From 423b354a90c4b1197ea549dbef5030c4a7a612a2 Mon Sep 17 00:00:00 2001
From: jbranchaud <jbranchaud@gmail.com>
Date: Fri, 2 Sep 2022 16:29:03 -0500
Subject: [PATCH] Add Create Union Type From Constants as a TypeScript TIL

---
 README.md                                     |  3 +-
 .../create-union-type-from-constants.md       | 37 +++++++++++++++++++
 2 files changed, 39 insertions(+), 1 deletion(-)
 create mode 100644 typescript/create-union-type-from-constants.md

diff --git a/README.md b/README.md
index d161f6b..6c27ecc 100644
--- a/README.md
+++ b/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).
 
-_1234 TILs and counting..._
+_1235 TILs and counting..._
 
 ---
 
@@ -1135,6 +1135,7 @@ _1234 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 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)
 - [Generate Inferred Type From Zod Schema](typescript/generate-inferred-type-from-zod-schema.md)
diff --git a/typescript/create-union-type-from-constants.md b/typescript/create-union-type-from-constants.md
new file mode 100644
index 0000000..b66c803
--- /dev/null
+++ b/typescript/create-union-type-from-constants.md
@@ -0,0 +1,37 @@
+# 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)