Add comments on dateNeutralized function and utility types

src/constants/types.ts

@@ -295,6 +295,25 @@ export enum LinkTitleError {
 
 export type SubscriptionLevel = 1 | 2 | 3
 
+/*****************************************
+ *          # Utility Types #            * 
+ *   Behold! The power of TypeScript :)  *
+ *****************************************/
+
+
+ /**
+  * JavaScript does not allow us to use enum as dictionary keys by default.
+  * Make it happy.
+  */
 export type EnumDictionary<T extends string | symbol | number, U> = {
   [K in T]: U;
-};
\ No newline at end of file
+}
+
+/**
+ * A union type contains all keys in type T, whose corresponding value extends type O.
+ * 
+ * i.e. All keys of type O
+ */
+export type AllKeysExtends<T, O> = {
+  [K in keyof T]: T[K] extends O ? K : never
+}[keyof T]
\ No newline at end of file

src/utils/functions.ts

+import { AllKeysExtends } from "constants/types"
 import moment from "moment"
 
 export function titleCase(string: string) {
@@ -85,15 +86,27 @@ export function emailFromUsername(username: string): string {
   return `${username}@ic.ac.uk`
 }
 
+/**
+ * TODO: not implemented
+ * 
+ * Show the full cohort string of the given cohortID
+ * i.e. 'c3' => 'Computing - Year 3'
+ * @param cohortID cohort id of the student (e.g. 'c3')
+ */
 export function showCohort(cohortID: string): string {
   return "Computing - Year 3"
 }
 
-export type AllKeysExtends<T, O> = {
-  [K in keyof T]: T[K] extends O ? K : never
-}[keyof T]
-
-
+/**
+ * Neutralize an object by parsing specified date members into Date objects. 
+ * 
+ * Can be used to neutralize the response object from the api request,
+ * as the raw response object stores all Dates members as strings.
+ * 
+ * This function would only work on the keys that are declared to be of 'Date' type. 
+ * @param dict The object to be neutralized
+ * @param dateKeys The keys you wish to neutralize.
+ */
 export function dateNeutralized<T>(dict: T, ...dateKeys: AllKeysExtends<T, Date>[]): T {
   for (const k of dateKeys) {
     const date = dict[k]