leaked any example

some-flow.js

// From the flow docs: https://flow.org/en/docs/types/any/#toc-avoid-leaking-any

// @flow
function fn(obj: any) /* (:any) */ {
  let foo /* (:any) */ = obj.foo;
  let bar /* (:any) */ = foo * 2;
  return bar;
}
// returns an any type (implicitly)

// Leaks any types into your code
let bar /* (:any) */ = fn({ foo: 2 });
let baz /* (:any) */ = "baz:" + bar;

// The solution is to explicitly type something
// @flow
function fn(obj: any) /* (:number) */ {
  let foo: number = obj.foo; // <-- here
  let bar /* (:number) */ = foo * 2;
  return bar;
}
// Now we return a number (implicitly)

// So what if there was a switch that made the first example a compiler error?
// Then, if you _meant_ to return any, you'd have to be explicit
// @flow
function fn(obj: any): any {
  let foo /* (:any) */ = obj.foo;
  let bar /* (:any) */ = foo * 2;
  return bar;
}