> It's always baffled me why more people don't seem interested in adopting design by contract

Because it's yet another layer on top of other layers, because it has performance implications ... A few languages such as Vala support some form of contracts, this kind of feature is more useful when part of the language itself, not some third party babel plugin that leads to codebase fragmentation as one just had yet another layer to the compilation pipeline. No offense, that's a nice thing you wrote, I'm just not going to use this kind of thing and add more complexity for very little gain. To me tests ARE the contracts.

I would only be willing to pay the cost of transpilation with something like Typescript which adds a signification value. Babel and co ? not worthwhile.