BadTypeScript.ts

// Example 1: `any` allows unsafe casts

var x : any = 1;
x();

// Example 2: subtyping on functions allows unsafe casts

interface I1 { x: number; }
interface I2 { x: number; y: number; }
function f(g: (x: I1) => void) { g({x: 1}); }
f(function (x: I2) { console.log(x.y); })

// Example 3: type system ignores closing over `this`:

class C {
  n: number;
  constructor () {
    this.n = 0;
  }
	
  next() {
    var k = this.n;
    this.n = k + 1;
    return k;
  }
}

function fc(g: () => number) {
  console.log(g());
  console.log(g());
  console.log(g());
}

var c = new C();
fc(c.next);
fc(c.next);
fc(c.next);