JavaScript Exercises: 29 Practice Problems with Solutions

The exercises

Showing 29 of 29 exercises.

1. BeginnerBasics & output

   1. Greet a name

   Store the name "Ada" in a variable and log "Hello, Ada!".

   Hint: Template literals use backticks and ${}.

   const name = ;
   // log the greeting

   Show solution

   const name = "Ada";
   console.log(`Hello, ${name}!`);

2. BeginnerBasics & output

   2. Swap two variables

   Given a = 3 and b = 8, swap them and log both values.

   Hint: Array destructuring: [a, b] = [b, a].

   let a = 3, b = 8;

   Show solution

   let a = 3, b = 8;
   [a, b] = [b, a];
   console.log(a, b);

3. BeginnerBasics & output

   3. Inspect types

   Log the typeof 5, "5", true, null and undefined.

   Hint: typeof null is famously "object".

   Show solution

   console.log(typeof 5, typeof "5", typeof true, typeof null, typeof undefined);

4. BeginnerNumbers & math

   4. Celsius to Fahrenheit

   Convert 37 degrees Celsius to Fahrenheit and log the result.

   Hint: F = C * 9 / 5 + 32.

   const c = 37;

   Show solution

   const c = 37;
   console.log(c * 9 / 5 + 32);

5. BeginnerConditionals

   5. Even or odd

   Log "even" or "odd" for the number 17.

   Hint: Use the remainder operator %.

   const n = 17;

   Show solution

   const n = 17;
   console.log(n % 2 === 0 ? "even" : "odd");

6. BeginnerConditionals

   6. Largest of three

   Log the largest of 12, 45 and 31 without using Math.max.

   Hint: Compare pairs with if/else.

   const a = 12, b = 45, c = 31;

   Show solution

   const a = 12, b = 45, c = 31;
   let max = a;
   if (b > max) max = b;
   if (c > max) max = c;
   console.log(max);

7. BeginnerLoops

   7. FizzBuzz to 20

   Log 1..20, replacing multiples of 3 with "Fizz", of 5 with "Buzz" and both with "FizzBuzz".

   Hint: Check the 15 case first.

   for (let i = 1; i <= 20; i++) {
   }

   Show solution

   for (let i = 1; i <= 20; i++) {
     if (i % 15 === 0) console.log("FizzBuzz");
     else if (i % 3 === 0) console.log("Fizz");
     else if (i % 5 === 0) console.log("Buzz");
     else console.log(i);
   }

8. BeginnerLoops

   8. Sum 1 to 100

   Use a loop to add every number from 1 to 100 and log the total.

   Hint: Keep a running total outside the loop.

   let total = 0;

   Show solution

   let total = 0;
   for (let i = 1; i <= 100; i++) total += i;
   console.log(total);

9. BeginnerLoops

   9. Multiplication table

   Log the 7 times table from 7x1 to 7x10 as "7 x 3 = 21".

   Hint: One loop, template literal inside.

   Show solution

   for (let i = 1; i <= 10; i++) console.log(`7 x ${i} = ${7 * i}`);

10. BeginnerStrings

    10. Reverse a string

    Reverse "javascript" and log it.

    Hint: split("") then reverse() then join("").

    const s = "javascript";

    Show solution

    const s = "javascript";
    console.log(s.split("").reverse().join(""));

11. BeginnerStrings

    11. Count vowels

    Count the vowels in "Programming is fun" and log the count.

    Hint: Loop the characters and test membership in "aeiou".

    const text = "Programming is fun";

    Show solution

    const text = "Programming is fun";
    let n = 0;
    for (const ch of text.toLowerCase()) if ("aeiou".includes(ch)) n++;
    console.log(n);

12. IntermediateStrings

    12. Palindrome check

    Write isPalindrome(s) that ignores case and spaces. Test it with "Never odd or even".

    Hint: Normalise first with replace(/\s/g, "").toLowerCase().

    function isPalindrome(s) {
    }

    Show solution

    function isPalindrome(s) {
      const t = s.toLowerCase().replace(/[^a-z0-9]/g, "");
      return t === t.split("").reverse().join("");
    }
    console.log(isPalindrome("Never odd or even"));

13. IntermediateStrings

    13. Title-case a sentence

    Turn "the quick brown fox" into "The Quick Brown Fox".

    Hint: Split on spaces, capitalise each word, join back.

    const s = "the quick brown fox";

    Show solution

    const s = "the quick brown fox";
    console.log(s.split(" ").map(w => w[0].toUpperCase() + w.slice(1)).join(" "));

14. BeginnerArrays

    14. Array statistics

    For [4, 9, 1, 7, 3] log the sum, the average, the smallest and the largest.

    Hint: reduce for the sum, Math.min/Math.max with spread.

    const nums = [4, 9, 1, 7, 3];

    Show solution

    const nums = [4, 9, 1, 7, 3];
    const sum = nums.reduce((a, b) => a + b, 0);
    console.log(sum, sum / nums.length, Math.min(...nums), Math.max(...nums));

15. IntermediateArrays

    15. Filter then map

    From 1..20 keep the even numbers and log their squares.

    Hint: Array.from({length: 20}, (_, i) => i + 1).

    Show solution

    const nums = Array.from({ length: 20 }, (_, i) => i + 1);
    console.log(nums.filter(n => n % 2 === 0).map(n => n * n));

16. IntermediateArrays

    16. Remove duplicates

    Remove duplicates from [1,2,2,3,4,4,4,5] keeping order.

    Hint: A Set preserves insertion order.

    const nums = [1, 2, 2, 3, 4, 4, 4, 5];

    Show solution

    const nums = [1, 2, 2, 3, 4, 4, 4, 5];
    console.log([...new Set(nums)]);

17. IntermediateObjects

    17. Group words by length

    Group ["hi","cat","dog","tree","a"] into an object keyed by word length.

    Hint: reduce into an object, creating the array when missing.

    const words = ["hi", "cat", "dog", "tree", "a"];

    Show solution

    const words = ["hi", "cat", "dog", "tree", "a"];
    const byLen = words.reduce((acc, w) => {
      (acc[w.length] ||= []).push(w);
      return acc;
    }, {});
    console.log(byLen);

18. IntermediateObjects

    18. Word frequency

    Count how often each word appears in "to be or not to be".

    Hint: Use a Map or plain object as a counter.

    const text = "to be or not to be";

    Show solution

    const text = "to be or not to be";
    const counts = {};
    for (const w of text.split(" ")) counts[w] = (counts[w] || 0) + 1;
    console.log(counts);

19. IntermediateObjects

    19. Sort objects by field

    Sort [{n:'A',score:70},{n:'B',score:91},{n:'C',score:55}] by score descending and log the names.

    Hint: sort((a, b) => b.score - a.score).

    const rows = [{ n: 'A', score: 70 }, { n: 'B', score: 91 }, { n: 'C', score: 55 }];

    Show solution

    const rows = [{ n: 'A', score: 70 }, { n: 'B', score: 91 }, { n: 'C', score: 55 }];
    console.log([...rows].sort((a, b) => b.score - a.score).map(r => r.n));

20. IntermediateFunctions

    20. Default and rest parameters

    Write sum(...nums) returning the total, and greet(name, greeting = "Hello").

    Hint: Rest parameters collect arguments into an array.

    Show solution

    const sum = (...nums) => nums.reduce((a, b) => a + b, 0);
    const greet = (name, greeting = "Hello") => `${greeting}, ${name}!`;
    console.log(sum(1, 2, 3), greet("Ada"));

21. AdvancedFunctions

    21. Closure counter

    Write makeCounter() returning a function that increments a private count each call.

    Hint: The inner function closes over a variable in the outer scope.

    function makeCounter() {
    }

    Show solution

    function makeCounter() {
      let count = 0;
      return () => ++count;
    }
    const next = makeCounter();
    console.log(next(), next(), next());

22. AdvancedFunctions

    22. Memoise a slow function

    Write memoize(fn) that caches results by argument, then memoise a recursive fibonacci.

    Hint: Keep a Map from argument to result.

    function memoize(fn) {
    }

    Show solution

    function memoize(fn) {
      const cache = new Map();
      return (n) => {
        if (cache.has(n)) return cache.get(n);
        const v = fn(n);
        cache.set(n, v);
        return v;
      };
    }
    const fib = memoize((n) => (n < 2 ? n : fib(n - 1) + fib(n - 2)));
    console.log(fib(40));

23. IntermediateClasses

    23. A small class

    Write a BankAccount class with deposit, withdraw (refusing overdrafts) and a balance getter.

    Hint: Use #balance for a private field.

    class BankAccount {
    }

    Show solution

    class BankAccount {
      #balance = 0;
      deposit(n) { this.#balance += n; return this; }
      withdraw(n) {
        if (n > this.#balance) throw new Error("Insufficient funds");
        this.#balance -= n;
        return this;
      }
      get balance() { return this.#balance; }
    }
    const acc = new BankAccount();
    acc.deposit(100).withdraw(30);
    console.log(acc.balance);

24. IntermediateErrors

    24. Handle a thrown error

    Parse the invalid JSON string "{oops}" inside try/catch and log a friendly message.

    Hint: JSON.parse throws a SyntaxError.

    const raw = "{oops}";

    Show solution

    const raw = "{oops}";
    try {
      console.log(JSON.parse(raw));
    } catch (err) {
      console.log("Could not read that data:", err.message);
    }

25. AdvancedAsync

    25. Await in sequence

    Write delay(ms) returning a Promise, then await it three times logging a step each time.

    Hint: new Promise(res => setTimeout(res, ms)).

    Show solution

    const delay = (ms) => new Promise((res) => setTimeout(res, ms));
    (async () => {
      for (let i = 1; i <= 3; i++) {
        await delay(100);
        console.log("step", i);
      }
    })();

26. AdvancedAsync

    26. Run promises in parallel

    Create three promises resolving to 1, 2 and 3 and log their sum using Promise.all.

    Hint: Promise.all takes an array and resolves to an array.

    Show solution

    const ps = [1, 2, 3].map((n) => Promise.resolve(n));
    Promise.all(ps).then((vals) => console.log(vals.reduce((a, b) => a + b, 0)));

27. AdvancedAlgorithms

    27. Binary search

    Implement binary search over a sorted array and return the index or -1.

    Hint: Track low and high, compare the middle.

    function search(arr, target) {
    }

    Show solution

    function search(arr, target) {
      let lo = 0, hi = arr.length - 1;
      while (lo <= hi) {
        const mid = (lo + hi) >> 1;
        if (arr[mid] === target) return mid;
        if (arr[mid] < target) lo = mid + 1;
        else hi = mid - 1;
      }
      return -1;
    }
    console.log(search([1, 3, 5, 7, 9, 11], 9));

28. AdvancedAlgorithms

    28. Flatten a nested array

    Flatten [1,[2,[3,[4]]],5] completely without using Array.prototype.flat.

    Hint: Recurse when an item is itself an array.

    function flatten(arr) {
    }

    Show solution

    function flatten(arr) {
      return arr.reduce((out, item) => out.concat(Array.isArray(item) ? flatten(item) : item), []);
    }
    console.log(flatten([1, [2, [3, [4]]], 5]));

29. AdvancedAlgorithms

    29. Debounce a function

    Write debounce(fn, ms) that only runs fn after calls stop for ms milliseconds.

    Hint: clearTimeout the previous timer on each call.

    function debounce(fn, ms) {
    }

    Show solution

    function debounce(fn, ms) {
      let t;
      return (...args) => {
        clearTimeout(t);
        t = setTimeout(() => fn(...args), ms);
      };
    }
    const log = debounce((v) => console.log("ran with", v), 50);
    log(1); log(2); log(3);