Add Narrow The Type Of An Array To Its Values 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).
 
-_1238 TILs and counting..._
+_1239 TILs and counting..._
 
 ---
 
@@ -1141,6 +1141,7 @@ _1238 TILs and counting..._
 - [Extract Object Type Values Into A Union Type](typescript/extract-object-type-values-into-a-union-type.md)
 - [Generate Inferred Type From Zod Schema](typescript/generate-inferred-type-from-zod-schema.md)
 - [Interfaces With The Same Name Are Merged](typescript/interfaces-with-the-same-name-are-merged.md)
+- [Narrow The Type Of An Array To Its Values](typescript/narrow-the-type-of-an-array-to-its-values.md)
 - [Re-Export An Imported Type](typescript/re-export-an-imported-type.md)
 - [Type Narrowing With Const VS Let Strings](typescript/type-narrowing-with-const-vs-let-strings.md)
 - [Type Narrowing With Similarly Shaped Objects](typescript/type-narrowing-with-similarly-shaped-objects.md)

typescript/narrow-the-type-of-an-array-to-its-values.md

@@ -0,0 +1,39 @@
+# Narrow The Type Of An Array To Its Values
+
+When an array of string values is defined in a TypeScript context, the inferred
+type will be `string[]`. That's because the values of an array can be
+reassigned. The most precise TypeScript can be is to say that it is an array of
+string.
+
+```typescript
+const actions = ['increase', 'decrease'];
+// typeof actions => string[]
+```
+
+We can freeze the array with its values using `as const`, the [const
+assertion](https://www.typescriptlang.org/docs/handbook/release-notes/typescript-3-4.html#const-assertions).
+This doesn't actually _freeze_ the array, but it does mark it as `readonly` for
+the TypeScript compiler.
+
+That means we can lean on the compiler for a lot more specific feedback.
+Consider for instance this `reducer` function.
+
+```typescript
+const actions = ['increase', 'decrease'] as const
+
+const reducer = (action: typeof actions[number]) => {
+  switch (action) {
+    case 'increase':
+      // do an increase
+    case 'decrease':
+      // do a decrease
+    case 'submit': // TYPE ERROR, "submit" is not comparable to type 'increase' | 'decrease'
+      // do a submit
+    default:
+      throw Error('Unrecognized action!')
+  }
+}
+
+// TYPE ERROR, "submit" is not comparable to type 'increase' | 'decrease'
+reducer('submit')
+```