Indeed, that is basically what you get from unions, except the exhaustiveness check. Unless I'm mistaken, if one were to later implement a new class that exten

Indeed, that is basically what you get from unions, except the exhaustiveness check. Unless I'm mistaken, if one were to later implement a new class that extends AppState, all existing code would compile, but possibly fail or misbehave at runtime, unless you meticulously checked every place that tries to determine something based on those derived types. In TypeScript, adding a new case for an union and not handling it everywhere is a compilation error on every incomplete usage site. For example, try deleting one of the arms of the switch in this playground: https://www.typescriptlang.org/play?ts=4.2.2#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-ACELg9noseuJIzGEwD1NYtJcAE1iDEqBFOD4KGxo5q8AQQXwClCdfHvS55otlhBzZb064GQIoL4Yp4YzWoAnXDdxC3A226QBRVi3LkYbGCiCPZ5YLWIlIpL7AhF9bE5JPjaUuNRYKGaoA https://www.typescriptlang.org/play?ts=4.2.2#code/C4TwDgpgBA... I have to say, the default diagnostic isn't brilliant, but some tooling will give a better error and actually point out the missing arms, instead of complaining about the return type.