Add Type Narrowing With Const VS Let Strings as a TypeScript TIL

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).
 
-_1236 TILs and counting..._
+_1237 TILs and counting..._
 
 ---
 
@@ -1142,6 +1142,7 @@ _1236 TILs and counting..._
 - [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)
 - [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)
 - [Type Promise Results With The Awaited Type](typescript/type-promise-results-with-the-awaited-type.md)
 - [Use An Array Check For Type Narrowing](typescript/use-an-array-check-for-type-narrowing.md)

typescript/type-narrowing-with-const-vs-let-strings.md

@@ -0,0 +1,40 @@
+# Type Narrowing With Const VS Let Strings
+
+[TypeScript's `typeof`
+operator](https://www.typescriptlang.org/docs/handbook/2/typeof-types.html) can
+be used to capture the type of a variable.
+
+For instance, we can use it with a string variable like so:
+
+```typescript
+let status = 'active';
+
+type Status = typeof status;
+//=> type Status = string;
+```
+
+The result is a type `Status` defined as a `string`.
+
+What if we were to do the same with a string variable defined with `const`
+(instead of `let`)?
+
+```typescript
+const status = 'active';
+
+type Status = typeof status;
+//=> type Status = 'active';
+```
+
+We get a different result. A much more specific result. The `Status` type
+definition in this case isn't just any `string`. TypeScript knows specifically
+that it is the string `'active'`.
+
+This is _Type Narrowing_ at work. With our `let` string, which can be redefined
+at anytime, the most TypeScript can tell us about it is that it is a `string`.
+Our `const` string, on the other hand, is frozen as `'active'`, so TypeScript
+can narrow the type from `string` to the string `'active'`.
+
+That can be handy for [defining a union type from existing `const`
+values](https://twitter.com/jbrancha/status/1565770454052249601).
+
+[source](https://twitter.com/jbrancha/status/1565752445187358721)