JavaScript interview questions and answers
Here are 50 JavaScript interview questions along with their answers. These cover various topics from basics to advanced concepts.
Basic Questions
What is JavaScript?
JavaScript is a high-level, interpreted programming language primarily used for creating interactive effects within web browsers.
What are the data types in JavaScript?
JavaScript has seven data types: undefined, null, boolean, number, string, symbol, and object.
What is the difference between == and ===?
== checks for equality with type coercion, while === checks for strict equality without type coercion.
What is a variable in JavaScript?
A variable is a container that holds data values. In JavaScript, you can declare variables using var, let, or const.
What is a function?
A function is a block of code designed to perform a particular task. It can be invoked when needed.
Intermediate Questions
What is a closure?
A closure is a function that retains access to its lexical scope, even when the function is executed outside that scope.
Explain the concept of hoisting.
Hoisting is JavaScript's default behavior of moving declarations to the top of the current scope before code execution.
What are arrow functions?
Arrow functions are a concise syntax for writing function expressions. They do not have their own this context.
What is an IIFE?
An Immediately Invoked Function Expression (IIFE) is a function that runs as soon as it is defined.
What is event delegation?
Event delegation is a technique of using a single event listener on a parent element to manage events for multiple child elements.
Advanced Questions
What is the this keyword?
The this keyword refers to the context in which a function is called. Its value depends on how the function is invoked.
What are Promises?
Promises are objects that represent the eventual completion (or failure) of an asynchronous operation and its resulting value.
What is async/await?
async/await is syntactic sugar over Promises that allows writing asynchronous code in a more synchronous manner.
What are modules in JavaScript?
Modules are reusable pieces of code that export and import functionalities, helping in organizing code into separate files.
What is the event loop?
The event loop is a mechanism that allows JavaScript to perform non-blocking I/O operations by offloading operations to the system.
Array and Object Questions
How do you create an array in JavaScript?
Arrays can be created using array literals [] or the Array constructor, e.g., let arr = [1, 2, 3];.
What is the difference between Array.prototype.map() and Array.prototype.forEach()?
map() creates a new array with the results of calling a function on every element, while forEach() executes a function for each element without returning a new array.
How do you remove duplicates from an array?
You can use a Set: const uniqueArray = [...new Set(array)];.
What is an object in JavaScript?
An object is a collection of properties, where each property is defined as a key-value pair.
How do you merge two objects?
You can use Object.assign() or the spread operator: const merged = { ...obj1, ...obj2 };.
DOM and Browser Questions
What is the DOM?
The Document Object Model (DOM) is an interface that browsers implement to structure a document as a tree of objects.
How do you select an element in the DOM?
You can use methods like document.getElementById(), document.querySelector(), or document.getElementsByClassName().
What is event bubbling?
Event bubbling is a propagation method where an event starts from the target element and bubbles up to its parent elements.
How do you prevent the default behavior of an event?
You can call event.preventDefault() within the event handler.
What are data attributes?
Data attributes are custom attributes that start with data- and can be used to store additional information on HTML elements.
Error Handling and Debugging
What is a try/catch block?
A try/catch block allows you to handle errors gracefully by attempting to execute code and catching exceptions if they occur.
What is a thrown error?
A thrown error is an error that is created and thrown using the throw statement, interrupting the normal flow of code execution.
How can you debug JavaScript code?
You can use browser developer tools, console.log(), or debugging statements to trace issues.
What is the difference between synchronous and asynchronous code?
Synchronous code runs in sequence, blocking further execution until the current task is completed, while asynchronous code allows tasks to run in parallel.
What is a stack trace?
A stack trace provides information about the function call stack at the point where an error occurred, helping in debugging.
Performance and Optimization
What is debouncing?
Debouncing is a technique used to limit the rate at which a function is executed, ensuring it is only called once after a specified delay.
What is throttling?
Throttling limits the number of times a function can be called over time, ensuring it only runs at specific intervals.
How can you optimize JavaScript performance?
You can optimize performance by minimizing DOM manipulation, using event delegation, and avoiding memory leaks.
What are Web Workers?
Web Workers allow you to run scripts in background threads, providing a way to execute code without blocking the UI.
What is lazy loading?
Lazy loading is a design pattern that postpones loading of non-essential resources until they are needed, improving initial load time.
Frameworks and Libraries
What is a JavaScript framework?
A JavaScript framework is a pre-built collection of JavaScript code designed to assist in building applications. Examples include React, Angular, and Vue.
What is the Virtual DOM?
The Virtual DOM is a lightweight representation of the actual DOM that allows libraries like React to perform efficient updates and rendering.
What are the advantages of using libraries like jQuery?
Libraries like jQuery simplify tasks such as DOM manipulation, event handling, and AJAX requests, providing cross-browser compatibility.
What is state management in frameworks?
State management refers to how an application handles and stores state, allowing components to react to changes and maintain consistency.
What is a Single Page Application (SPA)?
An SPA is a web application that loads a single HTML page and dynamically updates content as the user interacts with the app, without requiring a full page reload.
Miscellaneous Questions
What is JSON?
JSON (JavaScript Object Notation) is a lightweight data interchange format that is easy for humans to read and write and easy for machines to parse and generate.
How do you parse JSON in JavaScript?
You can use JSON.parse() to convert a JSON string into a JavaScript object.
What is the difference between null and undefined?
null is an assignment value that represents "no value," while undefined indicates a variable that has been declared but not assigned a value.
What is the purpose of bind()?
The bind() method creates a new function that, when called, has its this keyword set to a specified value.
What is the spread operator?
The spread operator (...) allows an iterable (like an array) to be expanded into individual elements.
Final Questions
What are higher-order functions?
Higher-order functions are functions that can take other functions as arguments or return them as results.
What is a promise chain?
A promise chain is a series of .then() calls that allows handling asynchronous operations in sequence.
What is the apply() method?
The apply() method calls a function with a given this value and arguments provided as an array.
What is a generator function?
A generator function is a function that can be paused and resumed, defined using the function* syntax and yielding values with yield.
What is the difference between slice() and splice()?
slice() returns a shallow copy of a portion of an array, while splice() changes the contents of an array by removing or replacing existing elements.
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