asdf.js

let foo: ?number;

function bar() {
  if (foo == null) {
    return;
  }
  
  setTimeout(() => {
    // I refined above, so this should work right?
    // (hint: wrong. If `foo` were const, then it would be a correct/more intuitive assumption)
    let someNum: number = foo;
  }, offset);
}

// ...in a galaxy far far away...
foo = null;