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

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

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+# 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)