Add Create A Union Type From An Array as a TypeScript TIL

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).
 
-_1241 TILs and counting..._
+_1242 TILs and counting..._
 
 ---
 
@@ -1138,6 +1138,7 @@ _1241 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)
+- [Create A Union Type From An Array](typescript/create-a-union-type-from-an-array.md)
 - [Create Union Type From Constants](typescript/create-union-type-from-constants.md)
 - [Extract Object Type Keys Into A Union Type](typescript/extract-object-type-keys-into-a-union-type.md)
 - [Extract Object Type Values Into A Union Type](typescript/extract-object-type-values-into-a-union-type.md)

typescript/create-a-union-type-from-an-array.md

@@ -0,0 +1,32 @@
+# Create A Union Type From An Array
+
+Let's say we have an array of _actions_ that our program knows how to handle.
+
+```typescript
+// inferred type: string[]
+const actions = ['increase', 'decrease', 'reset'];
+```
+
+The inferred type of that array is `string[]` which is a pretty wide type. We
+can't do much with it. We can prevent the widening of this array's inferred
+type using `as const`.
+
+```typescript
+// inferred type: readonly ['increase', 'decrease', 'reset']
+const actions = ['increase', 'decrease', 'reset'] as const;
+```
+
+That inferred type is specific enough that we can do something with it, like
+create a union type.
+
+```typescript
+const actions = ['increase', 'decrease', 'reset'] as const;
+
+type Actions = typeof actions[number];
+//=> type Actions = 'increase' | 'decrease' | 'reset'
+```
+
+We could use the `Actions` type to specify that a function only takes values
+that correspond to known actions, for instance.
+
+[source](https://bobbyhadz.com/blog/typescript-create-union-type-from-array)