If you don't specify types at all, is that really opting out? This is valid TypeScript code:

  function foo(a, b) {
      if (a != 3) {
          return a + b
      } else {
          return "hello"
      }
  }
  
  console.log(foo(2, 5)); //prints 7
  
  console.log(foo(3, 5)); //prints "hello"

You can compile it and run it on the typescript playground: https://www.typescriptlang.org/play/

This is not like C#, where would be required to specify that every type is "dynamic". Here, you can optionally choose to specify that the types are "Any", but it's still gradual typing.

EDIT: you said "implicit opt-out"... which sounds like a synonym for "opt-in". I don't think "implicit opt-out" is a term.

The "implicit opt-out" is noImplicitAny[1]. That code of yours won't compile with this opt-out enabled. Edit: the "dynamic opt-in" is `any` (and is `dynamic` in C#).

[1]: https://www.typescriptlang.org/docs/handbook/compiler-option...

If it was really not a statically typed language, I'd expect this code to at least run. But it detects a type failure at compilation time despite the lack of any declarations.

    function foo() {
        return 1;
    }

    let x = foo().substring(1, 2);

> I'd expect this code to at least run

It still compiles the code to JavaScript even with the error, and you can still run it, which makes this more of a warning than anything else.

That warning based on type inference seems like a positive feature... because there's no situation in which that code is right. You can add `: any` to the function declaration and it will stop complaining, but I wouldn't contend that the language is not dynamic because it encourages you not to make a mistake.

The distinction between "warning" and "error" is highly arbitrary anyway. In C++, you also get a warning rather than an error if you, say, return a reference to a local, or read from an uninitialized local. You still get a binary, and it still runs. But these are 100% bugs, which is why most projects enable treat-warnings-as-errors for a lot of that stuff in C++. I'd imagine it's common with TypeScript, as well.