Control Flow Improvements

In almost every release there are control flow improvements, in 4.7 there are improvements in computed properties work when narrowing. To learn more about narrowing see: example:type-widening-and-narrowing 

const dog = "stringer";

const dogsToOwnersOrID = {
  hayes: "The McShanes",
  poppy: "Pat",
  stringer: "Jane",
  otto: 1,
} as Record;
// A computed property is a property access which is not hardcoded, for example: 
const owner = dogsToOwnersOrID[dog];
//    ^?
// Prior to TypeScript 4.7, using a computed property would not reliably narrow the type of a value. 
if (typeof dogsToOwnersOrID[dog] === "string") {
  const str = dogsToOwnersOrID[dog].toUpperCase();
  //     ^?

  // In 4.6, this would still be 'string | number'.
}