Add Create Union Type From Constants as a TypeScript TIL

typescript/create-union-type-from-constants.md

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+# Create Union Type From Constants
+
+I like to capture what I call _magic strings_ in constants (variables, really)
+so that I can use them and reuse them with minimal maintenance. Minimal
+maintenance because if something about the value of the string changes, I only
+need to make that update in a single place.
+
+I want to extend this reuse to my type system as well.
+
+Let's say I have some constants defined like so:
+
+```typescript
+const UPGRADE = "upgrade";
+const DOWNGRADE = "downgrade";
+```
+
+I can create a union type from those values using the [`typeof`
+operator](https://www.typescriptlang.org/docs/handbook/2/typeof-types.html).
+
+```
+type IntervalChange = typeof UPGRADE | typeof DOWNGRADE;
+//=> type IntervalChange = 'upgrade' | 'downgrade'
+```
+
+I can then use both the constants and the type throughout my code with only one
+place to update.
+
+```typescript
+function checkForUpgrade(interval: string): IntervalChange {
+  // some logic
+  const result = ...;
+
+  return result ? UPGRADE : DOWNGRADE;
+}
+```
+
+[source](https://twitter.com/jbrancha/status/1565770454052249601?s=20&t=4seYY0mzoTFeDCiFwRB91g)