Is TypeScript really a superset of JavaScript

Question

I just started using TypeScript and sometimes get compiler errors "use of the undeclared variable". For example the following works in plain JavaScript :

var foo = {};
foo.bar = 42;

If I try to do the same in TypeScript it won't work and give me the mentioned error above. I have to write it like that:

var foo :any = {};
foo.bar = 42;

In plain JavaScript the type definition with any is neither required nor valid, but in TypeScript this seems to be mandatory. I understand the error and the reason for it, but I always heard in Videos and read in the documentation:

typescriptlang.org:

"TypeScript is a typed superset of JavaScript [...]"

Introduction Video @minute 3:20:

"All JavaScript code is TypeScript code, simply copy and paste"

Is that a thing that changed during the development of TypeScript or do I have to pass a specific compiler setting to make this work?

Answer 1

The purpose of TypeScript is to have a compiler and language that can enforce types better than vanilla Javascript. Syntactically, any regular Javascript is valid TypeScript. That does not imply that the compiler must be completely satisfied. Vanilla Javascript frequently contains code that is insecure in terms of type security. That doesn't make it invalid TypeScript code, but it's the reason TypeScript exists, and it's the compiler's job to point out those issues to you.

The languages are still subsets and supersets of one another.