When we put type members in a union, TypeScript will only allow us to use the common methods and properties that all members of the union share. Take this code:

const batteryStatus: boolean | number = false; batteryStatus.toString(); // No TypeScript error batteryStatus.toFixed(2); // TypeScript error

Since batteryStatus can be a boolean or a number, TypeScript will only allow us to call methods that both number and boolean share. They both share .toString(), so we’re good there. But, since only number has a .toFixed() method, TypeScript will complain if we try to call it.

This rule also applies to type objects that we define. Take this code:

type Goose = { isPettable: boolean; hasFeathers: boolean; canThwartAPicnic: boolean; } type Moose = { isPettable: boolean; hasHoofs: boolean; } const pettingZooAnimal: Goose | Moose = { isPettable: true }; console.log(pettingZooAnimal.isPettable); // No TypeScript error console.log(pettingZooAnimal.hasHoofs); // TypeScript error

Like before, since .isPettable is on both Goose and Moose types, TypeScript will allow us to call it. But since .hasHoofs is only a property on Moose, we cannot call that method on pettingZooAnimal. Any properties or methods that are not shared by all of the union’s members won’t be allowed and will produce a TypeScript error.



In the editor, we’ve written a program that should display a message about a social media event. The getFriendNameFromEvent() function takes an event, then returns the friend’s displayName or username.

Let’s start by adding a type to the getFriendNameFromEvent()’s event parameter. Use a union to allow event to be a Like or a Share, then save your code.


In getFriendNameFromEvent()‘s body, it returns event.displayName || event.username. Hover over event.displayName. You’ll see a pop-up with this error message:

Property 'displayName' does not exist on type 'Like | Share'. Property 'displayName' does not exist on type 'Share'.(2339)

This message is pointing out that there’s no displayName property on the Share type. In the Share type, add a displayName of type string, just like on the Like type.


Now run tsc in the terminal to verify there are no type errors.

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