noImplicitAny

There are some things that cannot be inferred or inferring them might result in unexpected errors. A fine example is function arguments. If you don't annotate them, its unclear what should and shouldn't be valid e.g.

function log(someArg) {
  sendDataToServer(someArg);
}
​
// What arg is valid and what isn't?
log(123);
log('hello world');

So if you don't annotate some function argument, TypeScript assumes any and moves on. This essentially turns off type checking for such cases, which is what a JavaScript dev would expect. But this can catch people that want high safety off guard. Hence there is an option, noImplicitAny, that when switched on will flag the cases where the type cannot be inferred e.g.

function log(someArg) { // Error : someArg has an implicit `any` type
  sendDataToServer(someArg);
}

Of course you can then go ahead and annotate:

function log(someArg: number) {
  sendDataToServer(someArg);
}

And if you truly want zero safety you can mark it explicitly as any:

function log(someArg: any) {
  sendDataToServer(someArg);
}