Add Narrow The Type Of An Array To Its Values as a TypeScript TIL

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

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+# 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')
+```