Add Extract Object Type Keys Into A Union Type as a TypeScript TIL

typescript/extract-object-type-keys-into-a-union-type.md

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+# Extract Object Type Keys Into A Union Type
+
+The [object type](https://www.typescriptlang.org/docs/handbook/2/objects.html)
+is a way of grouping types together to represent something more complex. For
+instance, it may be used to represent the types of events that a reducer can
+process.
+
+```typescript
+interface GlobalReducerEvent {
+    ADD_TODO: {
+        text: string
+    }
+    LOG_IN: {
+        email: string
+    }
+    DELETE_TODO: {
+        todo_id: number
+    }
+}
+```
+
+From this object type, I can extract a [union
+type](https://www.typescriptlang.org/docs/handbook/2/everyday-types.html#union-types)
+of the keys, perhaps as part of building [a more useful type
+mapping](https://dev.to/jbranchaud/breaking-down-a-complex-mapped-type-in5).
+
+This can be done with
+[`keyof`](https://www.typescriptlang.org/docs/handbook/2/keyof-types.html).
+
+```typescript
+type EventTypes = keyof GlobalReducerEvent;
+//=> 'ADD_TODO' | 'LOG_IN' | 'DELETE_TODO'
+```
+
+The `keyof` type operator extracts each key of the `GlobalReducerEvent` into a
+union type. This can be mixed into other types in all sorts of interesting
+ways.