TOP 475 javascript interview questions for candidate screening in 2025

Why JavaScript Interview Questions Matter More Than Ever

Finding that dream developer isn't just about knowing how to code—it's about proving they can think like a developer when the pressure's on. JavaScript interview questions have become the ultimate litmus test for hiring managers who need to separate the wheat from thechaff. Whether they're gunning for a front-end position or aiming for that coveted full-stack role, their ability to tackle these questions confidently can make or break the team. The good news? With the right preparation, they can walk into any interview room knowing they've got what it takes.

The JavaScript Landscape: Numbers That Tell a Story

Here's something that might surprise you: JavaScript is used in approximately 97% of websites globally. That's not a typo—nearly every website you visit runs on JavaScript in some capacity. Created by Brendan Eich back in 1995, this language has evolved from a simple scripting tool into the third biggest web technology after HTML and CSS.

The demand is absolutely real. Recruiters worldwide consistently report that full-stack and back-end developers are among the most in-demand IT roles, and guess what skill keeps popping up? JavaScript. It's no wonder that mastering javascript interview questions has become non-negotiable for developers who want to stay competitive.

Technical Foundations Every Developer Should Know

Let's talk specifics. JavaScript operates with seven primitive data types: BigInt, Boolean, Number, Null, String, Symbol, and Undefined—plus two compound types: Object and Array. This dynamically-typed language processes type checking during run-time, which is why interviewers love to test your understanding of these fundamentals.

The evolution story gets interesting with ES6 in 2015, which introduced game-changers like:

And yeah, if you still was not aware of it, make sure you know JavaScript is not related tightly to Java. It is a completely different language. Here are some Java interview quesions if you need them

• let and const keywords for better variable management
• Arrow functions for cleaner syntax
• Generator functions with the function* keyword
• Native support for modules

Industry experts consistently note that 80% of JavaScript interviews can be cracked by mastering key topics. The language's lightweight, interpreted nature makes it faster than many alternatives, while its ability to run both client-side and server-side (thanks to Node.js) has solidified its position as a must-know technology.

Your Path to Interview Success

Ready to dive deep? We've compiled the most comprehensive collection of javascript interview questions you'll find anywhere. These aren't just random questions—they're battle-tested scenarios that real hiring managers use to evaluate candidates at companies ranging from scrappy startups to major tech giants. Let's explore 475 best practice javascript interview questions of 2024 that will prepare you for whatever curveballs come your way.

Because you're reading this article, you might be interested in the following article as well: Self Paced Video Interview: Tips for Employers.

🏆 The list of TOP 475 javascript interview questions in 2025

What are the different data types present in JavaScript?

This question helps assess a candidate’s fundamental understanding of JavaScript — a must-have for any front-end or full-stack developer role. It’s especially useful for technical screening early in the process.

Comment:

JavaScript has two main categories of data types:

• Primitive types:
• String
• Number
• BigInt
• Boolean
• Undefined
• Null
• Symbol

• Non-primitive type:
• Object

Primitive types store a single value and are immutable. Objects, on the other hand, allow storage of collections and more complex entities.

Best practice is to look for candidates who not only list these types but also briefly explain how they're used. For example, someone who mentions that "undefined is the default value of a variable that has been declared but not assigned" shows better applied understanding.

Look out for vague or incorrect answers — they may signal limited hands-on experience or study. Ideally, the candidate should also know that arrays and functions are special types of objects in JavaScript.

Explain Hoisting in JavaScript

Hoisting is JavaScript's default behavior of moving declarations (not initializations) to the top of the current scope. This applies to both variables and functions.

For example:

console.log(a); // undefined
var a = 5;

Even though `a` is declared after the `console.log`, JavaScript "hoists" the declaration to the top, so it doesn't throw an error—it just logs `undefined`.

This also applies to function declarations:

sayHello();

function sayHello() {
  console.log("Hello!");
}

Here, the function call works even though the function is defined later in the code. Why? Because the function declaration is hoisted.

However, not everything is hoisted the same way. Variables declared with `let` and `const` are hoisted as well—but they’re not initialized. Trying to access them before their declaration will throw a ReferenceError. This is known as the temporal dead zone.

Best practice:

• Always declare variables and functions at the top of their scope to avoid confusion.
• Use `let` and `const` over `var` to prevent unexpected behavior.

---

Comment:

Hoisting is the default behaviour of JavaScript where all the variable and function declarations are moved to the top. This means that no matter where variables and functions are declared in the code, JavaScript moves their declarations to the top of their scope—either local or global. It's important for candidates to explain how `var`, `let`, and `const` behave differently and the concept of the temporal dead zone. Good responses also mention function hoisting and provide clear examples.

Why do we use the word "debugger" in JavaScript?

The word "debugger" in JavaScript is a built-in keyword that stops the execution of code and allows developers to examine what's going on at that specific moment. When the JavaScript engine hits the `debugger` statement, it pauses, giving developers the chance to inspect variables, check the call stack, and walk through their logic step-by-step.

This is extremely helpful during the development and testing phase since it allows developers to spot and fix bugs more easily without relying only on `console.log()` statements.

Comment

The debugger for the browser must be activated in order to debug the code. Built-in debuggers can be turned on or off depending on the browser settings. When the `debugger` keyword is used, code execution pauses at that line if the developer tools are open. This allows the programmer to evaluate values, step through functions, and check where things might be going wrong. Best practice is to remove `debugger` lines before pushing code to production, as leaving them in can affect performance and expose unnecessary details.

What is the difference between `==` and `===` operators?

When you ask a candidate this question, you're testing their technical knowledge of comparison operators, typically in JavaScript.

Correct Answer: Both `==` and `===` are comparison operators in JavaScript. The main difference is:

• `==` compares only values, performing type coercion if needed.
• `===` compares both values and types, so no conversion happens.

Example:

'5' == 5    // true
'5' === 5   // false

Comment:

This is a good screening question for developers, especially frontend or full-stack roles. The ideal candidate should clearly explain that `==` allows for type coercion, while `===` enforces both type and value match. Best practice in most modern JavaScript development is to use `===` to avoid unexpected bugs. Look for concise, clear, and confident answers — bonus points if they mention best practices or code safety.

Difference between var and let keyword in JavaScript

When interviewing frontend or full-stack developer candidates, asking specific JavaScript questions helps evaluate their understanding of core concepts and best practices. A common question is:

“What is the difference between `var` and `let` in JavaScript?”

Comment:

From the very beginning, the `var` keyword was used in JavaScript programming, whereas the `let` keyword was just added in 2015 with ES6. The keyword `var` has function scope — meaning it's accessible anywhere within the function it’s declared in. On the other hand, a variable declared using `let` is block-scoped, so its availability is limited to the braces (`{}`) where it is defined.

This question helps you assess:

• If the candidate is up-to-date with modern JavaScript
• Whether they know the difference in scope management
• How well they write clean, maintainable code

Best practice: Candidates who prefer `let` (or `const`) show more modern and safe coding habits, which can reduce bugs related to scope and hoisting. Look for candidates who not only know the difference but also understand why one is better in most situations.

Explain Implicit Type Coercion in JavaScript

Implicit type coercion in JavaScript happens automatically when the language converts a value from one data type to another during code execution.

For example:

• If you add a number to a string: `1 + '2'` becomes `'12'` (number `1` is converted to string `'1'`)
• When used in boolean context: `!!"hello"` becomes `true`, because non-empty strings are truthy.
• Equality checks like `0 == false` return `true`, because JavaScript converts both to the same type before comparing.

JavaScript does this behind the scenes to make things easier. But it can sometimes lead to confusing results if you're not careful. It’s best practice for developers to understand when and how this coercion happens to avoid bugs. In screening technical candidates, their ability to explain real-world examples of coercion and when it might be dangerous (like using `==` instead of `===`) shows solid understanding of JavaScript fundamentals.

Is JavaScript a statically typed or a dynamically typed language?

JavaScript is a dynamically typed language. This means that variable types are determined at runtime, not during compile-time like in statically typed languages such as Java or C#.

Comment:

This is a simple technical screening question that helps you quickly see if a candidate understands the fundamentals of JavaScript.

Best practice: Look for answers that not only mention “dynamically typed” but also explain what that means. A strong candidate might add that this allows flexibility but can also lead to runtime bugs if not handled carefully.

If a candidate says "statically typed" or seems unsure, it's a red flag for basic JavaScript knowledge.

What is NaN property in JavaScript?

NaN stands for "Not-a-Number", and it's a special property in JavaScript. It shows up when a value that’s supposed to be a number doesn’t make sense mathematically or can’t be parsed as a number. For example, trying to divide a string by a number or perform invalid math operations can return NaN.

Even though NaN means "Not-a-Number", funny enough, its type is still considered a Number in JavaScript.

Best Practice Tip:

To check if a value is really NaN, don’t use equality checks like `value === NaN`, because NaN is not equal to itself. Instead, always use the built-in `isNaN()` or better yet, `Number.isNaN()` for cleaner and more accurate results.

Example:

console.log(typeof NaN); // "number"
console.log(NaN === NaN); // false
console.log(isNaN("hello")); // true
console.log(Number.isNaN("hello")); // false

Comment:

NaN property represents the "Not-a-Number" value. It indicates a value that is not a legal number. The `typeof` of NaN will return Number, even though it’s not a real number. To properly check if a value is NaN, use `isNaN()` or `Number.isNaN()`—it’s the best practice approach in JavaScript.

Explain passed by value and passed by reference.

Comment: This is a great question to assess a candidate’s understanding of how data is handled in memory, especially in programming languages like JavaScript. In JavaScript:

• Primitive types (like numbers, strings, booleans) are passed by value. This means when you assign a primitive to another variable or pass it into a function, a copy of the value is made. Any changes to the new variable do not affect the original.

• Non-primitive types (like objects, arrays, functions) are passed by reference. This means the variable holds a reference to the memory location of the actual value. If you modify the object or array inside a function, the changes will reflect outside the function too.

Best practice: Listen for clear examples during the candidate's answer. They should be able to illustrate how modifying a primitive inside a function doesn’t change the original, while modifying an object does. This helps you check both technical understanding and communication skills.

What is an Immediately Invoked Function in JavaScript?

An Immediately Invoked Function Expression (IIFE) is a function in JavaScript that is executed right after it is defined. The syntax might look a bit unusual at first, but it’s a very powerful tool for controlling variable scope and avoiding global variable pollution.

Here’s what it looks like:

(function() {
  // code goes here
})();

Or with arrow functions:

(() => {
  // code goes here
})();

This pattern is wrapped in parentheses to convert the function into a function expression, and then it's immediately called with the trailing parentheses.

Why It Matters in Screening

Asking this question is a great way to assess a candidate’s understanding of scope, closures, and JavaScript function execution. If they can explain what an IIFE is and when to use it, it shows they have hands-on experience and a solid grasp of code structuring.

What to Listen For:

• A clear definition of IIFE
• Recognition that it executes immediately
• Mention of scope isolation and use in avoiding polluting the global scope
• Mention of use cases like module patterns or initializing code

Best practice calls for engineers to know common JavaScript patterns like IIFEs, particularly those working with older codebases or vanilla JS. This question is especially relevant for front-end developers or full-stack engineers.

What do you mean by strict mode in JavaScript and characteristics of JavaScript strict-mode?

Strict mode in JavaScript is a way to opt into a restricted version of the language. Introduced in ECMAScript 5, it helps developers write cleaner and more reliable code. By turning on strict mode, JavaScript treats certain errors more seriously and prevents potentially risky actions.

You activate strict mode by adding `'use strict';` at the beginning of a JavaScript file or function.

Key characteristics of strict mode:

• Eliminates silent errors. Mistakes that normally go unnoticed throw exceptions in strict mode.
• Disallows duplicate parameter names or property names, reducing confusion.
• Prevents use of reserved keywords as variable or function names (e.g., `let`, `class`, or `eval`).
• No accidental global variables. You can’t assign values to undeclared variables, which helps prevent bugs.
• Fixes `this` behavior. In strict mode, `this` is `undefined` when a function is invoked without context, instead of defaulting to the global object.

---

How this helps in candidate screening:

When asking this question, you're not just checking if the candidate knows about strict mode—you’re seeing if they understand its purpose and use cases. A strong answer should cover the reasons behind using it (like avoiding bugs and enforcing good coding practices), not just list its technical features.

Best practice: Look for candidates who can explain the why, not just the what. For developer roles, understanding strict mode often reflects a commitment to clean code and debugging discipline.

Explain Higher Order Functions in JavaScript

Higher-order functions are functions that either take other functions as arguments or return them as results. This is possible in JavaScript because functions are treated as first-class citizens, meaning you can assign them to variables, use them as parameters, and return them from other functions—just like any other value.

Some common examples of higher-order functions include:

• `map()`
• `filter()`
• `reduce()`

These are often used to iterate and transform arrays in a clean and readable way.

Using higher-order functions is a best practice in JavaScript when you want to write flexible and reusable code. They help improve modularity and reduce code duplication, which is essential in building scalable applications.

---

Comment:

Functions that operate on other functions, either by taking them as arguments or by returning them, are called higher-order functions. Higher-order functions are a result of functions being first-class citizens in JavaScript.

Explain "this" keyword

The "this" keyword in JavaScript refers to the object that is currently executing the code. Its value depends on how a function is called, not where or how it’s defined. This makes "this" powerful but sometimes confusing.

In an object method, "this" refers to the object the method belongs to. In a regular function, "this" is undefined in strict mode, or the global object in non-strict mode. In arrow functions, "this" is lexically bound, meaning it uses "this" from the outer function or scope.

Comment:

Understanding how "this" changes depending on the context is key. When screening candidates, look for answers that show they know:

• How "this" behaves in regular functions vs arrow functions
• How it changes in event handlers, object methods, or callbacks
• Ways to control "this" using bind, call, or apply

Best practice: Ask your candidate to give a short code example or explain a mistake they made with "this." It often reveals how deep their understanding really is.

What do you mean by Self Invoking Functions?

A self-invoking function is a function that runs automatically as soon as it is defined. In JavaScript, this is often used to execute code without cluttering the global scope. The function is wrapped in parentheses and immediately followed by another set of parentheses to invoke it.

Example:

(function() {
  console.log("This function runs itself!");
})();

This code will run immediately, without needing to be called later.

Comment:

Without being requested, a self-invoking expression is automatically invoked (initiated). If a function expression is followed by `()`, it will execute automatically. A function declaration cannot be invoked by itself. This is a common concept in JavaScript, often used to isolate code or avoid naming conflicts.

Best practice tip: When evaluating developer candidates, ask them to explain how and when to use self-invoking functions. Strong candidates will mention their use in creating private scopes and reducing variable clashes in larger applications.

Explain call(), apply(), and bind() methods

When interviewing candidates for JavaScript roles, understanding how they explain core concepts like `call()`, `apply()`, and `bind()` can reveal how deep their knowledge really is. These methods are essential for setting the value of `this` in function execution and are widely used in both vanilla JS and frameworks like React.

Best Practice Tip: Ask candidates to explain these with examples. Bonus if they can relate it back to real projects they've worked on.

---

call()

The `call()` method is used to invoke a function by explicitly setting the value of `this`. You pass the context (the owner object) as the first argument and then list the function’s arguments one by one.

Example:

function greet() {
  console.log(this.name);
}

const person = { name: 'Alice' };
greet.call(person); // Outputs: Alice

Why it matters: This demonstrates the candidate understands how function context works. A good answer includes both what it does and how it's used in practice.

---

apply()

`apply()` works almost exactly the same as `call()`. The main difference is that it takes the function arguments as an array or array-like object.

Example:

function introduce(city, country) {
  console.log(`${this.name} from ${city}, ${country}`);
}

const person = { name: 'Bob' };
introduce.apply(person, ['New York', 'USA']);

Why it matters: Candidates who know this can explain its benefits when arguments are already in array format, like when working with `Math.max.apply(null, array)`.

---

bind()

Unlike `call()` and `apply()` that execute the function immediately, `bind()` returns a new function with `this` bound to the provided object. It does not call the function right away.

Example:

function sayHello() {
  console.log(`Hello, ${this.name}`);
}

const user = { name: 'Charlie' };
const boundHello = sayHello.bind(user);
boundHello(); // Outputs: Hello, Charlie

Why it matters: `bind()` is especially valuable in event handling or when passing callbacks. Candidates who understand this tend to write cleaner and more maintainable code.

---

Best Practice: Make sure the candidate can compare the three methods clearly and knows when to use each one. Strong developers won’t just know the syntax—they’ll know the real-world use cases.

What is the difference between `exec()` and `test()` methods in JavaScript?

When evaluating candidates for JavaScript roles, a good screening question is:

"Can you explain the difference between the `exec()` and `test()` methods in JavaScript?"

This question helps assess a candidate’s understanding of regular expressions and their behavior in JavaScript.

---

Best Practice Answer:

• `exec()` is a method used with regular expressions to search a string for a match and return detailed results (like the full match and any capture groups).
• If it finds a match, it returns an array with that match and relevant groups.
• If no match is found, it returns null.

Example:

const regex = /hello/;
const result = regex.exec("hello world");
// result => ["hello"]

• `test()`, on the other hand, is a simpler method that checks for the presence of a pattern and returns a Boolean – `true` if the match exists and `false` if it doesn't.

Example:

const regex = /hello/;
const isMatch = regex.test("hello world");
// isMatch => true

---

What to look for in candidate answers:

• They should relate the use case of each method.
• Good candidates know that `exec()` returns more data, useful when you want to extract values.
• `test()` is ideal for simple checks like form validations or basic text searches.

Make sure candidates also mention that both methods are part of RegExp objects and are often used in validation, search operations, or parsing strings in JavaScript.

---

This question screens for practical string handling skills, which matter in real-world frontend/backend development.

What is currying in JavaScript?

Currying in JavaScript is a technique where a function, instead of taking all arguments at once, takes them one by one. Each time the function is called, it returns another function that accepts the next argument, and so on, until all arguments are provided and the final result is returned.

For example:

function add(a) {
  return function(b) {
    return a + b;
  };
}

const addFive = add(5);
console.log(addFive(10)); // Outputs 15

This approach helps in creating reusable functions by pre-filling some arguments.

Comment:

Currying is an advanced technique to transform a function of arguments `n`, to `n` functions of one or fewer arguments. By using the currying technique, we do not change the functionality of a function, we just change the way it is invoked.

Best practice tip for screening candidates: A good candidate should not only be able to define currying but also explain why and when it could be useful, like improving modularity or creating more readable and flexible code. Look for examples in their projects or during coding tests.

What are some advantages of using External JavaScript?

External JavaScript offers several key benefits that make frontend development smoother and more collaborative for teams.

• Code Reusability: With external JavaScript files, you can write code once and use it across multiple pages or projects. This improves efficiency and reduces repetition.

• Improved Code Readability: Separating your JavaScript code from the HTML keeps both files cleaner and easier to read. This is especially helpful when debugging or reviewing someone else’s code.

• Better Collaboration: Web designers and developers can work independently on HTML and JS files. This separation simplifies workflow, especially in teams where responsibilities are clearly divided.

• Faster Page Load and Caching: Browsers can cache external JS files. This means that after the first load, users won’t have to reload the entire script each time, resulting in faster performance.

Best Practice Tip: Always keep your JavaScript logic in external files unless you have a specific reason not to. This keeps your HTML clean and your codebase scalable.

Using external JavaScript not only makes your code reusable and maintainable but also creates a better experience for everyone involved—from developers to the end users.

Explain Scope and Scope Chain in JavaScript

When you're interviewing for a JavaScript-related role, asking "Can you explain Scope and Scope Chain in JavaScript?" is a strong way to judge a candidate's understanding of variable management and execution context.

A good response should cover these key points:

• Scope defines where variables and functions are accessible in code.
• There are three main types of scope:
• Global Scope: Accessible anywhere in the code.
• Function (or Local) Scope: Variables declared inside a function are only available within that function.
• Block Scope: Introduced with `let` and `const`. Variables inside `{}` blocks (like `if`, `for`) are only accessible within that block.

• Scope Chain is how JavaScript looks for variables. It starts from the current function and moves outward to parent scopes until it finds the variable or runs out of scopes.

Comment:

This is a core concept in JavaScript. Candidates who understand Scope and Scope Chain are usually more confident managing code behavior and debugging. Look for simple explanations and examples. Best practice is to test their skill further with a small code snippet during the interview to see how they apply their understanding.

Explain Closures in JavaScript

Closures in JavaScript happen when a function is able to access variables from its outer lexical scope, even after that outer function has finished executing. In simple terms, a closure lets a function remember and access its surrounding state.

For example:

function outer() {
  let count = 0;
  return function inner() {
    count++;
    return count;
  };
}

const counter = outer();
console.log(counter()); // 1
console.log(counter()); // 2

In the above, the `inner` function still has access to `count` even after `outer()` has completed. That’s a closure in action.

---

Comment:

Closures are an important part of JavaScript because they allow functions to maintain access to the variables they were created with, even after the parent function exits.

When screening candidates:

• Look for an answer that shows the candidate understands both the concept and its practical uses.
• Bonus if they mention use cases like:
• Data privacy using closures
• Function factories
• Event handlers remembering state

Best practice: Ask them to give a short code example and explain it. This reveals not just their understanding but their ability to communicate technical ideas clearly.

What are some advantages of JavaScript?

JavaScript offers several advantages, making it one of the most widely used languages in web development today.

• Client-side and server-side execution: JavaScript can run both in the browser (client-side) and on servers using platforms like Node.js. This makes it versatile and efficient for full-stack development.

• Ease of learning: It’s considered one of the simpler programming languages to pick up, especially for beginners entering the tech field.

• Enhanced web functionality: It adds interactivity and dynamic elements to web pages, such as sliders, animations, and real-time updates, improving user experience.

• Speed: Since JavaScript runs directly in the browser, it executes quickly for the end-user without needing to constantly go back and forth to a server.

• Rich ecosystem and frameworks: Libraries and frameworks like React, Vue.js, and Angular make development quicker and more powerful.

Comment:

This is a good answer if you're looking for candidates with front-end development experience or someone who understands the core benefits of JavaScript in real-world applications. Look for responses that go beyond the basics—bonus points if they mention performance optimization or how JavaScript fits into full-stack solutions. Best practice is to ask for examples of how they've used these advantages in projects.

What are object prototypes?

In JavaScript, every object has a hidden internal property known as `[[Prototype]]`, which links to another object. This is called the object’s prototype. A prototype acts like a template object that other objects can inherit properties and methods from.

When a property or method is accessed on an object, JavaScript first looks for it directly on the object. If it's not found, it checks the prototype, and then the prototype’s prototype—forming a chain known as the prototype chain. This continues until it's found or the chain ends.

Understanding prototypes is important because it’s how inheritance is handled in JavaScript.

---

Comment:

This is a strong technical question to test JavaScript fundamentals. A clear and confident answer shows the candidate understands how inheritance and property access work in JS. Look for candidates who can explain:

• That all objects in JavaScript have a prototype
• How the prototype chain works
• Why it's useful (e.g., shared methods or memory efficiency)

Best practice tip: Follow up by asking for an example of modifying a prototype or explain how a class in JavaScript uses a prototype under the hood.

What are callbacks?

In programming, especially in JavaScript, a callback is a function that is passed as an argument to another function and is executed after the original function completes. This helps in handling tasks that run over time, like API calls, file operations, or events.

Example:

function processUserInput(callback) {
  let name = prompt('Please enter your name.');
  callback(name);
}

function greet(name) {
  alert('Hello ' + name);
}

processUserInput(greet);

Comment:

A callback is a function that gets called once another function finishes its job. Think of it as telling someone, “Let me know when you're done, and I’ll take it from there.”

From a hiring perspective, if you're assessing candidates for software development roles, especially in languages like JavaScript, ask them:

• "Can you explain how callbacks work in JavaScript?"
• "Give an example of a callback you've used recently."

Best practice: Look for candidates who can clearly explain what callbacks do, how they've used them in real-world code, and understand both synchronous and asynchronous behavior. Clarity and practical experience are key indicators of a skilled developer.

What are the types of errors in JavaScript?

When you're screening a candidate for a role that requires JavaScript skills, understanding how they approach debugging and problem-solving is key. Ask them: "What are the types of errors in JavaScript?" This question checks both their technical understanding and their attention to detail.

Comment:

There are two main types of errors in JavaScript:

• Syntax Errors – These happen when there’s a mistake in the way the code is written. Think of missing parentheses, misspelled keywords, or incorrect punctuation. These errors stop the script from running altogether.

• Logical Errors – These occur when the syntax is correct, but the code doesn’t behave as expected. The program runs, but the output is wrong due to flawed reasoning or wrong assumptions.

Best practice tip: Look for candidates who don’t just name the types, but also explain how these impact debugging. Strong answers mention using browser developer tools, `console.log()`, or linters to catch these errors early. This shows hands-on experience, not just textbook knowledge.

What is memoization?

Memoization is a programming technique used to speed up programs by storing the results of expensive function calls. When a function is called with a specific set of inputs, the result is saved. If the function is called again with the same inputs, the saved result is returned instead of recalculating it.

Comment:

This is a great technical question to evaluate a candidate's understanding of optimization and performance. The ideal answer should include:

• An explanation of caching and how memoization stores results.
• How it avoids redundant calculations by returning precomputed results.
• An example, like using memoization in recursion-heavy problems (e.g., the Fibonacci sequence).

Best Practice: Follow up by asking the candidate to provide a code example or real-world scenario where they have applied memoization. This helps reveal if they're discussing practical knowledge or just theory.

What is recursion in a programming language?

Recursion is a technique in programming where a function calls itself to solve smaller instances of a problem. It keeps repeating the process until it reaches a basic condition, known as the base case, that stops the function from calling itself again.

---

How to evaluate the answer:

When asking this question, you're checking if the candidate:

• Understands the concept of recursion.
• Can explain it in simple terms (clear communication matters).
• Mentions the base case and how recursion stops.
• Might give a coding example (like calculating factorial or Fibonacci sequence).

Best practice: Look for candidates who can explain recursion without diving too deep into technical jargon. Bonus points if they also mention why or when recursion is useful, such as solving problems that can be broken down into similar sub-problems.

This question works well to test logical thinking and how well someone understands fundamental coding concepts.

What is the use of a constructor function in JavaScript?

Constructor functions in JavaScript are used to create multiple objects with the same structure and behavior. They act like templates, allowing developers to define properties and methods that can be reused. This helps keep the code clean, organized, and efficient.

When asking this question in an interview, you're evaluating if the candidate understands object-oriented programming principles in JavaScript. A strong answer should mention object creation, usage of the `new` keyword, and how constructor functions help with reusability and maintainability.

---

Comment:

Constructor functions are used to create objects in JavaScript. If we want to create multiple objects having similar properties and methods, constructor functions are used.

What is DOM?

The DOM (Document Object Model) is a programming interface for web documents. It represents the page so that programs can change the document structure, style, and content. When a browser loads an HTML or XML page, it creates a live tree structure—this is the DOM.

Comment:

This is a good screening question for roles like front-end developers or full-stack engineers. A strong candidate should mention that the DOM is an object-based representation of the structure and elements of a webpage. Look for answers that touch upon:

• How browsers create the DOM tree from HTML
• Interaction with the DOM using JavaScript
• How DOM manipulation affects page rendering and performance

Best practice tip: Use follow-up questions to dive deeper—ask about virtual DOM vs. real DOM or how to efficiently manipulate the DOM for performance.

Which method is used to retrieve a character from a certain index?

The method used in JavaScript to retrieve a character at a specific index from a string is `charAt()`.

This function takes an index as an argument and returns the character at that index. Remember that JavaScript strings are zero-indexed, meaning the first character is at index 0, the second at 1, and so on, up to (length - 1).

Example:

let name = "Candidate";
console.log(name.charAt(0)); // Output: 'C'

---

Comment: The `charAt()` method of the JavaScript string finds a character element at the supplied index. The index number begins at 0 and continues up to n-1, where n is the string length.

When screening candidates for frontend or full-stack developer roles, this type of question checks for basic string manipulation skills, a must-have for many coding tasks. Ideally, candidates should not only name the `charAt()` method but also describe how indexing works in strings.

Best practice: Follow up by asking how they would handle out-of-bound indexes to evaluate their attention to error cases.

What do you mean by BOM?

When screening tech candidates, especially front-end developers, one great technical question to ask is: “What do you mean by BOM?”

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Best Answer Expectation: A strong candidate should clearly state that BOM stands for Browser Object Model. They should explain that BOM allows JavaScript to interact with the browser outside of the HTML document. The candidate should mention that the window object is the top-level object in BOM, and that it gives access to various components such as:

• `window.document` (DOM access)
• `window.history`
• `window.location`
• `window.navigator`
• `window.screen`, etc.

A confident explanation may include how BOM is different from DOM and may even point out how BOM features like `alert()`, `setTimeout()`, or navigation redirects work.

---

Red Flags:

• Confusing BOM with DOM
• Inability to name core components (like window, location, history, etc.)
• Vague or memorized answers without context

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Why it matters: Understanding BOM is key for front-end developers. It reflects if they grasp how JS interacts with the browser environment. This isn’t just about theory—it’s about how they control browser behavior through code. Always follow up with a simple scenario that puts BOM to use, like asking how they’d navigate to another page using JS.

Pro Tip: Good candidates don’t just explain what BOM is—they give practical examples of when and why they’ve used it.

💡 Remaining 445 questions...

The online article only includes the first 30 questions to keep it digestible, but we’ve put together an ebook for you with all the questions we gathered through our extensive research.

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Real-World Success Stories with JavaScript Interview Questions

Learning from industry experts and educational institutions can provide valuable insights into effective javascript interview questions screening strategies. Several notable case studies highlight different approaches to mastering and evaluating JavaScript skills.

Yash Modi from Hi-Tech Institute of Engineering and Technology (HIET) has developed a comprehensive approach to JavaScript preparation. Modi recommends that candidates focus on mastering key JavaScript topics, which can help them successfully tackle approximately 80% of JavaScript interviews. His research suggests that candidates should supplement their learning with quality resources like the techSith YouTube channel, which provides practical, hands-on examples.

Another interesting case comes from S. Krishnan, who created an innovative JavaScript script specifically for Quora user infographics. This project showcased practical applications of JavaScript skills through real-world examples, featuring profiles of industry leaders including Quincy Larson, Gayle Laakmann McDowell, and James Altucher. This case demonstrates how javascript interview questions often focus on practical implementation rather than theoretical knowledge alone.

The learning platform Skillcrush provides valuable perspective on the JavaScript learning journey. Their analysis reveals that while mastering JavaScript isn't the easiest programming challenge, it's certainly achievable with the right approach. They emphasize that persistent practice over several months typically leads to solid proficiency levels that can help candidates excel in technical interviews.

To support comprehensive skill development, H2K Infosys has designed an industry-expert-led Java course that complements JavaScript learning. This program equips developers with both web development and backend programming expertise, creating well-rounded candidates who can handle diverse javascript interview questions with confidence.

Why Video Screening Software is Revolutionizing JavaScript Interviews

The recruitment landscape is rapidly evolving, and video screening software is becoming the go-to solution for evaluating candidates' JavaScript skills. Traditional phone screenings often fall short when assessing technical abilities, especially for complex javascript interview questions that require visual problem-solving.

Modern recruiters are discovering that video screening allows them to:

The ability to watch candidates work through javascript interview questions provides invaluable insights that resume reviews simply cannot offer. Video screening captures not just the final answer, but the entire problem-solving journey – from initial analysis to final implementation.

Companies are reporting faster hiring cycles and higher-quality hires when incorporating video screening into their JavaScript recruitment process. The technology eliminates scheduling conflicts, reduces interviewer bias, and creates a more comfortable environment for nervous candidates to showcase their true abilities.

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