{"id":213,"date":"2023-03-29T15:50:04","date_gmt":"2023-03-29T15:50:04","guid":{"rendered":"https:\/\/codeinterview.io\/blog\/?p=213"},"modified":"2024-09-02T10:28:21","modified_gmt":"2024-09-02T10:28:21","slug":"javascript-coding-interview-questions","status":"publish","type":"post","link":"https:\/\/codeinterview.io\/blog\/javascript-coding-interview-questions\/","title":{"rendered":"30 JavaScript Coding Interview Questions for Developers"},"content":{"rendered":"\n

JavaScript is a leading programming language for creating interactive and dynamic web pages. You will see a lot of popular frameworks and libraries based on JavaScript, such as VueJS framework<\/a>, jQuery, ReactJS, AngularJS and others. It is no surprise that JavaScript is the main programming language for the front-end development of software applications. Today we will guide you on how to design challenges for a JavaScript coding interview. <\/p>\n\n\n\n

We will suggest coding challenges for beginner, mid-level and expert JavaScript developers. Let’s start with coding challenges that are suitable for beginner-level developers.<\/p>\n\n\n\n

\"Developer<\/a><\/figure>\n\n\n\n

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Interview Questions for Beginner JavaScript Developers<\/strong><\/h2>\n\n\n\n

When preparing JavaScript coding interview questions for beginner-level JavaScript developers, focus on basic JavaScript IDE<\/a>, syntax, array and string manipulation, objects and JSON, DOM manipulation, error handling and asynchronous programming. Here are 10 sample questions in this regard:<\/p>\n\n\n\n

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Question 1<\/strong><\/h3>\n\n\n\n

What will be the output of the below code:<\/p>\n\n\n\n

const array = [10, 20, 30, 40];\nconst result = array.map((num) => num \/ 2).filter((num) => num >= 15);\nconsole.log(result);<\/code><\/pre>\n\n\n\n
Question 2<\/strong><\/h3>\n\n\n\n
Find the issue in the below code snippet.<\/p>\n\n\n\n
 let counter = 0;\nfor (var i = 1; i <= 10; i++) {\n  counter+= i;\n}\nconsole.log(counter);\nconsole.log(i);<\/code><\/pre>\n\n\n\n
Question 3<\/strong><\/h3>\n\n\n\n
Analyze the below code. Do you see any issue? If yes, what is that issue?<\/p>\n\n\n\n
const object1 = {\n  a: 10,\n  b: 20,\n  c: function () {\n\tconsole.log(this.a + this.b);\n  },\n};\nconst func = object1.c;\nfunc();<\/code><\/pre>\n\n\n\n
Question 4<\/strong><\/h3>\n\n\n\n
Create a JavaScript function that calculates the tip for a given bill amount and tip percentage. Bill amount and tip percentage will be input parameters while output will be calculated tip value.<\/p>\n\n\n\n
Question 5<\/strong><\/h3>\n\n\n\n
What will be the output of below code snippet:<\/p>\n\n\n\n
function greetHello(name) {\n  return `Hello, ${name}!`;\n}\nconsole.log(greetHello(\"Brian\"));<\/code><\/pre>\n\n\n\n
Question 6<\/strong><\/h3>\n\n\n\n
 Will the below code return any error? If yes, identify the error.<\/p>\n\n\n\n
function fetchData(callback) {\n  fetch('https:\/\/api.example.com\/data')\n\t.then(response => response.json())\n\t.then(data => callback(null, data))\n\t.catch(error => callback(error));\n}\n fetchData(function (error, data) {\n  if (error) {\n\tconsole.log('Error:', error);\n  } else {\n\tconsole.log('Data:', data);\n  }\n});<\/code><\/pre>\n\n\n\n
Question 7<\/strong><\/h3>\n\n\n\n
Implement a simple shopping cart system with features to add items, remove items and calculate the total price. Use objects to represent items, including properties for the item name, price and quantity. Implement features to add items to the cart, remove items and calculate the total cost.<\/p>\n\n\n\n
Question 8<\/strong><\/h3>\n\n\n\n
Analyze the below code snippet and advise what will be the output:<\/p>\n\n\n\n
 const person = {\n  firstName: \"Helen\",\n  lastName: \"Ryan\",\n  getFullName: function () {\n\treturn this.firstName + \" \" + this.lastName;\n  },\n};\nconsole.log(person.getFullName());<\/code><\/pre>\n\n\n\n
Question 9<\/h3>\n\n\n\n
Find the issue with the below code snippet:<\/p>\n\n\n\n
setTimeout(function () {\n  console.log(\"This will be executed after 3 seconds\");\n}, 3000);\nclearTimeout();<\/code><\/pre>\n\n\n\n
Question 10<\/strong><\/h3>\n\n\n\n
What issue exists in the below code:<\/p>\n\n\n\n
const testArray = [1, 2, 3];\ntestArray = [4, 5, 6];\nconsole.log(testArray);<\/code><\/pre>\n\n\n\n
<\/p>\n\n\n\n
Interview Questions for Mid-level JavaScript Developers<\/h2>\n\n\n\n
When designing interview questions for mid-level JavaScript developers, you should prepare challenges to test the understanding of advanced JavaScript concepts<\/a> and problem-solving skills. Some areas that should be considered for evaluation include functional programming, asynchronous programming, promises, working with APIs and advanced JavaScript features. Find below top 10 coding challenges which are suited to mid-level JavaScript developers:<\/p>\n\n\n\n
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Question 1<\/strong><\/h3>\n\n\n\n
What is the issue in the below code:<\/p>\n\n\n\n
 const fetchData = async () => {\n  const response = await fetch(\"https:\/\/api.samplewebsite.com\/data\");\n  const data = await response.json();\n  console.log(data);\n};\nfetchData();<\/code><\/pre>\n\n\n\n
Question 2<\/strong><\/h3>\n\n\n\n
What will be the output of the below code:<\/p>\n\n\n\n
const promise1 = Promise.resolve(\"One\");\nconst promise2 = new Promise((resolve) => setTimeout(() => resolve(\"Two\"), 1000));\nconst promise3 = Promise.reject(\"Three\");\n Promise.allSettled([promise1, promise2, promise3]).then((results) => console.log(results));<\/code><\/pre>\n\n\n\n
Question 3<\/strong><\/h3>\n\n\n\n
Develop a simple URL shortener service using JavaScript. Implement a function that takes a long URL as an input parameter and the output will be a shortened URL. Create a reverse function as well. The reverse function takes the shortened URL and returns the original long URL. You can use simple in-memory objects to store the mapping between long and short URLs.<\/p>\n\n\n\n
Question 4<\/strong><\/h3>\n\n\n\n
Implement an autocomplete feature for a search input field. Given an array of words, write a function that suggests words based on the current input. The output of the function will be an array of suggested words that start with the input characters, limiting the number of suggestions (e.g., a maximum of 7 suggestions).<\/p>\n\n\n\n
Question 5<\/strong><\/h3>\n\n\n\n
What is the issue in the below code:<\/p>\n\n\n\n
const obj = {\n  name: \"Conner\",\n  age: 27,\n  greet: () => {\n\tconsole.log(`Hey, my name is ${this.name}`);\n  },\n};\nobj.greet();<\/code><\/pre>\n\n\n\n
Question 6<\/strong><\/h3>\n\n\n\n
What will be the output of below code snippet:<\/p>\n\n\n\n
const object1 = {\n  prop1: \"value1\",\n  prop2: {\n\tprop3: \"value3\",\n  },\n};\n const newObj = { ...obj };\nnewObj.prop2.prop3 = \"newValue3\";\nconsole.log(object1.prop2.prop3);<\/code><\/pre>\n\n\n\n
Question 7<\/strong><\/h3>\n\n\n\n
Will the below code return any error? If yes, what will be the error?<\/p>\n\n\n\n
class Bird {\n  constructor(name) {\n\tthis.name = name;\n  }\n  speak() {\n\tconsole.log(`${this.name} makes a noise.`);\n  }\n}\n class Crow extends Bird{\n  speak() {\n\tsuper.speak();\n\tconsole.log(`${this.name} sings.`);\n  }\n}\n const crow = new Crow(\"Tim\");\ncrow.speak();<\/code><\/pre>\n\n\n\n
Question 8<\/strong><\/h3>\n\n\n\n
Develop a function that throttles another function, allowing it to be called at most once every specified interval (e.g., 300ms). The throttling function will have two input parameters. One will be the function to be throttled and the second will be the interval in milliseconds. The throttled function should be called with the same arguments as the original function.<\/p>\n\n\n\n
Question 9<\/strong><\/h3>\n\n\n\n
What is wrong with the below code:<\/p>\n\n\n\n
const arr = [1, 2, 3, 4, 5];\nconst sum = arr.reduce((total, num) => total + num);\nconsole.log(sum \/ arr.length);<\/code><\/pre>\n\n\n\n
Question 10<\/strong><\/h3>\n\n\n\n
Design a simple meeting scheduler that finds the first available time slot for a meeting between two people. Given two arrays of busy time intervals and a meeting duration, create a function that returns the earliest available time slot for the meeting when both people will be available. Each interval is represented as an array of two integers, where the first integer is the start time and the second integer is the end time.<\/p>\n\n\n\n
<\/p>\n\n\n\n
Interview Questions for Expert JavaScript Developers<\/strong><\/h2>\n\n\n\n
When preparing coding challenges for the JavaScript coding interview, you should test the advanced features of the language and performance optimization techniques. Some of the areas to evaluate include advanced JavaScript features, code architecture, design patterns, performance optimization and security. Below we have presented 10 JavaScript coding interview challenges for expert JavaScript developers:<\/p>\n\n\n\n
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Question 1<\/strong><\/h3>\n\n\n\n
Is there any security vulnerability in the below code? If yes, identify it:<\/p>\n\n\n\n
const username = document.getElementById('username').value;\nconst password = document.getElementById('password').value;\nfetch('https:\/\/api.examplewebsite.com\/login', {\n  method: 'POST',\n  body: JSON.stringify({ username, password })\n})\n.then(response => response.json())\n.then(data => console.log(data))\n.catch(error => console.log(error));<\/code><\/pre>\n\n\n\n
Question 2<\/strong><\/h3>\n\n\n\n
Identify the output of the below code.<\/p>\n\n\n\n
const testArray = [1, 2, 3, 4, 5];\nconst res = testArray.reduce((acc, curr) => {\n  if (curr % 2 === 0) {\n\treturn acc + curr;\n  }\n  return acc;\n}, 0);\nconsole.log(res);<\/code><\/pre>\n\n\n\n
Question 3<\/strong><\/h3>\n\n\n\n
What is the possible performance issue in the below code?<\/p>\n\n\n\n
const arr = [];\nfor (let i = 0; i < 1000000; i++) {\n  arr.push(Math.floor(Math.random() * 1000));\n}<\/code><\/pre>\n\n\n\n
Question 4<\/strong><\/h3>\n\n\n\n
Suggest the output of the below code:<\/p>\n\n\n\n
const arr = [1, 2, 3];\nconst object1 = { x: 1, y: 2, z: 3 };\nconsole.log([...arr, ... object1]);<\/code><\/pre>\n\n\n\n
Question 5<\/strong><\/h3>\n\n\n\n
Design a social media platform that contains features like sign up, creating a profile and posting status updates. Users should be able to follow other users and view their posts on a newsfeed.<\/p>\n\n\n\n
Question 6<\/strong><\/h3>\n\n\n\n
What is wrong with the below call to the API?<\/p>\n\n\n\n
fetch('https:\/\/api.example.com\/data')\n  .then(response => {\n\tif (!response.ok) {\n  \tthrow new Error('Network response was not ok');\n\t}\n\treturn response.json();\n  })\n  .then(data => console.log(data))\n  .catch(error => console.log(error));<\/code><\/pre>\n\n\n\n
Question 7<\/strong><\/h3>\n\n\n\n
What will be the output of below code snippet?<\/p>\n\n\n\n
const promise1 = Promise.resolve(One);\nconst promise2 = Promise.resolve(Two);\nPromise.all([promise1, promise2]).then(([result1, result2]) => console.log(result1 + ' ' + result2));<\/code><\/pre>\n\n\n\n
Question 8<\/strong><\/h3>\n\n\n\n
Design an online code editor where users can write, save and run JavaScript code. The editor should include features like syntax highlighting, auto-completion and error checking.<\/p>\n\n\n\n
Question 9<\/strong><\/h3>\n\n\n\n
The below code snippet uses closures to implement a counter. How will you optimize it to minimize memory usage:<\/p>\n\n\n\n
 function counter() {\n  let count = 0;\n  return function() {\n\tcount++;\n\tconsole.log(count);\n  }\n}\nconst increment = counter();\nincrement(); \/\/ 1\nincrement(); \/\/ 2\nincrement(); \/\/ 3<\/code><\/pre>\n\n\n\n
Question 10<\/strong><\/h3>\n\n\n\n
Develop a fitness tracker application where users can enter their daily exercise routines and track their progress over time. The application should allow users to set goals, view their progress and receive reminders.<\/p>\n\n\n\n
\"Test<\/a><\/figure>\n\n\n\n
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Conclusion<\/strong><\/h2>\n\n\n\n
Evaluating JavaScript developers at varying skill levels requires well-designed code challenges. Syntax, data types and functions are some basic concepts to test first. More sophisticated topics like object-oriented programming, algorithms and data structures should be included when testing mid-level engineers. Complex issues like performance optimization, security and design patterns should be used to evaluate expert-level developers.<\/p>\n\n\n\n
This article has presented numerous examples of coding challenges to help hiring managers spot promising JavaScript developers at different levels of experience.<\/p>\n\n\n\n
Further reading: <\/em><\/p>\n\n\n\n