Programs
Updated on December 27, 2024
JavaScript is a programming language that is in demand right now, and possessing this skill is a crucial factor to look for in a developer if you want to land your dream job. To nail a JavaScript coding interview, one needs to be prepared with both theoretical knowledge and how to provide practical answers to coding problems. This article presents the top 90 most common JavaScript coding interview questions with their answers for beginner, intermediate and advanced levels.
Data types, variables and scoping, array, string manipulation, OOP (Object Oriented Programming), error handling, control flow, DOM manipulation, and asynchronous programming are basic questions in JavaScript. The basic JavaScript coding interview questions are:
When the Interviewer asks these questions, the interviewer is looking for a basic understanding of JavaScript.
const reverseString =(str) => str.split(“”).reverse().join(“”);
The NaN function checks if the passed value is NaN(not a number) and if it’s of the type ‘number’. In JavaScript, the value NaN is a type of number. If the argument is not a number, it returns true; otherwise, it returns false.
JavaScript is faster than ASP script & ASP script is a server-side scripting language. The server-side scripting language ASP always depends on the server — other languages seen in this section can be developed on notebooks, resulting in their deployment on the server in general.
The negative infinity is a constant value with the lowest possible value. What it means is that no figure is smaller than this quantity. It can be generated using a self-made function or by an arithmetic operation. In JavaScript, when giving it a value to calculate, it will display the NEGATIVE_INFINITY as -Infinity.
We can break the javascript source code into several lines in a string statement. This can be broken by using the backslash n ‘n’.
console.log(“Voldemort Computer Science n for Robots, Harry Potter”)
JavaScript was developed by Netscape Communication corporation. It was created by Brendan Eich in 1995 while working at Netscape.
The NULL value represents no value or no object. It is known as an empty value object.
The prompt box is a dialogue box which prompts the user to enter some text in the message it provides within the prompt box. The user uses it if you want to input a value before you go to a page. If the input consists of a string, it returns it; otherwise, it returns null.
JavaScript functions are important objects. It behaves similarly to objects, can be assigned to variables, passed to other functions, and returned from functions. Like objects, they also have their properties, and ‘this’ stores the current JavaScript program’s execution context. As a result, this value changes depending on how the function is defined, invoked and the default execution context when it is used inside a function.
Yes, JavaScript supports automatic type conversion.
Variable typing is the type of the variable used to store a number and use that same variable to assign a “string”.
raj_potter = 55 ;
System = “computer”.
A closure is a function doing something different from its outer scope. It remembers where it is now, even after the outer function ends.
A for loop executes a block source code a specific number of times.
This executes different source code blocks based on conditions.
Let fruits = [“Apple”, “Banana”, “Cherry”];
The DOM is a programming interface for web documents, representing documents written in HTML or XML. It includes Events, methods, and properties.
Using the following methods document.getElementById(), document.querySelector(), or document.getElementByClassName().
Event building is where an event propagates from the target element to its parent elements.
ES6 modules allow source code to be organized into separate files using import and export statements.
The promises represent an asynchronous operation’s eventual completion (or failure).
In JavaScript, two operators are used to compare values: the equality operator (==) and the strict equality operator (===), but their difference is in how they convert their data types. The equality operator (==) performs type coercion. This means that it converts its operands to the same type before performing the comparison and, hence, sometimes can produce some rather unexpected results. For example, 5 == “5” is true because the element on the right-hand side of the ‘==’ operator is converted into a number, in this case, 5. On the other hand, the strict equality operator (===) tests for value equality and the type of the operated arguments. This helps in comparison that will not compare the type of 5 and “5”, even though 5 === “5” returns false. It is always advisable to use === to compare things because there is often an implicit type of coercion in the case of loose comparison.
push(): Adds an element to the end.
pop(): Removes the last element.
shift(): Removes the first element.
map(), filter(), reduce(): Perform transformations and computations.
let person = { name: “John”, age: 30 };
Every JavaScript has a prototype, another object from which it inherits properties and methods.
This is a condition in JavaScript in which an event triggered in a child control of another control is passed up through the parent, parent, and all other ancestors in the DOM model. This means the event is the first run here. The target element is the element in which the event happened, and then, with the parent and grandparent, it is stopped up to the root element ten.
Synchronous: The tasks are executed sequentially.
Asynchronous: This task can run independently without waiting for others to complete.
The ES6 modules organise source code into separate files using import and export statements.
The arrow function is introduced in ES6. The arrow function allows us to write the shorter function syntax.
const add = (a, b) => a + b;
JSON.stringify(): This converts an object into a JSON string.
JSON. parse(): This converts a JSON string into a JavaScript object.
Another function is called with a callback function passed as an argument and is executed when the other function call completes.
Key features of Smalltalk include dynamic typing, object orientation based on the prototype, first-class functions, and support for event-driven and asynchronous programming.
In other words, it’s a JavaScript technique where instead of attaching multiple event listeners to multiple child elements, we attach only one event listener to a parent element called the event delegation.
The value that promise represents might be present now, in the future, or never. It is used to work on asynchronous operations. Promises have three states: They have methods that are pending, resolved (fulfilled), and rejected.
The bind() method creates a new function that, when called, has this keyword set to the provided value and prepends any provided.
In modern JavaScript, you can create a deep copy using JSON.parse(JSON.stringify(object)) or libraries like loadash or structuredClone.
The “null” is a deliberate assignment value representing no value or object. The “undefined” means a variable has been declared but not assigned a value.
The apply() method is similar to call() because it is invokable. The only difference is that the application accepts the arguments as an array or an array-like object, whereas the call accepts the arguments anyway.
A promise chain is a series of then() calls on a promise. Each then() returns a new promise, allowing you to chain multiple asynchronous operations sequentially.
Generators are special functions that pause execution using yield and resume where they left off. They provide a way to iterate over data asynchronously.
Backtick (`) enclosed template literals have expressions inside of expressions, multi-line string, and string interpolation using `${}`.
A setTimeout() is a function that executes some function or a segment of source code after a given time(ms).
The setInterval() executes a given function or source code snippet repeatedly with a fixed time delay between each call.
This keyword refers to the object that executes the source code in the current context, and its value is based on when it is used.
The errors in JavaScript can be handled using try, catch, and finally, blocks. Throw can be used to manually throw errors.
The spread operator expands an iterable (like an array or object) into individual elements or properties.
The async makes a function return a promise, which makes the function wait until the promise is complete.
Destructuring is a simple way to make values from arrays or objects into variables.
Object.freeze() converts an object so it cannot be modified because its properties and values cannot be modified.
An IIFEE is a function defined and executed immediately after its creation. It is often used to create a local scope to avoid polluting the global scope.
The reduce() method iterates over an array and accumulates a single result by applying a callback function.
You can clone an object using Object. assign({}, originalObject) or the spread operator({…originalObject}).
A WeekMap is a collection of key-value pairs where the keys are objects, and the values can be any type. This does not prevent garbage collection of keys.
The core of a JavaScript event loop is powerful machinery that allows the creation of non-blocking, asynchronous programs by orchestrating some tasks performed in that environment in a thread fashion.
The new keyword is used to create a new instance of an object, initializing it using constructor functions.
A Proxy is an object that wraps another object and intercepts operations like property access, assignment, etc., enabling custom behaviour.
We can prevent multiple calls by using techniques such as debouncing or throttling.
The setImmediate() executes an asynchronous function after the current event loop cycle. It is similar to setTimeout() with a delay of 0.
The find() method searches the first element in an array for which all the members satisfy the given testing function. If an argument is not passed in this method, it returns the root object; otherwise, it returns the value of the first element for which the specified criteria are met or returns undefined.
const numbers = [10, 20, 30];
const result = numbers.find(num => num > 15);
A HOF(Higher Order function) is a function that takes another function as an argument and must return another function or both. These functions are typical of the functional programming paradigm in JavaScript, especially with methods such as map, filter or reduce.
Host is a JavaScript mechanism that moves variables, function declarations, or classes to the top of their scope during execution. For example, variable declarations are hoisted but not their assignments.
A WeakSet is similar to a Set but only holds objects, not primitive values. Its references are weak, meaning that if no other references exist, the objects in a WeakSet can be collected as garbage.
The typeof operators is used to determine the type of a value.
The async and await simplify working with promises by turning asynchronous sources into code written synchronously.
Memoization is a concept that involves saving the result of expensive function calls in a cache so that the function can be quickly retrieved when it is called again with the same argument.
Every JavaScript object has a prototype property that allows for inheritance. When a method or property is not found on an object, JavaScript looks up the chain to its prototype.
Use the in-operator or hasOwnProperty Method.
The call() and apply() methods in JavaScript invoke functions with a specified value. The primary difference lies in how they handle arguments. The call() method arguments individually, while applying () takes them as an array. The methods are instrumental in borrowing methods from other objects or invoking functions in a specific context.
ES2017 introduced async/await, a syntactic sugar based on Promises that makes asynchronous code appear more like synchronous code.
async just tells the compiler that this is an asynchronous function and returns you a Promise out of the box. In an async function, the await keyword pauses execution until a Promise is resolved or rejected. By doing this, we have cleaner and more readable code and don’t have to have nested .then() chains. Secondly, error handling is simplified, using await with try…catch blocks.
The name implies memory management in JavaScript, performed automatically using a garbage collector. This process recovers objects not used or referred to and frees memory.
Almost all real implementations of garbage collection algorithms, like mark-and-sweep garbage collection, start with the object graph from the roots, such as global variables, and mark all accessible objects. Then, unmarked objects are cleaned up and considered unreachable. Memory-wise, developers can optimize memory usage by carefully managing these object references, avoiding references caused by circular references or keeping more references than necessary.
The function syntax defines the generator functions in JavaScript, which allows functions to yield multiple values over time rather than return a single value. The generator function is called and returns an iterator object. The iterator’s next() method executes the generator functions up to the next yield statement, pausing execution and returning the yielded value. Generators are useful for implementing lazy evaluation, asynchronous workflows, and custom iterables.
Hoisting is a JavaScript feature that elevates all variable and function declarations to the top of the relevant scope during the compile-state phase. If we declare a variable using the var or a function declaration, it gives us a reference to the variable or function, so we don’t have to define it in the code.
It is a method introduced in Promise that executes multiple promises simultaneously and waits until all of them are fulfilled (or one is rejected). It returns the new Promise, which resolves with an array of results from those input Promises or is rejected if any of those input Promises is rejected.
However, we can use a throttle function to limit the frequency at which we call a function. This ensures that the function is called at the right times regardless of how often the event is triggered (for example, when we scroll or resize). It helps us only execute the function if necessary.
JavaScript is a non-blocking, asynchronous programming language, and the best way to have asynchronous code is to use the Event loop. It constantly checks whether the call stack is empty and processes the task queue. The event loop pushes the top task in the queue onto the call stack for execution unless there are tasks (which) are in the queue.
In synchronized code, each operation of code will be executed sequentially. The next operation can’t start until the previous one has finished. If one operation takes a long time, delays can result.
On the other hand, asynchronous code executes other code even while certain operations (such as network requests or timers) run in the background. JavaScript does this through callbacks, Promises or async/await.
Example of Synchronous Source Code
The JavaScript event loop handles asynchronous operations. It allows you to perform I/O operations (like open(), etc.) from JavaScript without disturbing the main program. A message queue continuously checks for pending events, which are then pushed to the call stack for execution when the call stack is empty.
The call stack, message queue, web APIs (I/O operations), and the event loop enable JavaScript to perform tasks in parallel without wasting time on the main thread.
A higher-order function is a function that takes one or more functions as arguments or returns a function as output. It’s also a common concept in JavaScript for callbacks and function composition.
To create a new Object using an existing object as that new object’s [[Prototype]]. It is useful for setting up inheritance between objects. This is useful for setting up inheritance between objects.
A higher-order function is a function that takes another function as an argument, returns another function, or both. This will help us to write more abstract, more reusable code.
Event delegation is a technique in JavaScript where a single event listener is added to a parent element instead of attaching multiple event listeners to individual child elements. The event listener checks the event. target to determine which child element triggered the event.
The DOM Input Checkbox Property sets or returns a checkbox field’s checked status, reflecting the HTML Checked attribute.
document.getElementById(“container”).checked;
console.log(1 < 2 < 3);
console.log(3 > 2 > 1);
Output
true
false
Output
Answer
3
This can be done by using the target attribute in the hyperlink.
< a href=”/herovird.html” target=”newframe”> New Page </a>
JavaScript is dynamically typed (Variable type changes at the time of execution) and interpreted language (written and added as part of HTML to a web page, and it gets interpreted by the environment in which the page is currently loaded). It is a lightweight cross-platform scripting language used to perform real-time updates for form validations, animations, and user interactions in the browser, and it is very useful.
To learn coding interviews and become a good web developer, you must master JavaScript. With ES6 and beyond, you can focus on core concepts, such as closures, asynchronous programming and object manipulation, and properly show how you solve practical challenges.
Saying you need to write clean, efficient, maintainable code and demonstrate a problem-solving mindset will help you succeed. Daily practice and continual learning will also improve your technical skills and develop your confidence to try and solve complex technical challenges in real-world projects and interviews. Explore the Certificate Program in Application Development powered by Hero Vired to advance your application development skills. With expert-led training and hands-on projects, it’s the perfect way to build a solid foundation for your tech career.
Updated on December 27, 2024
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