Is TypeScript really a superset of JavaScript?

Question

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

var f         


        

Answer 1

The definition

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

is true. Because any JavaScript code can passed to the TypeScript compiler.

So it's sort of a Layer on top of JavaScript. So, of course the underlaying Layer (JavaScript) can be passed through the layers to the top (TypeScript), but not the other way around.

Why not?

Think of it as a bike (JavaScript) and a motorcycle (TypeScript). The basics are the same (two wheels, a frame), but the motorcycle as an engine and some enhanced features.

So, you can use your motorcycle (TypeScript) as a bike (JavaScript), but you cannot use a bike as a motorcycle.

EDIT:

If your compiler throws a warning, why does it make the statement wrong? It just says: Hey, you are using TypeScript, and it's more strict than what you gave me.

See this example, it compiles perfectly to JavaScript, but throws a warning.