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Practice JavaScript functions with 20+ hands-on programs covering arrow functions, closures, HOFs, recursion, IIFE, callbacks, and real-world coding challenges with solutions.
Overview
This module contains 20+ hands-on practice programs covering every topic in the JavaScript Functions module. Work through each problem before looking at the solution. The best way to learn is to struggle first, then study the answer.
Topics covered:
- Function Declaration & Expression
- Arrow Functions
- Callbacks
- Higher-Order Functions (map, filter, reduce)
- Closures
- IIFE
- Recursion
Program 1 – Basic Function Declaration
Task: Write a function greet that takes a name and returns "Hello, [name]! Welcome to JavaScript.".
js example
function greet(name) {
return `Hello, ${name}! Welcome to JavaScript.`;
}
console.log(greet("Riya"));
console.log(greet("Arjun"));Output
Hello, Riya! Welcome to JavaScript. Hello, Arjun! Welcome to JavaScript.
Program 2 – Arrow Function Conversion
Task: Convert this regular function to an arrow function:
js example
// Original
function square(n) {
return n * n;
}Solution:
Example
const square = n => n * n;
console.log(square(4));
console.log(square(9));
Output
16 81
Program 3 – Default Parameters
Task: Write a function calculateTax(amount, rate = 0.18) that computes tax.
js example
function calculateTax(amount, rate = 0.18) {
return amount * rate;
}
console.log(calculateTax(1000)); // uses default 18%
console.log(calculateTax(2000, 0.28)); // uses custom 28%Output
180 560
Program 4 – Rest Parameters
Task: Write a function sum that accepts any number of arguments and returns their total.
Example
function sum(...numbers) {
return numbers.reduce((total, n) => total + n, 0);
}
console.log(sum(1, 2, 3));
console.log(sum(10, 20, 30, 40, 50));
console.log(sum(7));
Output
6 150 7
Program 5 – Function Expression as Callback
Task: Use Array.forEach with a function expression to print items with their index.
Example
const fruits = ["apple", "banana", "mango", "orange"];
fruits.forEach(function (fruit, index) {
console.log(`${index + 1}. ${fruit}`);
});
Output
1. apple 2. banana 3. mango 4. orange
🟡 Intermediate Level
Program 6 – Higher-Order Functions: Map + Filter + Reduce Chain
Task: Given a list of students with names and scores, find the average score of students who scored above 60.
Example
const students = [
{ name: "Riya", score: 88 },
{ name: "Arjun", score: 45 },
{ name: "Priya", score: 72 },
{ name: "Sam", score: 35 },
{ name: "Kavya", score: 91 },
{ name: "Rohan", score: 65 }
];
const passedScores = students
.filter(s => s.score > 60)
.map(s => s.score);
const average = passedScores.reduce((sum, score) => sum + score, 0) / passedScores.length;
console.log("Passed students:", passedScores);
console.log("Average score:", average.toFixed(2));
Output
Passed students: [88, 72, 91, 65] Average score: 79.00
Program 7 – Closure: Counter Factory
Task: Build a createCounter function that returns an object with increment, decrement, and reset methods. Each counter should be independent.
Example
function createCounter(initialValue = 0) {
let count = initialValue;
return {
increment() { return ++count; },
decrement() { return --count; },
reset() { count = initialValue; return count; },
value() { return count; }
};
}
const counterA = createCounter(10);
const counterB = createCounter(0);
console.log(counterA.increment()); // 11
console.log(counterA.increment()); // 12
console.log(counterB.increment()); // 1
console.log(counterA.decrement()); // 11
console.log(counterA.reset()); // 10
console.log(counterB.value()); // 1
Output
11 12 1 11 10 1
Program 8 – Arrow Function: this Context
Task: Demonstrate the difference between regular function and arrow function this binding inside an object.
Example
const user = {
name: "Priya",
age: 25,
// Regular function – `this` = user object ✅
greetRegular: function () {
return `Hi, I'm ${this.name}, age ${this.age}`;
},
// Arrow function – `this` = outer scope (window/global) ❌
greetArrow: () => {
return `Hi, I'm ${this.name}, age ${this.age}`; // undefined
}
};
console.log(user.greetRegular());
console.log(user.greetArrow());
Output
Hi, I'm Priya, age 25 Hi, I'm undefined, age undefined
Takeaway: Use regular functions for object methods that need this.
Program 9 – Function Factory with Closures
Task: Create a makeAdder function that returns an adder function with a preset value.
js example
function makeAdder(base) {
return function (number) {
return base + number;
};
}
const add5 = makeAdder(5);
const add10 = makeAdder(10);
const add100 = makeAdder(100);
console.log(add5(3)); // 5 + 3
console.log(add10(7)); // 10 + 7
console.log(add100(42)); // 100 + 42
console.log(add5(add10(2))); // 5 + (10 + 2) = 17Output
8 17 142 17
Program 10 – Custom map and filter
Task: Implement your own myMap and myFilter functions.
Example
function myMap(arr, fn) {
const result = [];
for (const item of arr) {
result.push(fn(item));
}
return result;
}
function myFilter(arr, fn) {
const result = [];
for (const item of arr) {
if (fn(item)) result.push(item);
}
return result;
}
const nums = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
const evenSquares = myFilter(myMap(nums, n => n * n), n => n % 2 === 0);
console.log(evenSquares);
Output
[4, 16, 36, 64, 100]
Program 11 – Callback: Custom forEach
Task: Write your own forEach function that works like Array.forEach.
Example
function myForEach(arr, callback) {
for (let i = 0; i < arr.length; i++) {
callback(arr[i], i, arr);
}
}
myForEach(["a", "b", "c"], (item, index) => {
console.log(`Index ${index}: ${item}`);
});
Output
Index 0: a Index 1: b Index 2: c
Program 12 – IIFE: Configuration Object
Task: Use an IIFE to create a configuration object with private defaults.
Example
const AppConfig = (function () {
// private
const MAX_RETRIES = 3;
const BASE_URL = "https://api.myapp.com";
const VERSION = "v2";
return {
getUrl: () => `${BASE_URL}/${VERSION}`,
getRetries: () => MAX_RETRIES,
getFullPath: (endpoint) => `${BASE_URL}/${VERSION}/${endpoint}`
};
})();
console.log(AppConfig.getUrl());
console.log(AppConfig.getRetries());
console.log(AppConfig.getFullPath("users"));
console.log(AppConfig.BASE_URL); // private – undefined
Output
https://api.myapp.com/v2 3 https://api.myapp.com/v2/users undefined
🔴 Advanced Level
Program 13 – Recursion: Factorial
Task: Write a recursive factorial(n) function.
js example
function factorial(n) {
if (n < 0) throw new Error("No factorial for negative numbers");
if (n <= 1) return 1;
return n * factorial(n - 1);
}
console.log(factorial(0));
console.log(factorial(5));
console.log(factorial(10));Output
1 120 3628800
Program 14 – Recursion: Fibonacci with Memoization
Task: Write an efficient Fibonacci function using memoization.
Example
function fibonacci(n, memo = {}) {
if (n in memo) return memo[n];
if (n === 0) return 0;
if (n === 1) return 1;
memo[n] = fibonacci(n - 1, memo) + fibonacci(n - 2, memo);
return memo[n];
}
for (let i = 0; i <= 10; i++) {
process.stdout.write(fibonacci(i) + " ");
}
console.log();
console.log("fib(50):", fibonacci(50));
Output
0 1 1 2 3 5 8 13 21 34 55 fib(50): 12586269025
Program 15 – Recursion: Flatten Nested Array
Task: Write a recursive function to flatten a deeply nested array.
Example
function deepFlatten(arr) {
return arr.reduce((flat, item) => {
return Array.isArray(item)
? flat.concat(deepFlatten(item))
: flat.concat(item);
}, []);
}
const nested = [1, [2, 3], [4, [5, [6, 7]]], 8];
console.log(deepFlatten(nested));
Output
[1, 2, 3, 4, 5, 6, 7, 8]
Program 16 – Memoize Higher-Order Function
Task: Build a general memoize function that caches results of any function.
Example
function memoize(fn) {
const cache = new Map();
return function (...args) {
const key = JSON.stringify(args);
if (cache.has(key)) {
console.log(`[Cache hit] args: ${key}`);
return cache.get(key);
}
const result = fn(...args);
cache.set(key, result);
return result;
};
}
const expensiveAdd = memoize((a, b) => {
// simulate slow computation
return a + b;
});
console.log(expensiveAdd(5, 3));
console.log(expensiveAdd(5, 3)); // from cache
console.log(expensiveAdd(10, 2));
Output
8 [Cache hit] args: [5,3] 8 12
Program 17 – Currying
Task: Write a curry function that converts a 3-argument function into curried form.
Example
function curry(fn) {
return function curried(...args) {
if (args.length >= fn.length) {
return fn(...args);
}
return function (...moreArgs) {
return curried(...args, ...moreArgs);
};
};
}
const add = (a, b, c) => a + b + c;
const curriedAdd = curry(add);
console.log(curriedAdd(1)(2)(3));
console.log(curriedAdd(1, 2)(3));
console.log(curriedAdd(1)(2, 3));
console.log(curriedAdd(1, 2, 3));
Output
6 6 6 6
Program 18 – Debounce
Task: Build a debounce function using closures. Debounce ensures a function only fires after the user stops calling it for a set delay.
Example
function debounce(fn, delay) {
let timeoutId;
return function (...args) {
clearTimeout(timeoutId);
timeoutId = setTimeout(() => {
fn.apply(this, args);
}, delay);
};
}
const handleSearch = debounce(function (query) {
console.log(`Searching for: ${query}`);
}, 300);
// Simulating rapid calls (only last one fires after 300ms)
handleSearch("J");
handleSearch("Ja");
handleSearch("Jav");
handleSearch("Java");
handleSearch("JavaScript");
// After 300ms:
Output
Searching for: JavaScript
Program 19 – Function Composition (pipe)
Task: Build a pipe function that applies functions left-to-right.
Example
function pipe(...fns) {
return function (value) {
return fns.reduce((acc, fn) => fn(acc), value);
};
}
const processName = pipe(
str => str.trim(),
str => str.toLowerCase(),
str => str.split(" ").map(w => w[0].toUpperCase() + w.slice(1)).join(" "),
str => `Hello, ${str}!`
);
console.log(processName(" riya sharma "));
console.log(processName(" ARJUN VERMA "));
Output
Hello, Riya Sharma! Hello, Arjun Verma!
Program 20 – Recursive Tree Traversal
Task: Count total nodes in a nested tree structure.
Example
function countNodes(node) {
if (!node) return 0;
if (!node.children || node.children.length === 0) return 1;
let total = 1; // count current node
for (const child of node.children) {
total += countNodes(child);
}
return total;
}
const tree = {
name: "root",
children: [
{
name: "A",
children: [
{ name: "A1", children: [] },
{ name: "A2", children: [] }
]
},
{
name: "B",
children: [
{ name: "B1", children: [] }
]
},
{
name: "C",
children: []
}
]
};
console.log("Total nodes:", countNodes(tree));
Output
Total nodes: 7
🏆 Challenge Problems
Challenge 1 – Private Score Tracker
Build a score tracker using closures with add, subtract, reset, and history (returns all past scores).
Example
function createScoreTracker() {
let score = 0;
const history = [];
return {
add(points) {
score += points;
history.push({ action: `+${points}`, score });
return score;
},
subtract(points) {
score -= points;
history.push({ action: `-${points}`, score });
return score;
},
reset() {
score = 0;
history.push({ action: "reset", score });
return score;
},
getScore: () => score,
getHistory: () => [...history]
};
}
const game = createScoreTracker();
console.log(game.add(10));
console.log(game.add(5));
console.log(game.subtract(3));
console.log(game.getScore());
console.log(game.getHistory());
Output
10
15
12
12
[
{ action: '+10', score: 10 },
{ action: '+5', score: 15 },
{ action: '-3', score: 12 }
]Challenge 2 – Recursive Palindrome Check
Example
function isPalindrome(str) {
const clean = str.toLowerCase().replace(/[^a-z0-9]/g, "");
function check(s) {
if (s.length <= 1) return true;
if (s[0] !== s[s.length - 1]) return false;
return check(s.slice(1, -1));
}
return check(clean);
}
console.log(isPalindrome("racecar"));
console.log(isPalindrome("A man a plan a canal Panama"));
console.log(isPalindrome("hello"));
Output
true true false
Challenge 3 – Once Function (Runs Only Once)
js example
function once(fn) {
let called = false;
let result;
return function (...args) {
if (!called) {
called = true;
result = fn(...args);
}
return result;
};
}
const initializeApp = once(function () {
console.log("App initialized!");
return "initialized";
});
console.log(initializeApp()); // runs
console.log(initializeApp()); // returns cached result
console.log(initializeApp()); // returns cached resultOutput
App initialized! initialized initialized initialized
Key Takeaways
- ✅ Practice every topic: declaration, expression, arrow, callback, HOF, closure, IIFE, recursion
- ✅ Map/filter/reduce chains are a staple of real-world JavaScript
- ✅ Closures power counters, factories, debounce, memoize, and module patterns
- ✅ Recursion needs a base case and must shrink the problem with each call
- ✅ Higher-order patterns (curry, compose, pipe) are interview and codebase staples
- ✅ Build these programs from scratch before checking solutions
- ✅ The
once,memoize, anddebouncepatterns appear frequently in real codebases
Exam Focus
Revise definitions, diagrams, examples, and short-answer points for JavaScript Functions Practice Programs – 20+ Exercises with Solutions.
Interview Use
Prepare one clear explanation, one practical example, and one common mistake for this JavaScript Master Course topic.
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