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

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

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+# 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)