TS compilation - "noImplicitAny" doesn't work

I have the code

let z;
z = 50;
z = 'z';

and my tsconfig.json is:

{
  "compilerOptions": {
    "target": "es5",
    "module": "commonjs",
    "sourceMap": false,
    "noEmitOnError": true,
    "strict": true,
    "noImplicitAny": true
  }
}

But what the hell it is that there's no exceptions throgh compiling to js?

Best Regards, Crova