diff --git a/src/12-testing/KATA - Solution/tests/UserService.test.ts b/src/12-testing/KATA - Solution/tests/UserService.test.ts
index 4eab43d..ec76858 100644
--- a/src/12-testing/KATA - Solution/tests/UserService.test.ts	
+++ b/src/12-testing/KATA - Solution/tests/UserService.test.ts	
@@ -20,10 +20,10 @@ describe('UserService', () => {
 
     expect(user).toEqual({ id: 1000, username: 'Test User' });
     expect(apiService.fetchUser).toHaveBeenCalledWith(1000);
+    expect(apiService.fetchUser).toHaveBeenCalledTimes(1);
   });
 
   test('getUserById should throw an error if ID is missing', () => {
-    const userService = new UserService(null as unknown as ApiService);
     expect(() =>
       userService.getUserById(undefined as unknown as number)
     ).toThrow('ID is required');
@@ -32,10 +32,15 @@ describe('UserService', () => {
   test('getAllUsers should return a list of users', async () => {
     apiService.fetchAllUsers.mockResolvedValue([
       { id: 2000, username: 'User 1' },
+      { id: 2001, username: 'User 2' },
     ]);
     const users = await userService.getAllUsers();
 
-    expect(users).toEqual([{ id: 2000, username: 'User 1' }]);
+    expect(users).toEqual([
+      { id: 2000, username: 'User 1' },
+      { id: 2001, username: 'User 2' },
+    ]);
+    expect(users.length).toBe(2);
     expect(apiService.fetchAllUsers).toHaveBeenCalled();
   });
 });
diff --git a/src/13-funktionale-programmierung/.vscode/settings.json b/src/13-funktionale-programmierung/.vscode/settings.json
new file mode 100644
index 0000000..c6c736a
--- /dev/null
+++ b/src/13-funktionale-programmierung/.vscode/settings.json
@@ -0,0 +1,9 @@
+{
+  "github.copilot.enable": {
+    "*": false,
+    "plaintext": false,
+    "markdown": false,
+    "scminput": false
+  },
+  "deno.enable": true
+}
diff --git a/src/13-funktionale-programmierung/composition.ts b/src/13-funktionale-programmierung/composition.ts
new file mode 100644
index 0000000..b6f6e54
--- /dev/null
+++ b/src/13-funktionale-programmierung/composition.ts
@@ -0,0 +1,86 @@
+function toUpperCase(str: string): string {
+  return str.toUpperCase();
+}
+
+function addExclamation(str: string): string {
+  return str + '!';
+}
+
+function addPrefix(str: string): string {
+  return 'abc ' + str;
+}
+
+function greet(name: string): string {
+  return addExclamation(toUpperCase('Hello ' + name));
+}
+
+console.log(greet('Lars'));
+
+function compose<T>(...functions: ((arg: T) => T)[]): (arg: T) => T {
+  return (arg: T) => functions.reduceRight((result, fn) => fn(result), arg);
+}
+
+const enhanceText = compose(addPrefix, toUpperCase, addPrefix, addExclamation);
+
+console.log(enhanceText('hello'));
+
+type GenericFn = (a: any) => any;
+
+function composeGeneric(...fns: GenericFn[]) {
+  return function composed(inital: any) {
+    return fns.reduceRight((value, fn) => fn(value), inital);
+  };
+}
+
+const addHello = (str: string) => `Hello ${str}`;
+const makeBold = (str: string) => `<b>${str}</b>`;
+const toObj = (str: string) => ({
+  html: str,
+});
+
+const composedFn = composeGeneric(toObj, makeBold, addHello);
+console.log(composedFn('World'));
+
+function pipe<T>(...functions: ((arg: T) => T)[]): (arg: T) => T {
+  return (arg: T) => functions.reduce((result, fn) => fn(result), arg);
+}
+
+const enhanceTextPipe = pipe(
+  (str: string) => 'Hello ' + str,
+  addPrefix,
+  toUpperCase,
+  makeBold,
+  addExclamation
+);
+console.log(enhanceTextPipe('Max'));
+
+class StringProcessor {
+  constructor(private value: string) {}
+
+  toUpperCase(): this {
+    this.value = this.value.toUpperCase();
+    return this;
+  }
+
+  addExclamation(): this {
+    this.value = this.value + '!';
+    return this;
+  }
+
+  prependHello(): this {
+    this.value = 'Hello ' + this.value;
+    return this;
+  }
+
+  getResult(): string {
+    return this.value;
+  }
+}
+
+const result = new StringProcessor('alice')
+  .toUpperCase()
+  .prependHello()
+  .addExclamation()
+  .getResult();
+
+console.log(result);
diff --git a/src/13-funktionale-programmierung/currying.ts b/src/13-funktionale-programmierung/currying.ts
new file mode 100644
index 0000000..897bd01
--- /dev/null
+++ b/src/13-funktionale-programmierung/currying.ts
@@ -0,0 +1,25 @@
+function add(a: number, b: number): number {
+  return a + b;
+}
+
+function multiply(a: number, b: number) {
+  return a * b;
+}
+
+console.log(add(2, 3));
+
+function curriedAdd(a: number): (b: number) => number {
+  return (b: number) => a + b;
+}
+
+const addTwo = curriedAdd(2);
+console.log(addTwo(3));
+
+function curry<T, U, R>(fn: (a: T, b: U) => R): (a: T) => (b: U) => R {
+  return (a: T) => (b: U) => fn(a, b);
+}
+
+const curriedMultiply = curry(multiply);
+
+const multiplyByThree = curriedMultiply(3);
+console.log(multiplyByThree(4));
diff --git a/src/13-funktionale-programmierung/higher-order-functions.ts b/src/13-funktionale-programmierung/higher-order-functions.ts
new file mode 100644
index 0000000..5c5662b
--- /dev/null
+++ b/src/13-funktionale-programmierung/higher-order-functions.ts
@@ -0,0 +1,33 @@
+function applyFunction(fn: (x: number) => number, value: number): number {
+  return fn(value);
+}
+
+function double(x: number): number {
+  return x * 2;
+}
+
+console.log(applyFunction(double, 5));
+
+const numbers = [1, 2, 3, 4];
+const doubleNumbers = numbers.map((n) => n * 2);
+console.log(doubleNumbers);
+
+function createMultipler(factor: number): (x: number) => number {
+  return (x: number) => x * factor;
+}
+
+const triple = createMultipler(3);
+
+console.log(triple(5));
+
+function compose<T>(...functions: ((arg: T) => T)[]): (arg: T) => T {
+  return (arg: T) => functions.reduceRight((result, fn) => fn(result), arg);
+}
+
+const processNumber = compose(
+  (x: number) => x + 1,
+  (x: number) => x * 2,
+  (x: number) => x - 3
+);
+
+console.log(processNumber(50));
diff --git a/src/13-funktionale-programmierung/index.ts b/src/13-funktionale-programmierung/index.ts
new file mode 100644
index 0000000..bfca2bb
--- /dev/null
+++ b/src/13-funktionale-programmierung/index.ts
@@ -0,0 +1,41 @@
+const numbers: number[] = [1, 2, 3, 4, 5];
+
+const doubleFn = (num: number): number => num * 2;
+
+const doubledNumbers: number[] = numbers.map(doubleFn);
+
+console.log(doubledNumbers);
+console.log(doubleFn(52));
+
+const filterEven = (num: number): boolean => num % 2 === 0;
+console.log(numbers.filter(filterEven));
+
+const sumFn = (total: number, current: number) => total + current;
+console.log(numbers.reduce(sumFn, 0));
+
+function doubleArray<T extends number>(arr: T[]): T[] {
+  return arr.map(doubleFn) as T[];
+}
+
+type Celsius = number;
+type Fahrenheit = number;
+
+function scaleTemperatures<T extends number>(
+  temps: T[],
+  factor: number,
+  offset: number = 0
+): T[] {
+  return temps.map((temp) => (temp * factor + offset) as T);
+}
+
+const temperaturesC: Celsius[] = [10, 20, 30];
+const temperaturesF: Fahrenheit[] = scaleTemperatures(temperaturesC, 1.8, 32);
+
+console.log(temperaturesF);
+
+const result: number = numbers
+  .filter(filterEven)
+  .map(doubleFn)
+  .reduce(sumFn, 0);
+
+console.log(result);
diff --git a/src/13-funktionale-programmierung/pure-functions.ts b/src/13-funktionale-programmierung/pure-functions.ts
new file mode 100644
index 0000000..043ed64
--- /dev/null
+++ b/src/13-funktionale-programmierung/pure-functions.ts
@@ -0,0 +1,48 @@
+function add(a: number, b: number): number {
+  return a + b;
+}
+
+console.log(add(2, 3));
+console.log(add(2, 3));
+console.log(add(2, 3));
+console.log(add(2, 3));
+
+let total = 0;
+
+function addToTotal(value: number): number {
+  total += value;
+  return total;
+}
+
+console.log(addToTotal(5));
+console.log(addToTotal(5));
+console.log(addToTotal(5));
+console.log(addToTotal(5));
+
+function addItem<T>(array: T[], item: T): T[] {
+  return [...array, item];
+}
+
+const numbers = [1, 2, 3];
+console.log(numbers);
+const newNumbers = numbers;
+
+numbers.push(4);
+
+// const newNumbers = addItem(numbers, 99);
+
+console.log(numbers);
+console.log(newNumbers);
+
+const person = { name: 'Alice', age: 25 };
+person.age = 26;
+console.log(person);
+
+const updatedPerson = { ...person, age: 26 };
+console.log(updatedPerson);
+
+type User = Readonly<{ name: string; age: number }>;
+const user: User = { name: 'charly', age: 21 };
+// user.age = 22;
+const updateUser = { ...user, age: 22 };
+console.log(updateUser);