting because automated formatters (that are great) undo it.