Even developers who "don't make type errors" have to deal with bad and unexpected data, which may be in a different type. If you don't validate that all untrus

Even developers who "don't make type errors" have to deal with bad and unexpected data, which may be in a different type. If you don't validate that all untrusted data is off the appropriate type, security vulns or "garbage in - garbage out" bugs can result. Then if you do manually validate the types of untrusted data, then you have to wonder of having built-in type support might be faster. "Bad data" doesn't just come from the outside world. As teams start to use shared libraries written by other people, it's also valuable adds guards to ensure that shared functions are being called with the expected types. My own experience is that it doesn't feel like type-related bugs come up often on my team, but when they do the problems they cause can be bad enough to make us wish more of our code had been ported to TypeScript. Classic example: a CSV parser parsed numbers as strings because a particular spreadsheet quoted the number column. Result: "2"+"2" = "22"