I have many concerns about typing. *Types are opinionated*
Choice of type system significantly affects code that people write. As observed in many codebases, adoption of TypeScript favors object oriented style over functional style. Choosing type system descending from Java type system is message "creators of language endorse OOP as main JavaScript paradigm". It's fine for TypeScript to take opinionated approach, but it's unlikely with TC39. If you don't see limitations of eg. TypeScript type system, try to express in its terms: "function takes subclass of Foo as parameter Ctor and returns subclass of Ctor extended by some methods". Not to mention reflection here (think of Bluebird's promisification)... Functional languages usually provide better type systems, but complexity can be prohibitive. Moreover, I don't know any programming language that combines type system as strong as eg. Idris and object oriented programming. *Runtime type checking* Runtime type checking would incur significant cost on browsers (and JavaScript parse time is already an issue for smartphone CPUs). It's especially important to remember about dynamic nature of JavaScript. TypeScript can rely on assumption that no one creates accessors on Object.prototype or Array prototype, but it's unacceptable for language feature. Therefore, chance of someone calling Object.defineProperty(Array.prototype, 7,...) requires type checking on every property assignment. There hundreds of ways to screw type system, including changes in inheritance in runtime. foo instanceof Foo; // true foo instanceof Foo; // false foo instanceof Foo; // true Ups, someone overwritten Function.prototype[Symbol.hasInstance] with () => Math.random() > 0.5. *Compilation time type checking* Compilation time type checking requires tooling. If language relies on external tool to strip type definitions, why whole typing can't be left to external tool? External tools can iterate faster than language and can compete. *People hates writing types* That's why C++ have auto, Kotlin have val, and it's significant reason why dynamically typed languages became popular. *Type inference* is awesome, but integrating it with existing JavaScript code in backwards compatible way seems to be hard.
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