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

Author: jbranchaud

README.md

@@ -10,7 +10,7 @@ pairing with smart people at Hashrocket.
 
 For a steady stream of TILs, [sign up for my newsletter](https://crafty-builder-6996.ck.page/e169c61186).
 
-_1209 TILs and counting..._
+_1210 TILs and counting..._
 
 ---
 
@@ -1115,6 +1115,7 @@ _1209 TILs and counting..._
 
 - [Add Types To An Object Destructuring](typescript/add-types-to-an-object-destructuring.md)
 - [Compiler Checks For Unused Params And Variables](typescript/compiler-checks-for-unused-params-and-variables.md)
+- [Extract Object Type Keys Into A Union Type](typescript/extract-object-type-keys-into-a-union-type.md)
 - [Interfaces With The Same Name Are Merged](typescript/interfaces-with-the-same-name-are-merged.md)
 - [Re-Export An Imported Type](typescript/re-export-an-imported-type.md)
 - [Type Narrowing With Similarly Shaped Objects](typescript/type-narrowing-with-similarly-shaped-objects.md)

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

@@ -0,0 +1,37 @@
+# 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.