Cons of using Typescript:

* Lack of purity. You can unintentionally trigger side effects from a function that's a supposed to simply return a value.

    function add(x: number, y: number): number {
      fireTheMissiles()
      return x + y
    }

* It's impossible to validate that external JSON data is the correct type at runtime. If a JSON API returns something weird, you're going to get a runtime error.