[z3t4]

Personally I'm a fan of sprinkling the code with manual checks, conversions, and early friendly error messages. Rather then spraying it with type annotations. Practically when developing and there's an error, you add a check as early as possible, that would catch the bug, like at the beginning of the function, throwing a human readable error with some added debug data. For example, lets say you have done some detective work to locate the source of a bug, and figured out the earliest possible place to detect it is in the foo() function, where the bug was caused by a missing "baz" property ...

    function foo(bar) {
      if(foo.baz == undefined) throw new Error("foo need to have a baz! foo.baz=" + foo.baz + " foo=" + JSON.stringify(foo, null, 2));
    }

Typescript could probably detect the bug in the first place if you had used interface or what not, and typed everything. Static typing (and also tests) can however slow you down when you are quickly iterating and re-writing. And at the end of the day when you have a working prototype/MVP, and have added a few manual checks that will throw early if there's a bug. Are you gonna spend your time type annotating it all (or writing tests) or are you going to spend that time marketing ?

If the product receive continuous work there will however be a tipping point where tests are needed. And the checks you added will be of help as errors will come early. But when you have tests in place, is it really worth it to spray-paint the code with annotation and make it static ? There are already tools today that can infer most types, and give you auto-completion already.