30+ Javascript Developer Interview Questions and Answers

Promises and async/await are used in JavaScript to handle asynchronous operations and avoid callback hell.

• Promises are objects representing the eventual completion or failure of an asynchronous operation.

• Async/await is a syntactic sugar built on top of promises, making asynchronous code look synchronous.

• Promises can be chained using .then() and .catch() to handle success and error cases.

• Async functions return a promise, allowing you to use await to pause execution until the p...read more

var, let, and const are used for variable declaration in JavaScript. ES6 concepts include arrow functions and block-scoped variables.

• var is function-scoped, let and const are block-scoped

• let and const support block-level scoping, reducing the risk of variable hoisting

• const variables cannot be reassigned, but their properties can be modified

• ES6 introduces arrow functions for concise syntax and lexical scoping

• State and props are key concepts in React for managing component data...read more

forEach and map are array methods in JavaScript. forEach executes a provided function once for each array element. Map creates a new array with the results of calling a provided function on every element in the calling array.

• forEach does not return anything, it just iterates over the array and executes the provided function for each element.

• Map returns a new array with the results of calling a provided function on every element in the calling array.

• Map is useful when you want...read more

1. Yes, .map and .filter are pure functions. 2. Network calls should not be blocked in Node.js. 3. For..in.. loop is for iterating through objects.

• 1. Yes, .map and .filter are pure functions as they do not modify the original array and always return a new array with the desired transformations or filtered elements.

• 2. Network calls should not be blocked in Node.js as it can lead to performance issues and hinder the event-driven nature of Node.js.

• 3. For..in.. loop is used for i...read more

Check if a number is even by using the modulo operator.

• Use the modulo operator (%) to check if the number divided by 2 has a remainder of 0.

• If the remainder is 0, then the number is even.

• Example: if(num % 2 === 0) { // number is even }

Designing a NLP engine involves defining the problem, selecting appropriate algorithms, and training the model.

• Define the problem and identify the data sources

• Select appropriate algorithms for tasks such as tokenization, part-of-speech tagging, and named entity recognition

• Train the model using labeled data and evaluate its performance

• Continuously improve the model by incorporating feedback and updating the algorithms

The map method creates a new array with the results of calling a provided function on every element in the original array.

• The map method does not modify the original array.

• The provided function takes three arguments: currentValue, index, and array.

• The map method returns a new array with the same length as the original array.

• Example: const numbers = [1, 2, 3]; const doubledNumbers = numbers.map(num => num * 2); // [2, 4, 6]

Event delegation is a technique where a single event listener is added to a parent element to handle events on its child elements.

• Improves performance by reducing the number of event listeners

• Useful for dynamically added elements

• Event bubbling is utilized

• Example: Click event on a list of items

Callback hell refers to the situation where multiple nested callbacks make the code difficult to read and maintain.

• Occurs when multiple asynchronous operations are nested within each other

• Can lead to deeply nested code structure which is hard to follow

• Can be avoided by using promises or async/await

The '==' operator checks for equality after type coercion, while '===' checks for equality without type coercion.

• The '==' operator compares two values after converting them to a common type.

• The '===' operator compares two values without type conversion, so they must be of the same type to be considered equal.

• Example: 1 == '1' will return true because the values are equal after type coercion, but 1 === '1' will return false because they are of different types.

Event bubbling is the propagation of an event from the innermost element to the outermost element in the DOM tree.

• When an event occurs on an element, it first runs the handlers on that element, then on its parent, and so on up to the document object.

• It allows for event delegation, where a single event handler can be attached to a parent element to handle events for all its children.

• Event.stopPropagation() can be used to stop the event from bubbling up the DOM tree.

Promises are used for asynchronous programming in JavaScript. var is function-scoped, let is block-scoped, and const is block-scoped and cannot be reassigned.

• Promises are used to handle asynchronous operations in JavaScript.

• var is function-scoped, let is block-scoped, and const is block-scoped and cannot be reassigned.

• Example: var x = 10; let y = 20; const z = 30;

ServiceNow is a cloud-based platform that provides IT service management, IT operations management, and IT business management solutions.

• ServiceNow is commonly used for IT service management, helping organizations streamline their IT processes.

• It offers features such as incident management, change management, problem management, and asset management.

• ServiceNow also provides IT operations management capabilities, including monitoring, event management, and automation.

• The platf...read more

Dynamic forms can be created using JavaScript and emberJS to allow for interactive and customizable user input.

• Use JavaScript to dynamically generate form elements based on user input or predefined conditions

• Leverage emberJS to handle form data binding and validation

• Implement event listeners and callbacks to update form elements and perform actions based on user input

A reusable piece of UI that can be composed with other components to form a complete UI.

• Components are the building blocks of React applications.

• They can be either functional or class-based.

• They can have state, props, and lifecycle methods.

• They can be reused and composed to create complex UIs.

• Examples include buttons, forms, and navigation menus.

Yes, Agile is a project management methodology that emphasizes iterative development and collaboration.

• Agile is a project management methodology that focuses on iterative development, collaboration, and flexibility.

• It involves breaking down projects into smaller tasks and completing them in short iterations called sprints.

• Agile promotes frequent communication and feedback among team members to adapt to changing requirements.

• Common Agile frameworks include Scrum, Kanban, and E...read more

Use pagination, lazy loading, virtual scrolling, and server-side processing for efficient loading of large data in UI reports.

• Implement pagination to display data in smaller chunks

• Utilize lazy loading to load data only when needed

• Implement virtual scrolling to render only the visible portion of data

• Use server-side processing to offload heavy data processing to the server

Types of loops in JavaScript include for, while, and do-while loops.

• For loop: Executes a block of code a specified number of times.

• While loop: Executes a block of code while a specified condition is true.

• Do-while loop: Executes a block of code once, and then repeats the loop as long as a specified condition is true.

Callback functions are functions passed as arguments to other functions to be executed later.

• Callback functions are commonly used in asynchronous programming.

• They are executed after a certain task is completed.

• Example: setTimeout function in JavaScript takes a callback function as an argument.

JS libraries are pre-written code that can be used to perform specific tasks in JavaScript.

• Libraries can save time and effort by providing pre-written code for common tasks

• Popular JS libraries include jQuery, React, and Angular

• Libraries can be installed using package managers like npm or yarn

Yes, I am proficient in JavaScript and have experience in developing web applications using it.

• Proficient in JavaScript programming language

• Experience in developing web applications using JavaScript

• Familiar with popular JavaScript libraries and frameworks like React, Angular, and Node.js

Multiple inheritance is when a class can inherit attributes and methods from more than one parent class.

• Allows a class to inherit from multiple parent classes

• Can lead to the diamond problem where ambiguity arises if two parent classes have a method with the same name

• Languages like JavaScript do not support multiple inheritance but can achieve similar functionality using mixins

Ember apps can be deployed using various methods such as hosting on a server, using a cloud platform, or deploying to a content delivery network (CDN).

• Ember apps can be deployed by hosting them on a server, such as Apache or Nginx.

• They can also be deployed using cloud platforms like Heroku or AWS.

• Another option is to deploy the app to a content delivery network (CDN) for faster and more efficient distribution.

• Deployment can be automated using tools like Ember CLI or continuou...read more

A polyfill is a code that adds a feature which the browser may not support. Promise.all() polyfill can be used for older browsers.

• Create a function that takes an array of promises as input

• Return a new promise that resolves when all promises in the input array have resolved

• If any promise in the input array rejects, reject the new promise with the reason of the first promise that rejects

• Use setTimeout and recursion to check if all promises have resolved

• If the browser supports P...read more

Promise in JS is an object representing the eventual completion or failure of an asynchronous operation.

• Promises are used to handle asynchronous operations in JavaScript.

• They can be in one of three states: pending, fulfilled, or rejected.

• Promises can be chained using .then() method to handle success and failure cases.

• Example: new Promise((resolve, reject) => { ... }).then(result => { ... }).catch(error => { ... });

Types of operators include arithmetic, assignment, comparison, logical, bitwise, and more.

• Arithmetic operators perform mathematical operations (+, -, *, /, %)

• Assignment operators assign values to variables (=, +=, -=, *=, /=)

• Comparison operators compare values (==, !=, >, <, >=, <=)

• Logical operators combine multiple conditions (&&, ||, !)

• Bitwise operators manipulate individual bits (&, |, ^, ~, <<, >>)

• Unary operators operate on a single operand (++, --)

Async await is a feature in JavaScript that allows for asynchronous code to be written in a synchronous manner.

• Async await is built on top of promises and provides a cleaner syntax for handling asynchronous operations.

• It allows you to write asynchronous code that looks like synchronous code, making it easier to read and maintain.

• The 'async' keyword is used to define a function as asynchronous, while the 'await' keyword is used to pause the execution of the function until a pr...read more

Use console.log() to quickly output code to the console in JavaScript.

• Use console.log() followed by the value or variable you want to output.

• You can output multiple values by separating them with commas.

• console.log('Hello, world!');

The event loop is a mechanism in JavaScript that allows for asynchronous operations to be executed in a non-blocking manner.

• The event loop continuously checks the call stack for any functions that need to be executed.

• If the call stack is empty, the event loop checks the task queue for any pending tasks.

• The event loop prioritizes tasks in the microtask queue over the macrotask queue.

Call stack is a data structure that stores information about the active subroutines of a computer program.

• Call stack is a LIFO (Last In, First Out) data structure.

• It keeps track of function calls in a program.

• When a function is called, it is added to the top of the call stack. When it returns, it is removed.

The modulo operator returns the remainder of a division operation.

• Use the % symbol to perform the modulo operation.

• Example: 10 % 3 will return 1 (10 divided by 3 is 3 with a remainder of 1).

let is block scoped, var is function scoped, const is block scoped and cannot be reassigned.

• let: block scoped variable declaration

• var: function scoped variable declaration

• const: block scoped constant declaration, cannot be reassigned