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

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

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+# Extract Object Type Values Into A Union Type
+
+Let's say we have an object type like the following:
+
+```typescript
+type GlobalReducerEvent = {
+    ADD_TODO: {
+        text: string
+    }
+    LOG_IN: {
+        email: string
+    }
+    DELETE_TODO: {
+        todo_id: number
+    }
+}
+```
+
+We can use the [indexed access
+type](https://www.typescriptlang.org/docs/handbook/2/indexed-access-types.html)
+to access the value indexed at one the type keys. This works a lot like
+JavaScript's object access.
+
+```typescript
+type AddTodoType = GlobalReducerEvent['ADD_TODO'];
+/*
+type AddTodoType = {
+  text: string
+}
+*/
+```
+
+Where it gets more interesting and differs from JavaScript's object access is
+when we do an indexed access of a union type.
+
+```typescript
+type TypesForEvents = GlobalReducerEvent[keyof GlobalReducerEvent];
+/*
+type TypesForEvents = {
+  text: string
+} | {
+  email: string
+} | {
+  todo_id: number
+}
+*/
+```
+
+The result is a union of each of the values whose keys had matches in the
+union.
+
+Where this gets more interesting and useful is when we use it as part of a
+mapped type like I explore in [Breaking Down a Complex Mapped
+Type](https://dev.to/jbranchaud/breaking-down-a-complex-mapped-type-in5).