Right: when you actually run your program. Your program might get run when you test it in a browser, or when a user visits your website, or when you run a unit test. If you’re disciplined and write plenty of unit tests and end-to-end tests, smoke test your code before pushing it, and test it internally for a while before shipping it to users, you will hopefully find out about your error before your users do. But what if you don’t?
That’s where TypeScript comes in. Even cooler than the fact that TypeScript gives you helpful error messages is when it gives them to you: TypeScript gives you error messages in your text editor, as you type. That means you don’t have to rely on unit tests or smoke tests or coworkers to catch these sorts of issues: TypeScript will catch them for you and warn you about them as you write your program.”