Creating Union Types

I want to start with a concept we’ve already seen, but which I want to talk about in more depth because it winds up being important for understanding other TypeScript techniques. The feature is called a union type, and we used it to create the type checking for our action types in our various reducers in Chapter 11, ​Managing State in React​ and in Chapter 12, ​Using Redux to Manage State​.

You create a union type by combining existing types with the pipe operator ( | ). The existing types can be built-in or specific to your code. One of our union types, for example, looks like this:

​ ​export​ type VenueAction =
​  | ClearHolds
​  | HoldTicket

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