ts: Migrate ‘user_sort.js’ to TypeScript.

Also fixes a bug in the people.ts User; `delivery_email` is never undefined.

tools/test-js-with-node

@@ -269,7 +269,7 @@ EXEMPT_FILES = make_set(
         "web/src/user_groups.ts",
         "web/src/user_profile.js",
         "web/src/user_settings.ts",
-        "web/src/user_sort.js",
+        "web/src/user_sort.ts",
         "web/src/user_status.ts",
         "web/src/user_status_ui.js",
         "web/src/user_topic_popover.js",

web/src/people.ts

@@ -23,7 +23,7 @@ export type ProfileData = {
 
 export type User = {
     user_id: number;
-    delivery_email?: string | null;
+    delivery_email: string | null;
     email: string;
     full_name: string;
     date_joined: string;

web/src/user_sort.ts

@@ -1,4 +1,6 @@
-export function compare_a_b(a, b) {
+import type {User} from "./people";
+
+export function compare_a_b(a: number | string, b: number | string): number {
     if (a > b) {
         return 1;
     } else if (a === b) {
@@ -7,7 +9,7 @@ export function compare_a_b(a, b) {
     return -1;
 }
 
-export function sort_email(a, b) {
+export function sort_email(a: User, b: User): number {
     const email_a = a.delivery_email;
     const email_b = b.delivery_email;
 
@@ -27,10 +29,10 @@ export function sort_email(a, b) {
     return compare_a_b(email_a.toLowerCase(), email_b.toLowerCase());
 }
 
-export function sort_role(a, b) {
+export function sort_role(a: User, b: User): number {
     return compare_a_b(a.role, b.role);
 }
 
-export function sort_user_id(a, b) {
+export function sort_user_id(a: User, b: User): number {
     return compare_a_b(a.user_id, b.user_id);
 }