Introduce way to type narrow a string property

Co-authored-by: Janne Savolainen <janne.savolainen@live.fi>

Signed-off-by: Iku-turso <mikko.aspiala@gmail.com>

src/common/utils/type-narrowing.ts

@@ -35,6 +35,15 @@ export function hasTypedProperty<S extends object, K extends PropertyKey, V>(val
   return hasOwnProperty(val, key) && isValid(val[key]);
 }
 
+/**
+ * Narrows `val` to include the property `key` with type string
+ * @param val the value that we are trying to type narrow
+ * @param key The key to test if it is present on the object (must be a literal for tsc to do any meaningful typing)
+ */
+export function hasStringProperty<S extends object, K extends PropertyKey>(val: S, key: K): val is (S & { [key in K]: string }) {
+  return hasOwnProperty(val, key) && isString(val[key]);
+}
+
 /**
  * Narrows `val` to include the property `key` with type `V | undefined` or doesn't contain it
  * @param val the value that we are trying to type narrow