> ...there is an argument that states that the adoption of TypeScript would not have been as high if a sound type system is 100% guaranteed. This is disproved a

> ...there is an argument that states that the adoption of TypeScript would not have been as high if a sound type system is 100% guaranteed. This is disproved as the dart language is finally gaining popularity as Flutter is now widely used. In my experience TypeScript's unsoundness is a feature, not a bug. You cannot compare it to languages that were designed from the ground up to be sound. JavaScript was fundamentally not designed to be used in a 100% sound way, and trying to do so introduces an enormous developer burden in terms of idioms and language features that you can't use, and syntactic hoops you have to jump through. You find yourself constantly fighting against the language. I know this because at my company we started with Flow, which is (very proudly) sound. It was impossible to be productive. For a small example, every reference to document.body had to have a null-check, even though we could guarantee by where we placed our <script> tags that the body would never ever be undefined. The experience was rife with situations like this, which created tons of cruft and made code harder to read. "any" is nearly always bad. But the other escape hatches, like casting, have proven essential to building a reasonable codebase that doesn't try to ignore the reality of JavaScript. > Runtime type checking would be beneficial when dealing with JSON payloads returned from API calls, for example. This I agree with, but I don't think it belongs in the core TypeScript project. We used Flow Runtime with Flow (https://gajus.github.io/flow-runtime/ https://gajus.github.io/flow-runtime/) which creates runtime assertions automatically based on your static types, on a file-by-file basis. It was wonderful, and is the only thing I miss about Flow, although it was a third-party project and there's no reason something similar couldn't be made for TypeScript. I would be giddy to hear that someone was building one.