My solution

.vscode/settings.json

@@ -0,0 +1,5 @@
+{
+  "files.exclude": {
+    "**/node_modules": false,
+  }
+}

exercises/exercise-00/index.ts

@@ -26,7 +26,13 @@ Run this exercise:
 
 */
 
-const users: unknown[] = [
+interface Users {
+    name: string,
+    age: number,
+    occupation: string
+}
+
+const users: Users[] = [
     {
         name: 'Max Mustermann',
         age: 25,
@@ -39,7 +45,7 @@ const users: unknown[] = [
     }
 ];
 
-function logPerson(user: unknown) {
+function logPerson(user: Users) {
     console.log(` - ${chalk.green(user.name)}, ${user.age}`);
 }
 

exercises/exercise-01/index.ts

@@ -40,7 +40,9 @@ interface Admin {
     role: string;
 }
 
-const persons: User[] /* <- Person[] */ = [
+type Person = User | Admin;
+
+const persons: Person[] = [
     {
         name: 'Max Mustermann',
         age: 25,
@@ -63,7 +65,7 @@ const persons: User[] /* <- Person[] */ = [
     }
 ];
 
-function logPerson(user: User) {
+function logPerson(user: Person) {
     console.log(` - ${chalk.green(user.name)}, ${user.age}`);
 }
 

exercises/exercise-02/index.ts

@@ -65,7 +65,7 @@ const persons: Person[] = [
 
 function logPerson(person: Person) {
     let additionalInformation: string;
-    if (person.role) {
+    if ('role' in person) {
         additionalInformation = person.role;
     } else {
         additionalInformation = person.occupation;

exercises/exercise-03/index.ts

@@ -48,11 +48,11 @@ const persons: Person[] = [
     { type: 'admin', name: 'Bruce Willis', age: 64, role: 'World saver' }
 ];
 
-function isAdmin(person: Person) {
+function isAdmin(person: Person): person is Admin{
     return person.type === 'admin';
 }
 
-function isUser(person: Person) {
+function isUser(person: Person): person is User {
     return person.type === 'user';
 }
 

exercises/exercise-04/index.ts

@@ -47,6 +47,8 @@ interface Admin {
 
 type Person = User | Admin;
 
+type PartialUser = Partial<User>;
+
 const persons: Person[] = [
     { type: 'user', name: 'Max Mustermann', age: 25, occupation: 'Chimney sweep' },
     {
@@ -95,7 +97,7 @@ function logPerson(person: Person) {
     console.log(` - ${chalk.green(person.name)}, ${person.age}, ${additionalInformation}`);
 }
 
-function filterUsers(persons: Person[], criteria: User): User[] {
+function filterUsers(persons: Person[], criteria: PartialUser): User[] {
     return persons.filter(isUser).filter((user) => {
         let criteriaKeys = Object.keys(criteria) as (keyof User)[];
         return criteriaKeys.every((fieldName) => {

exercises/exercise-05/index.ts

@@ -49,6 +49,8 @@ interface Admin {
 
 type Person = User | Admin;
 
+type FilterCriteria<T> = Partial<Omit<T, 'type'>>;
+
 const persons: Person[] = [
     { type: 'user', name: 'Max Mustermann', age: 25, occupation: 'Chimney sweep' },
     { type: 'admin', name: 'Jane Doe', age: 32, role: 'Administrator' },
@@ -64,11 +66,17 @@ function logPerson(person: Person) {
     );
 }
 
-function filterPersons(persons: Person[], personType: string, criteria: unknown): unknown[] {
+function getObjectKeys<O>(obj: O): (keyof O)[] {
+    return Object.keys(obj) as (keyof O)[];
+}
+
+function filterPersons(persons: Person[], personType: 'user', criteria: FilterCriteria<User>): User[];
+function filterPersons(persons: Person[], personType: 'admin', criteria: FilterCriteria<Admin>): Admin[];
+function filterPersons(persons: Person[], personType: 'user' | 'admin', criteria: FilterCriteria<Person>): Person[] {
     return persons
         .filter((person) => person.type === personType)
         .filter((person) => {
-            let criteriaKeys = Object.keys(criteria) as (keyof Person)[];
+            let criteriaKeys = getObjectKeys(criteria);
             return criteriaKeys.every((fieldName) => {
                 return person[fieldName] === criteria[fieldName];
             });

exercises/exercise-06/index.ts

@@ -88,7 +88,7 @@ const users: User[] = [
     }
 ];
 
-function swap(v1, v2) {
+function swap<T1, T2>(v1: T1, v2: T2) : [T2, T1] {
     return [v2, v1];
 }
 

exercises/exercise-07/index.ts

@@ -41,7 +41,7 @@ interface Admin {
     role: string;
 }
 
-type PowerUser = unknown;
+type PowerUser = Omit<User & Admin, 'type'> & { type: 'powerUser' };
 
 type Person = User | Admin | PowerUser;
 
@@ -99,4 +99,4 @@ console.log(chalk.yellow('Power users:'));
 persons.filter(isPowerUser).forEach(logPerson);
 
 // In case if you are stuck:
-// https://www.typescriptlang.org/docs/handbook/release-notes/typescript-2-8.html#predefined-conditional-types
+// https://www.typescriptlang.org/docs/handbook/utility-types.html

exercises/exercise-08/index.ts

@@ -63,50 +63,39 @@ const users: User[] = [
     { type: 'user', name: 'Kate Müller', age: 23, occupation: 'Astronaut' }
 ];
 
-type AdminsApiResponse = (
+type ApiResponse<T> = (
     {
         status: 'success';
-        data: Admin[];
+        data: T;
     } |
     {
         status: 'error';
         error: string;
     }
-);
+)
 
-function requestAdmins(callback: (response: AdminsApiResponse) => void) {
+function requestAdmins(callback: (response: ApiResponse<Admin[]>) => void) {
     callback({
         status: 'success',
         data: admins
     });
 }
 
-type UsersApiResponse = (
-    {
-        status: 'success';
-        data: User[];
-    } |
-    {
-        status: 'error';
-        error: string;
-    }
-);
-
-function requestUsers(callback: (response: UsersApiResponse) => void) {
+function requestUsers(callback: (response: ApiResponse<User[]>) => void) {
     callback({
         status: 'success',
         data: users
     });
 }
 
-function requestCurrentServerTime(callback: (response: unknown) => void) {
+function requestCurrentServerTime(callback: (response: ApiResponse<number>) => void) {
     callback({
         status: 'success',
         data: Date.now()
     });
 }
 
-function requestCoffeeMachineQueueLength(callback: (response: unknown) => void) {
+function requestCoffeeMachineQueueLength(callback: (response: ApiResponse<string>) => void) {
     callback({
         status: 'error',
         error: 'Numeric value has exceeded Number.MAX_SAFE_INTEGER.'

exercises/exercise-09/index.ts

@@ -71,8 +71,19 @@ type ApiResponse<T> = (
     }
 );
 
-function promisify(arg: unknown): unknown {
-    return null;
+type PromisifyOldDefinition<T> = (callback: (response: ApiResponse<T>) => void) => void;
+type PromisifyNewDefinition<T> = () => Promise<T>;
+
+function promisify<T>(oldFunction: PromisifyOldDefinition<T>): PromisifyNewDefinition<T> {
+    return () => new Promise((resolve, reject) => {
+        oldFunction((response) => {
+            if (response.status === 'success') {
+                resolve(response.data)
+            } else {
+                reject(response.error)
+            }
+        });
+    });
 }
 
 const oldApi = {

exercises/exercise-10/declarations/str-utils/index.d.ts

@@ -1,4 +1,7 @@
 declare module 'str-utils' {
-    // export const ...
-    // export function ...
+    export function strToLower(str: string): string;
+    export function strToUpper(str: string): string;
+    export function strReverse(str: string): string;
+    export function strInvertCase(str: string): string;
+    export function strRandomize(str: string): string;
 }

exercises/exercise-11/declarations/stats/index.d.ts

@@ -1,3 +1,13 @@
 declare module 'stats' {
-    export function getMaxIndex(input: unknown, comparator: unknown): unknown;
+    type Comparator<I> = (a: I, b: I) => number;
+    type StatIndexFunction = <I>(input: I[], comparator: Comparator<I>) => number;
+    type StatElementFunction = <I>(input: I[], comparator: Comparator<I>) => I;
+
+    export const getMaxIndex: StatIndexFunction;
+    export const getMinIndex: StatIndexFunction;
+    export const getMedianIndex: StatIndexFunction;
+    export const getMaxElement: StatElementFunction;
+    export const getMinElement: StatElementFunction;
+    export const getMedianElement: StatElementFunction;
+    export const getAverageValue: <I, O>(items: I[], getValue: (item: I) => O) => O;
 }

exercises/exercise-11/index.ts

@@ -42,7 +42,7 @@ Exercise:
     Provide type declaration for that module in:
     declarations/stats/index.d.ts
 
-Higher difficulty bonus excercise:
+Higher difficulty bonus exercise:
 
     Avoid duplicates of type declarations.
 

exercises/exercise-12/module-augmentations/date-wizard/index.ts

@@ -2,5 +2,10 @@
 import 'date-wizard';
 
 declare module 'date-wizard' {
-    // Add your module extensions here.
+    interface DateDetails {
+        hours: number;
+        minutes: number;
+        seconds: number;
+    }
+    export function pad(num: number): string;
 }