let anything: any;
if (isFoo(anything)) {
  declare anything: Foo;
  anything.foo // valid and type-checkable
}

interface Foo {
  foo: number;
}

function isFoo(something: any): something is Foo {
    return typeof something.foo === "number"; 
}

let node: Node
if (node.nodeType === 1) {
   declare node: Element;
   node.addEventListener // valid
}

// Added on August 2, 2016
// Dog|House case from #9999
function fn(x: any) {
  if (x instanceof Animal) {
    declare x: Dog; // User knows better than the type system, so let it go explicitly
    x.woof(); // type-checked
  } else {
    declare x: House;
    // handle House case
  }
}

// Added on August 8, 2016
try {
  foo();
}
catch (err) {
  // when I'm absolutely sure that my error object will always be an instance of Error
  declare err: Error;
  console.error(err.messge) // compiler warns
}

{
  declare anything: Foo;
  // `anything` is Foo in this block.
  // if type of `anything` is not narrow-able into Foo, it should be an error.
}