TypeScript vs. PureScript vs. ReasonML vs. Elm

I think it's unfair to categorise TypeScript as the easiest for interop if you've asserted that static type safety is important to you. If the typings aren't already available you'll need to create a type declaration for them - meaning it's a lot closer to PureScript and ReasonML than it might otherwise appear - and I say "need" because otherwise you'll have `any` flowing through your codebase which defeats the purpose.

Your conclusion is reasonable given your desire to gradually migrate a preexisting codebase, but I'd personally come to a very different answer were this for a greenfield project. TypeScript is wonderful and I use it at work every day, but its limitations are frustrating and it's fundamentally unsafe.