User-defined type guard functions

TypeScript 1.6 adds a new way to narrow a variable type inside an if block, in addition to typeof and instanceof. A user-defined type guard functions is one with a return type annotation of the form x is T, where x is a declared parameter in the signature, and T is any type. When a user-defined type guard function is invoked on a variable in an if block, the type of the variable will be narrowed to T.

Examples

function isCat(a: any): a is Cat {
  return a.name === 'kitty';
}
var x: Cat | Dog;
if(isCat(x)) {
  x.meow(); // OK, x is Cat in this block
}