Fully typecheked JS with TS lib thing

blah.js

/** "TypeDef" lib */
function Obj<T>(a: T): T {
    return undefined;
}

function Str(): string {
    return undefined;
}

function Optional<T>(a: T) {
    return undefined as (T | undefined);
}

function Throwing<T>(a: T):T {
    if (a === void 0) {
        throw new Error('Type check error');
    }
    return undefined;
}

function Logging<T>(a: T):T {
    if (a === void 0) {
        console.error('Type check error');
    }
    return undefined;
}

/** =============== */

function meh(opts = Optional("hello")) {

}

function foo(x = Throwing("")) {
    
}

const SharedType = Obj({ message: Str() })

function Greeter(greeting = SharedType) {
    this.greeting = greeting;
}

Greeter.prototype.greet = function() {
    return "Hello, " + this.greeting;
}

// Oops, we're passing an object when we want a string. This will print 
// "Hello, [object Object]" instead of "Hello, world" without error.
let greeter = new Greeter({message: "world"});  

let button = document.createElement('button');
button.textContent = "Say Hello";
button.onclick = function() {
    alert(greeter.greet());
};

document.body.appendChild(button);