Is it possible to safely use Switch over FlowType union types (String Enums)?

Question

In the following example, since I'm using matching over type of Message using the switch statement, I would like flow to recognise my incorrect case of 'ENUM_TYPO'. It currently doesn't.

type Message = 'BROADCAST_MESSAGE' | 'PRIVATE_MESSAGE';

const message: Message = 'BROADCAST_MESSAGE';

switch (message) {
  case 'ENUM_TYPO':
    // Do Broadcast
    break;
  default:
    break;
}

Answer 1

As of v0.32.0, Flow does not complain about unreachable code, unless it's something like

// @flow
function foo() {
  throw new Error();
  return 123; // This will error
}. 

However, consider the following code

// @flow
function foo(x: string): Object {
  if (x === 123) {
    return x;
  }
  return {};
}

Will currently will not error on this code. Flow does in fact notice that x === 123 will never be true. Inside the if block, Flow will refine the type of x to the empty type, since it doesn't believe that this code will ever be reached. That is why it doesn't complain about the return x statement.

One of the members of the Flow team is almost done with adding reachability analysis to Flow. Once this improvement lands (I'm guessing v0.34.0?), Flow will complain when it sees a conditional that it thinks will always fail. This will help you with your example, since switch statement cases are basically strict equality checks.