I recently ported some (quite complex) code I wrote from JavaScript to Typescript. The code has about a 2:1 test to code ratio, and a fuzzer for correctness. While porting, I ended up adding a couple “useless” assert(typeof x === ...) calls to quieten the compiler, which felt useless because my code was correct. Lo and behold, the assertion tripped in my test suite - apparently I was sometimes treating a string as an object and didn’t notice. Which was a serious issue; and could end up being a security problem for some people. My fuzzer didn’t find it because it never occurred to me to add string method names in my random data generator.
Generally I find that the bugs that are easy to find with tests and the bugs that are easy to find with static types are different. You can eventually find all bugs with a sufficiently large test suite; and with enough PhDs you can apparently formally prove everything. But you get the best bang for buck with a little of each. A few tests is much better than no tests. But in the same spirit, I find no matter how big my test suite, there’s a good chance static types will improve my code.
Typescript is far from perfect, but I sleep better at night with a type checker checking my code.
