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Level up your JavaScript skills with 10 coding challenges

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The best way to learn to code in JavaScript is to practice, practice, practice. That’s why today we will go over 10 coding challenges to level up your JavaScript skills . These exercises are useful for any JavaScript developer. If you’re an absolute beginner, it will be helpful to check out our beginner’s guide before diving in.

JavaScript challenges can be difficult enough without us trying to make them harder for ourselves. One of the ways to make them easier is to follow principles outlined in a short book by George Polya called How To Solve It . These steps are as follows:

• Understand the Problem
• Make a Plan
• Execute Plan
• Refactor - is There a Way to Make it Better?

As we challenge ourselves in the next ten problems, keep these steps in mind. Keep in mind that there are several ways to solve these problems. As long as you can explain your thought-process in good detail, your answer is probably fine!

Today we will cover:

• Longest String in Array
• Most Commonly Used Char in String

Two Strings Are Anagrams of Each Other

String permutation is palindrome, balanced brackets, armstrong numbers, sorting objects, reverse a linked list.

• Remove Kth-Node From Linked List
• Cyclic Linked List

Wrapping up

Longest string in an array.

Write a function that accepts an array of strings. Return the longest string.

What is our Input?

• String[] = an array of strings

What is our Output?

• String = the longest of the strings in the array

Questions to ask the interviewer:

Do we need to consider if there are no strings in the array?

• For this challenge, no

Do we need to consider times when all of the strings are the same length?

• Yes. Return the first string.

Longest String Plan

For this problem, we need to:

• Initialize and Declare Variable for Longest String
• Loop through the array
• Find the length of each string
• Determine if that length is the largest we have seen so far.
• Replace the longest/largest variable with the arr[index] as long as length is greater than the variable.
• Return longest/largest string.

Possible Solution

Explanation.

There are two different ways listed here as a possible solution to this problem: one uses the ES5 Syntax and uses the ES6 Syntax.

In both solutions, we initially need to declare a longest variable and initialize it to an empty string.

In the ES5 solution, we are utilizing a traditional for loop structure to look at each index in the array. We are using the array method forEach to iterate over the array in the ES6 solution.

As we iterate through the data structure, we want to take a look at the length of each string. In JavaScript, we can do that with the length property.

In the ES5 solution, we utilized a conditional statement to see if the current ( arr[i] ) string’s length is longer than (but not equal to) the current longest string( longest ). The ES6 solution takes another approach with using the Math object property max as a shortcut for the conditional logic in the ES5 version.

If the string is longer, we assign it to the longest variable we already created.

Return the longest as our function’s terminal statement in both cases.

Most Commonly Used Character in String

Write a function that takes a string, and returns the character that is most commonly used in the string.

• String = one String

What is Our Output?

• String = the single character that shows up the most in the given string

Do we need to consider if there is no string given?

Do we need to consider times when more than one character shows up the same number of times?

• No. Return the last one.

Most Used Char Plan

To find the character most often used in a string, we need to be able to count how many of each kind we have. We can do this using an object that has key:value pairs.

• Define and Initialize an object
• Map character count to that object.
• Loop through the object to find the max count.
• Assign new max count value to maxCharCount and new max key to maxChar .
• Return maxChar

Create a charCount object. This object can be named whatever you’d like as long as it’s consistent.

Use a standard for loop to iterate through the string, and check to see if it exists in the charCount object. If it does, add 1 to CharCount[key] and reassign it to the same variable. The pipes here will cover any instance where the key does not yet exist in the object and initialize the count to 1.

Loop through the object using a for...in loop. This type of loop allows us to iterate over each property in an object to look at its value. In this instance we want to find the largest number.

The wording for this problem indicates that they want us to find the last max number (in the case there is more than one character with the same amount of appearances), so we use >= to reassign the maxChar if needed.

Return our maxChar .

Create a function that takes in two strings as two parameters and returns a boolean that indicates whether or not the first string is an anagram of the second string.

• String , String = two Strings
• Boolean are either true or false . True if they are anagrams of each other, and false if they are not.

What is an anagram?

• An anagram is another word or phrase formed by rearranging letters of the first word or phrase.

Do we need to consider whitespace?

• Trim whitespace prior to comparison.

Anagrams Plan

If an anagram is just a bunch of letters jumbled around, we can see if the strings are anagrams of each other by:

Comparing the length: if they are not same length, they are not an anagram of the other

Sorting the string and using an equality operator to see if it’s equal.

There are a couple of different ways to solve this problem.

• Compare the length of the first string to the length of the second string. If they are not equal, return false .
• If the strings pass the initial check, we sort each individual string, assign it to a variable, and then return the result of setting the variables equal to each other. If they are equal, it will return true . If not, it will return false .

This particular solution considers white space by sorting it to the end of the string, so it doesn’t necessarily matter if it’s counted or not.

Given a string, write a function that will return whether or not that string is a palindrome.

• Boolean means either true or false . true if string is a palindrome; false if it is not.
• Do not trim whitespace.

Does the palindrome need to be in the dictionary?

• No, it does not.

Palindromes Plan

• Reverse the string. We need to keep the old reference to the string to use later. To do this, we need to split string into an array.
• Reverse the string.
• Join the string.
• See if new variation of string is equal to an old variation of string.

This one is deceptively simple. You just need to have knowledge of array methods and how to work with strings in conjunction with those methods. Practice how this could possibly be done without the methods so you know how these methods work under the hood.

Keep the learning going.

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The Complete Guide to Modern JavaScript

Given a string possibly containing three types of braces ( {} , [] , () ), write a function that returns a Boolean indicating whether the given string contains a valid nesting of braces.

• Boolean means either true or false . true if string has balanced brackets; false if it does not.

What do you mean by balanced brackets?

• A string is considered balanced if it has as many opening brackets of a given type as it has closing brackets of that same type. No bracket can be left unmatched. A closing bracket also cannot match a corresponding opening bracket that comes after it. Brackets also cannot overlap each other.

Will the string only have brackets in it?

• No. All chars can be used.

Balanced Brackets Plan

Because we are looking for nested balanced brackets, we are looking to use a data structure to store the open brackets we have seen so far.

As we come to a closing bracket, we need to look to see if the last bracket added to the data structure matches the opposite of the current bracket.

If your data structure is empty by the time we get to the end of the string, we have balanced brackets.

The process for this particular problem is more about checking for failures. We use a data structure called a stack that takes a first in, last out approach to do that.

When we come across an open bracket, we push it on to the stack. When we come across a closed one, we check to see if the last index of the stack is the opposite the current character.

If the stack is empty by the time we get to the end of our logic, we have balanced brackets.

An Armstrong number is an n-digit number that is equal to the sum of the n t h nth n t h powers of its digits. Determine if the input number is an Armstrong number. Return either true or false.

• Number = n-digit number
• Boolean means either true or false . true if number is Armstrong Number; false if it is not.

May I have an example please?

• 1 3 + 5 3 + 3 3 = 153 1^3 + 5^3 + 3^3 = 153 1 3 + 5 3 + 3 3 = 153

Armstrong Number Plan

Remember that in JavaScript type coercion exists. When working with Numbers is JS, remember to check the typeof the number. If it is a string, you’ll need to plan for that. This is really important when working with math in JS.

To solve this problem, we change the number to a string, figure out the power by getting the length of the number, and then use a for loop to add up the individual numbers ^ length of the number passed in.

We then check to see if the sum is equal to the original number. If it is, it is an Armstrong number.

Given an array of objects, sort the objects by population size. Return the entire object.

• An array of objects
• An array of objects, sorted by population

SortObjects Plan

JavaScript has a built-in array method called sort() that we can use for this particular exercise.

What’s interesting about this particular method is that in JavaScript, if you don’t use a callback compare function to sort the array of objects, the default sort method is used to sort by characters rather than number. For example:

We need to use a compare function as a callback passed into the sort method to make sure that the actual numbers are treated properly.

To sort an array of objects by a certain property, we have to use the compare function as a parameter in the built-in sort method in JavaScript.

The compare function takes in two parameters: here they are named a and b . They represent two objects next to each other in our array. If a and b are both objects, we can use dot notation to access the property population .

If we want to be sure that the array has objects sorted by population in increasing order, we evaluate a.population - b.population . If it’s -1, it will put the list in decreasing order by population. The opposite would put it in increasing order. If the expression happens to evaluate to 0, it means that the two patterns are equal to the other.

Create a Node class and a LinkedList class with these methods:

• insertFirst(data)
• insertLast(data)
• Nothing. We are building two classes from scratch that have methods.
• This is a class that will create instances of a LinkedList. We will use it to test the reverse a linked list functionality.

Questions to Possibly Ask Interviewer

Are we to assume this is a singly-linked list that actually exists?

• No. Handle all possible errors if you can.

Reverse a Linked List Plan

The first thing we need to investigate is how a linked list works. Once we have that figured out, it might be a lot easier to write out the code for it.

Linked lists are different from arrays in that they don’t require a continuous place of residence in memory. Linked Lists can be broken up into single nodes if needed as long as the linked list’s next value is not null.

Let’s plan to initialize three variables: prev as NULL, curr as head, and next as NULL. Iterate trough the Linked List. In a while loop, do the following.

• Store next node
• next = curr.next
• Change next of current (this is where the reversing happens)
• curr.next = prev

We start with the Node class. It will have a data property and a next property. The next property acts as a pointer that will direct us to the next node in our linked list if it exists, or null.

For the Linked List class, we check for failures in our methods before we handle the success. With Linked Lists, we will check to see if the actual list exists by checking to see if the head exists and perform the appropriate logic for each method.

When we reverse a linked list, juggling variables around is the rule of the day. We have one variable, node, that is declared and then initialized to head. We declare tmp and prev without initializing them.

While the node exists, we redirect the pointers by reassigning variables. The result is a reversed linked list.

Remove Kth-Node from Linked List

Using the Node and LinkedList classes that you created in the previous question, add methods to it so that, when given an kth-integer, you can delete that k t h kth k t h node from the linked list

• We will add to the Linked List and Node classes that were written in the previous question.
• This is a class that will create instances of a LinkedList. We will use it to test delete a k t h kth k t h -node functionality.

Is this a zero-based indexing?

LinkedList Delete K-th Node Plan

When given K we have to be able to delete that node in a linked list. To handle edge cases, we need to have some knowledge of the size of the list, the node prior to the Kth node and the node after the Kth node. We essentially need to redirect the pointers for those nodes to not include the one we want to be deleted.

To do that, we’ll loop through the list, keep track of which node we’re on, and change the pointers to skip the Kth-node. It’ll still be in memory, ready to be written over, but it will be essentially deleted from our list.

Circular Linked List

Given a linked list, return true if the list is circular, false if it is not.

• A new LinkedList()

A Boolean that tells us if the linked list is circular or not.

What does it mean for a linked list to be circular?

• It means that a next node value could point to a node that we have already seen in our linked list.

We will have two pointers: one slow and one fast. The fast one will move twice as much as the slow one on each iteration. If it comes to be that the fast.next.next value doesn’t exist, we can assume it’s a linear linked list. If the slow pointer and the fast pointer ultimately end up equal, we know we have a circular component to our list.

Here we declare and initialize two pointers, one slow and one fast, that are both pointed at the head. While the next two nodes exist, we’ll reassign the pointers: the slow will move to the next node and the fast will move two nodes.

If the slow ever is the same value as the fast, we have a circular linked list. If ever one of the two nodes past the current node doesn’t exist, we’ll break the loop and return false.

Congrats! You’ve gone through 10 JavaScript coding challenges! One of the best things to do when encountering a problem is to use Georges Polya’s practices. Once we understand the problem, we make a plan and try to reiterate an unsolved problem. This way, you are more likely to break down the problem into smaller manageable components.

Solving these problems takes practice. You’ll develop an intuition for algorithms the more time you put into practicing. So don’t worry if it doesn’t come to you right away. Keep practicing!

Educative’s course The Complete Guide to Modern JavaScript walks you through the basics of the language and all the new features introduced up to 2020. After following this course, functions and variables let , const , generators, promises and async won’t be a problem anymore.

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15 Best Platforms To Practice JavaScript self.__wrap_b=(t,n,e)=>{e=e||document.querySelector(`[data-br="${t}"]`);let a=e.parentElement,r=R=>e.style.maxWidth=R+"px";e.style.maxWidth="";let o=a.clientWidth,c=a.clientHeight,i=o/2-.25,l=o+.5,u;if(o){for(;i+1 {self.__wrap_b(0,+e.dataset.brr,e)})).observe(a):process.env.NODE_ENV==="development"&&console.warn("The browser you are using does not support the ResizeObserver API. Please consider add polyfill for this API to avoid potential layout shifts or upgrade your browser. Read more: https://github.com/shuding/react-wrap-balancer#browser-support-information"))};self.__wrap_b(":R4mr36:",1)

HackerEarth

Freecodecamp, geeksforgeeks, educative.io, codeacademy, javascript questions – by lydia hallie, javascript coding practice – by learning zone, 123 essential javascript questions – by yuri katkov, javascript interview question – by sudheer j.

Learning any programming language is not enough, and we need to practice.

If you are learning JavaScript, I have curated the ten best platforms to practice JavaScript for free.

Codedamn is an interactive programming platform; you will learn and build projects side by side. It offers over 500+ coding problems for you to practice.

You can solve 3-5 questions daily without waiting another day to solve more questions.

Codedamn has community support, if you get stuck on a particular problem, you can ask for help, and developers will be ready to help you anytime.

The problem’s difficulty level ranges from easy to hard. But you can go at your pace.

Here is the link to solve 500+ JavaScript problems .

If you think these problems are too advanced, you can learn about JavaScript basics, which codedamn offers for free.

Link to learn JavaScript: JavaScript basics

On codewars, you will get all types of questions ranging from easy to complex.

You will get instant feedback on your answers. Your rank will level up as you progress.

If you get stuck, codewars have a senior developers community that will help you.

Link to practice on codewars

codin games is a gamified learning platform. On codin games, you will code and execute the correct code to save yourself from obstacles and enemies.

You can compete individually or by teaming up with other fellow developers.

You can compare your and other developer code to improve your JavaScript coding skills.

You can play and build your developer profile, and You can even apply for jobs. You need to solve complex problems and be consistent on the platform to stand out from others.

Link to practice on the codin game.

If you don’t know Solo Learn, you are not a developer. Earlier, Solo learn was only available as a mobile application, and now you can also learn on the web.

Solo learn offers a JavaScript course and exercise to improve your JavaScript coding skills.

At the end of each topic, you will get practice problems you need to solve to unlock the next section.

Link to practice on Solo learn .

Edabit has interactive exercises ranging from easy to complex, and you can filter the challenges using tags. You can practice your JavaScript coding skills for free.

If you get stuck on any question, you can unlock the solution, but you will lose the XPs you have gained.

Alternatively, the resources tab contains the concepts to learn to solve that problem.

Link to practice JavaScript at edabit.

Hacker earth has a set of questions based on if you are preparing for an interview or you are a beginner at programming ,

It has a codemonk section where you will get questions based on Data Structures and algorithms. Best suited if you know DSA and algorithms, or you can practice easy and medium-difficulty problems.

On hacker earth, you can participate in challenges, try to build your developer profile, and stand out from other developers. And apply for jobs at companies.

Link to practice at HackerEarth

On programiz, you can practice JavaScript for free, and You will get the list of concepts to learn and practice problems at the end of each concept to retain what you have learned.

There are separate JavaScript examples sections available there, and you will find the JavaScript-related question and some general programming questions based on the topics you have learned.

Link to practice JavaScript examples at programiz.

Freecodecamp is popular among developers because of its ocean of totally free courses.

Freecodecamp offers 100s of exercises related to JavaScript, ranging from basic concepts, regular expressions, array methods, objects, data structure, and algorithms.

These exercises are for beginners to advanced javascript developers.

Link to practice on freecodecamp

To practice on GeeksforGeeks, you need to create an account once you create an account with them, head over to the below link to practice JavaScript.

This platform best suits you if you have Data Structure and algorithms knowledge.

You can filter out the problems based on the companies that have asked about that problem or on the concepts like Arrays, Objects, and Strings.

You can even choose the problem difficulty level from school to pro level.

Link to practice on GeeksforGeeks

The Educative has an assessment test. You can take the JavaScript assessment test, which takes around 20 minutes to complete.

The assessment test consists of all the topics, from fundamental concepts like variables, data types, and conditional statements to advanced concepts like arrays and objects.

Link to the assessment test

On Codecademy, you will find a javascript project-based course that is very helpful if you want to learn and build projects at the same time using javascript.

Link to javascript courses

I have curated four GitHub repositories that are helpful. You will gain problem-solving skills, and some repositories will help you prepare for the interview.

This repo has more than 500+ questions with answers, and this repo is available in 18 other languages. In this repo, you will find basic to very advanced questions, and some are very difficult.

Link to repo

This repo has more than 100+ javascript exercises with the answers. This repo will give you insights into How the recruiter will ask questions in a javascript-related interview.

You will find 123 essential javascript interview-related questions, which you can practice on your code editor or online to crack practical interviews. This repo has 4K stars, and you will find some exciting questions that might help you build problem-solving skills.

You will find 1000+ questions ? related to javascript on this repo, which you can practice. Questions range from the difficulty level of beginners to advanced.

I hope you found this article helpful, You can choose any platform and practice it to improve your developer profile, gain problem-solving skills and move to the next platform.

Thanks for reading.

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JavaScript Online Compiler

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Our JavaScript online Compiler is the perfect solution for beginner developers looking to streamline their workflow and improve productivity. With our cutting-edge online editor, you can write, compile, and view the live results of your code all in one place. Our JavaScript Compiler is designed to make your life easier, whether you're a seasoned developer or just starting out.

The JavaScript Online Compiler offers an intuitive user interface, making it easy for beginners to learn and practice their coding skills. With ready-to-use templates, you can quickly kick-start your projects and learn by example. These templates cover a wide range of topics and are tailored to help you grasp essential JavaScript concepts effortlessly.

One of the key features of our JavaScript Online Compiler is the live view coding result. As you write your code, you can instantly see the outcome, making debugging and testing a breeze. This real-time feedback allows you to iterate faster and catch errors before they become a headache.

In addition to the live view, our JavaScript Compiler offers a rich library of resources to help you level up your coding skills. With comprehensive documentation, video tutorials, and a thriving community of developers, you'll never feel lost or unsupported.

Don't waste any more time juggling between different tools and platforms. Give our JavaScript Compiler a try and experience the convenience of coding, compiling, and viewing your JavaScript projects all in one place. Join thousands of satisfied developers who have already discovered the power and efficiency of our JavaScript Online Compiler. Get started today and unlock your full potential as a JavaScript developer!

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Collaborate with your team in real-time. Make coding interviews, teach students or your coworker.

NPM Packages

Play with any of the 2 million NPM packages. React, Vue, Lodash, RxJs, p5.js, three.js, etc.

The Fastest Live View

See results as you type, instantly. Speed-up learning and code debugging. PlayCode is the fastest coding experience ever.

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PlayCode will find and show you all the errors and help you fix them. It makes the process of "write, run, fix errors, again" instant.

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Find errors instantly, even when you have 1000 lines of code. The minimap will show all errors and warnings.

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Javascript Compiler

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Javascript Online

Edit and run JavaScript online right in your browser and see the results instantly. PlayCode's JavaScript online editor offers a seamless experience for quick prototyping and testing.

HTML, CSS, JS Integration

Not just limited to JavaScript, PlayCode also supports HTML and CSS, allowing you to build complete web projects.

Create multiple JS files, use preprocessors, save your code, format, and customize the editor to fit your needs.

Build apps, demos, or amazing projects with the JavaScript compiler. Share your JS code by simply copying the browser URL.

Use console to debug javascript code in playcode.

In addition to supporting core JavaScript, PlayCode JavasScript Compiler also provides access to popular libraries and frameworks such as jQuery, Angular, and Backbone.js. This allows you to expand your skillset and explore a variety of tools and technologies commonly used in web development.

The javascript compiler supports creating whole apps with HTML, CSS, version control, and app hosting using Vanilla Names. It also includes a JavaScript console for debugging and the ability to use JavaScript and CSS npm packages such as Vue.js, React, Lodash, RxJS, Solid-js, and many more.

With PlayCode, you can join a vibrant community of JavaScript enthusiasts, developers, and learners. Share your projects, seek help, and learn from others' experiences, making your journey into JavaScript a collaborative and engaging experience.

• JavaScript Coding Challenges for Beginners

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Mastering these coding challenges may not get you a job at Google... but you'll be one step closer to building your own JavaScript game at codeguppy.com

YouTube Channel

After you read this article, please remember to check out also the “Coding Adventures” YouTube channel. You’ll find there a series of fun and engaging coding lessons and cool projects.

Channel main page Coding lessons playlist Coding projects playlist

The coding challenges in this article are intended for code newbies, therefore the solutions are implemented using only simple / classical programming elements. Each solution is acompanied by an online link that helps you quickly run it in a code playground.

If you prefer to open all solutions in a single, unified coding playgroud please open the 50 Coding Challenges project.

The code is making use of the codeguppy specific function println() to print the results. If you want to run these solutions outside CodeGuppy, just replace println() with console.log() then run them using your browser console tool or node.js.

Coding challenge #1: Print numbers from 1 to 10

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Coding challenge #2: Print the odd numbers less than 100

Coding challenge #3: print the multiplication table with 7, coding challenge #4: print all the multiplication tables with numbers from 1 to 10, coding challenge #5: calculate the sum of numbers from 1 to 10, coding challenge #6: calculate 10, coding challenge #7: calculate the sum of odd numbers greater than 10 and less than 30, coding challenge #8: create a function that will convert from celsius to fahrenheit, coding challenge #9: create a function that will convert from fahrenheit to celsius, coding challenge #10: calculate the sum of numbers in an array of numbers, coding challenge #11: calculate the average of the numbers in an array of numbers, coding challenge #12: create a function that receives an array of numbers and returns an array containing only the positive numbers, coding challenge #13: find the maximum number in an array of numbers, coding challenge #14: print the first 10 fibonacci numbers without recursion, coding challenge #15: create a function that will find the nth fibonacci number using recursion, coding challenge #16: create a function that will return a boolean specifying if a number is prime, coding challenge #17: calculate the sum of digits of a positive integer number, coding challenge #18: print the first 100 prime numbers, coding challenge #19: create a function that will return in an array the first "nprimes" prime numbers greater than a particular number "startat", coding challenge #20: rotate an array to the left 1 position, coding challenge #21: rotate an array to the right 1 position, coding challenge #22: reverse an array, coding challenge #23: reverse a string, coding challenge #24: create a function that will merge two arrays and return the result as a new array, coding challenge #25: create a function that will receive two arrays of numbers as arguments and return an array composed of all the numbers that are either in the first array or second array but not in both, coding challenge #26: create a function that will receive two arrays and will return an array with elements that are in the first array but not in the second, coding challenge #27: create a function that will receive an array of numbers as argument and will return a new array with distinct elements, coding challenge #28: calculate the sum of first 100 prime numbers, coding challenge #29: print the distance between the first 100 prime numbers, coding challenge #30-a: create a function that will add two positive numbers of indefinite size. the numbers are received as strings and the result should be also provided as string., coding challenge #30-b: create a function that will add two positive numbers of indefinite size. the numbers are received as strings and the result should be also provided as string., coding challenge #31a. create a function that will return the number of words in a text, coding challenge #31b. create a function that will return the number of words in a text, coding challenge #32. create a function that will capitalize the first letter of each word in a text, coding challenge #33. calculate the sum of numbers received in a comma delimited string, coding challenge #34. create a function that will return an array with words inside a text, coding challenge #35. create a function to convert a csv text to a “bi-dimensional” array, coding challenge #36. create a function that converts a string to an array of characters, coding challenge #37. create a function that will convert a string in an array containing the ascii codes of each character, coding challenge #38. create a function that will convert an array containing ascii codes in a string, coding challenge #39. implement the caesar cypher, coding challenge #40. implement the bubble sort algorithm for an array of numbers, coding challenge #41. create a function to calculate the distance between two points defined by their x, y coordinates, coding challenge #42. create a function that will return a boolean value indicating if two circles defined by center coordinates and radius are intersecting, coding challenge 43. create a function that will receive a bi-dimensional array as argument and a number and will extract as a unidimensional array the column specified by the number, coding challenge #44. create a function that will convert a string containing a binary number into a number, coding challenge #45. create a function to calculate the sum of all the numbers in a jagged array (array contains numbers or other arrays of numbers on an unlimited number of levels), coding challenge #46-a. find the maximum number in a jagged array of numbers or array of numbers, coding challenge #46-b. find the maximum number in a jagged array of numbers or array of numbers, coding challenge #47. deep copy a jagged array with numbers or other arrays in a new array, coding challenge #48. create a function to return the longest word(s) in a string, coding challenge #49. shuffle an array of strings, coding challenge #50. create a function that will receive n as argument and return an array of n unique random numbers from 1 to n., coding challenge #51. find the frequency of characters inside a string. return the result as an array of objects. each object has 2 fields: character and number of occurrences., coding challenge #52. calculate fibonacci(500) with high precision (all digits), coding challenge #53. calculate 70 with high precision (all digits).

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JavaScript Online Compiler

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Solve common problems in your JavaScript code

The following links point to solutions to common problems you may encounter when writing JavaScript.

Common beginner's mistakes

Correct spelling and casing.

If your code doesn't work and/or the browser complains that something is undefined, check that you've spelt all your variable names, function names, etc. correctly.

Some common built-in browser functions that cause problems are:

Semicolon position

You need to make sure you don't place any semicolons incorrectly. For example:

There are a number of things that can go wrong with functions.

One of the most common errors is to declare the function, but not call it anywhere. For example:

This code won't do anything unless you call it with the following statement:

Function scope

Remember that functions have their own scope — you can't access a variable value set inside a function from outside the function, unless you declared the variable globally (i.e. not inside any functions), or return the value from the function.

Running code after a return statement

Remember also that when you return from a function, the JavaScript interpreter exits the function — no code after the return statement will run.

In fact, some browsers (like Firefox) will give you an error message in the developer console if you have code after a return statement. Firefox gives you "unreachable code after return statement".

Object notation versus normal assignment

When you assign something normally in JavaScript, you use a single equals sign, e.g.:

With Objects , however, you need to take care to use the correct syntax. The object must be surrounded by curly braces, member names must be separated from their values using colons, and members must be separated by commas. For example:

Basic definitions

• What is JavaScript?
• What is a variable?
• What are strings?
• What is an array?
• What is a loop?
• What is a function?
• What is an event?
• What is an object?
• What is JSON?
• What is a web API?
• What is the DOM?

Basic use cases

• How do you add JavaScript to your page?
• How do you add comments to JavaScript code?
• How do you declare a variable?
• How do you initialize a variable with a value?
• How do you update a variable's value? (also see Assignment operators )
• What data types can values have in JavaScript?
• What does 'loosely typed' mean?
• What types of number do you have to deal with in web development?
• How do you do basic math in JavaScript?
• What is operator precedence, and how is it handled in JavaScript?
• How do you increment and decrement values in JavaScript?
• How do you compare values in JavaScript? (e.g. to see which one is bigger, or to see if one value is equal to another).
• How do you create a string in JavaScript?
• Do you have to use single quotes or double quotes?
• How do you escape characters in strings?
• How do you join strings together?
• Can you join strings and numbers together?
• How do you find the length of a string?
• How do you find what character is at a certain position in a string?
• How do you find and extract a specific substring from a string?
• How do you change the case of a string?
• How do you replace one specific substring with another?
• How do you create an array?
• How do you access and modify the items in an array? (this includes multidimensional arrays)
• How do you find the length of an array?
• How do you add items to an array?
• How do you remove items from an array?
• How do you split a string into array items, or join array items into a string?

Debugging JavaScript

• What are the basic types of error?
• What are browser developer tools, and how do you access them?
• How do you log a value to the JavaScript console?
• How do you use breakpoints and other JavaScript debugging features?

For more information on JavaScript debugging, see Handling common JavaScript problems . Also, see Other common errors for a description of common errors.

Making decisions in code

• How do you execute different blocks of code, depending on a variable's value or other condition?
• How do you use if ...else statements?
• How do you nest one decision block inside another?
• How do you use AND, OR, and NOT operators in JavaScript?
• How do you conveniently handle a large number of choices for one condition?
• How do you use a ternary operator to make a quick choice between two options based on a true or false test?

Looping/iteration

• How do you run the same bit of code over and over again?
• How do you exit a loop before the end if a certain condition is met?
• How do you skip to the next iteration of a loop if a certain condition is met?
• How do you use while and do...while loops?

Intermediate use cases

• How do you find functions in the browser?
• What is the difference between a function and a method?
• How do you create your own functions?
• How do you run (call, or invoke) a function?
• What is an anonymous function?
• How do you specify parameters (or arguments) when invoking a function?
• What is function scope?
• What are return values, and how do you use them?
• How do you create an object?
• What is dot notation?
• What is bracket notation?
• How do you get and set the methods and properties of an object?
• What is this , in the context of an object?
• What is object-oriented programming?
• What are constructors and instances, and how do you create them?
• What different ways are there to create objects in JavaScript?
• How do you structure JSON data, and read it from JavaScript?
• How can you load a JSON file into a page?
• How do you convert a JSON object to a text string, and back again?
• What are event handlers and how do you use them?
• What are inline event handlers?
• What does the addEventListener() function do, and how do you use it?
• Which mechanism should I use to add event code to my web pages?
• What are event objects, and how do you use them?
• How do you prevent default event behavior?
• How do events fire on nested elements? (event propagation, also related — event bubbling and capturing)
• What is event delegation, and how does it work?

Object-oriented JavaScript

• What are object prototypes?
• What is the constructor property, and how can you use it?
• How do you add methods to the constructor?
• How do you create a new constructor that inherits its members from a parent constructor?
• When should you use inheritance in JavaScript?
• How do you manipulate the DOM (e.g. adding or removing elements) using JavaScript?

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