feat(deep-object-assign): add pure version (#383)

I thought we mutate the items during updates in Data Sets but apparently
we don't, we clone them (I really should do some interoperability
testing before I open PRs). This is basically `deepObjectAssign({},
base, update)` but with proper typesafety and more clearly communicating
the intention of not modifying the arguments.

src/deep-object-assign.ts

@@ -21,6 +21,21 @@ export type Assignable<T> = T extends undefined
   ? { [Key in keyof T]?: Assignable<T[Key]> | undefined }
   : T | undefined;
 
+/**
+ * Pure version of deepObjectAssign, it doesn't modify any of it's arguments.
+ *
+ * @param base - The base object that fullfils the whole interface T.
+ * @param updates - Updates that may change or delete props.
+ *
+ * @returns A brand new instance with all the supplied objects deeply merged.
+ */
+export function pureDeepObjectAssign<T>(
+  base: T,
+  ...updates: Assignable<T>[]
+): T {
+  return deepObjectAssign({} as any, base, ...updates);
+}
+
 /**
  * Deep version of object assign with additional deleting by the DELETE symbol.
  *