Preparing for an interview can be daunting particularly when preparing for javascript interview questions. you can approach it with confidence if you follow the proper strategy.
One of the most popular programming languages is JavaScript, and being proficient in its foundations is essential for acing technical interviews.
From the fundamentals to more complex ideas, we’ll take you through the key subjects you should learn and offer advice on how to ace the interview.
Fresher level Questions
What is JavaScript?
JavaScript is a high-level, interpreted programming language that is employed for generating interactive effects in web browsers. It is mainly employed for client-side scripting but can also be executed server-side with the help of Node.js.
What are the various data types in JavaScript?
- Primitive: String, Number, Boolean, Null, Undefined, Symbol, BigInt
- Non-primitive: Object
What is the difference between null and undefined?
- null: Denotes an explicit absence of any value.
- undefined: Is a declared variable but not yet assigned a value.
What is let and const in JavaScript?
- let: Defines a block-scoped variable that can be reassigned.
- const: Defines a block-scoped variable that cannot be reassigned.
What are arrays in JavaScript?
Arrays are ordered lists of elements that may contain many values. They may contain any data type and are zero-indexed.
What is the purpose of typeof operator?
The typeof operator is employed to determine the data type of a specified variable or expression.
What is a function in JavaScript?
A function is an area of reusable code that undertakes a function. Functions are called with the use of the function name and parentheses.
Write a function to determine whether a number is even or odd?
function isEvenOrOdd(number) {
return number % 2 === 0? 'Even' : 'Odd';
}Write a function to get the largest number from an array?
function findLargestNumber(arr) {
return Math.max(.arr);
}Write a function to reverse a string?
function reverseString(str) {
return str.split('').reverse().join('');
}Write a function to get the number of vowels in a string?
function countVowels(str) {
const vowels = ['a', 'e', 'i', 'o', 'u'];
let count = 0;
for (let char of str.toLowerCase()) {
if (vowels.includes(char)) count++;
}
return count;
}Write a function to calculate the factorial of a number?
function factorial(num) {
if (num === 0) return 1;
return num * factorial(num - 1);
}What are objects in JavaScript?
Objects are groups of properties, and each property is a key (or name) and a value. Objects are utilized to hold more complicated data.
What is the push() method in JavaScript?
The push() function inserts one or more items at the end of an array and returns the array’s new length.
What is JavaScript’s pop() method?
The pop() method takes the last element out of an array and returns the last element
Write a function to remove duplicate values from an array?
function removeDuplicates(arr) {
return [.new Set(arr)];
}Write a function to check if a string is a palindrome?
function isPalindrome(str) {
const reversed = str.split('').reverse().join('');
return str === reversed;
}Write a function to find the sum of numbers from 1 to N?
function sumOfNumbers(n)
return (n * (n + 1)) / 2;
}Create a function to capitalize the first letter of every word of a string?
function capitalizeWords(str) {
return str.split(' ').map(word => word.charAt(0).toUpperCase() + word.slice(1)).join(' ');
}Create a function to test whether or not a number is prime?
function isPrime(num) {
if (num <= 1) return false;
for (let i = 2; i <= Math.sqrt(num); i++) {
if (num % i === 0) return false;
}
return true;Intermediate Level Questions
What is a closure in JavaScript?
A closure is a function which keeps access to its lexical environment even after the function has been called outside the scope of the environment. It “remembers” the scope it was born in.
Define the difference between == and ===?
==: Equality operator compares for value equality but with type coercion.
===: Strict equality operator compares for both value and type equality without type coercion.
How can a variable be declared in JavaScript?
- var: Function-scoped or globally scoped.
- let: Block-scoped and reassignment allowed.
- const: Block-scoped and cannot be reassigned after initialization.
What is NaN in JavaScript?
NaN is short for “Not-a-Number” and is a special numeric value that signifies an invalid or unrepresentable number, for example, the result of 0/0.
How are null and undefined different?
null is a deliberate assignment of no value.
undefined indicates that a variable has been declared but not initialized.
What is the typeof operator used for?
The typeof operator is employed to return a string describing the type of a variable or expression (e.g., typeof 123 returns “number”).
What is an object in JavaScript?
An object is a group of properties, each with a key (or property name) and a value. Objects are used to store data and functions of different types.
What are arrays in JavaScript?
Arrays are array-like objects that can hold more than one value (of different types) in a single variable. They are indexed and support methods for iteration and manipulation
What is a closure in JavaScript?
A closure is the combination of a function and the lexical environment within which that function was declared.
In simpler terms, a closure gives a function access to variables from its outer (enclosing) function, even after the outer function has finished executing.
What is the Closure Scope Chain?
A closure scope chain refers to the series of scopes that a function has access to, starting from the most specific, which is its very own scope, and up to the global scope.
Every nested function creates a chain of access to scopes whereby the inner functions can access variables in their own scope, their outer function’s scope, and the global scope.
What do closures have to do with JavaScript functions such as event handlers?
Closures are useful in event-based programming, where the function that handles an event (like a click) can “remember” the state at the time it was created.
This allows us to attach dynamic behavior to elements while maintaining access to variables and functions defined earlier in the code.
Can closures cause memory leaks?
Yes, if not used carefully, closures can cause memory leaks. Since closures maintain references to their outer function’s scope, they can inadvertently prevent garbage collection, particularly if they reference large data structures. It’s important to manage closures properly to avoid this issue.
How do closures help with data encapsulation?
Closures allow us to have private variables and functions. For example, we can create with closures a function that has private state and only expose public methods that interact with that state.
It is the very basis of encapsulation in object-oriented programming
What is lexical scoping?
Lexical scoping refers to the way JavaScript resolves variable names based on the location where they are declared in the source code.
A nested function has access to variables in its own scope, its outer (enclosing) function’s scope, and global scope
Can closures be used to create private methods in JavaScript?
Yes! JavaScript doesn’t have native private methods (until ES2022), but closures allow you to simulate private methods by keeping them inside a function and exposing public methods to interact with them.
For example
const counter = (function() {
let privateCount = 0;
return {
increment: function() {
privateCount++;
},
decrement: function() {
privateCount--;
},
getCount: function() {
return privateCount;
}
};
})();
counter.increment();
console.log(counter.getCount()); // 110. What are some real-life examples of closures?
- Event handling: Closures allow event handlers (such as click or hover) to “remember” the context or data when they are created.
- Function factories: Closures are great for creating functions on the fly, with specific data that needs to be “remembered” and used later (e.g., makeAdder in the examples).
- Emulating private data: As mentioned, closures can simulate private variables and methods, a pattern commonly used in JavaScript modules.
What does a closure do if it attempts to reach out to a variable after its enclosing function has already returned?
A closure may still reach variables that are inside the outer function. This is true because JavaScript still holds onto a reference for the lexical environment of the closure so that the variables can still be accessed even if the enclosing function has completed.
What is the DOM in JavaScript?
The DOM (Document Object Model) is a way for JavaScript to manipulate the style, structure, and content of HTML documents.
What is event delegation?
Event delegation is a method where an event listener is placed on a parent element to handle events on child elements for performance improvement.
What is the difference between synchronous and asynchronous programming?
Synchronous: Code runs sequentially, statement by statement.
Asynchronous: The code is free to run separate from the general program flow so that other activities can be undertaken.
Advance level Questions
Define bind() method?
bind() defines a new function that, upon invocation, will have its this keyword fixed and the specified arguments pre-fixed.
How are slice() and splice() different?
- slice(): Creates a shallow copy of an array section.
- splice(): Alters the content of an array by deleting or swapping elements.
How do you verify if an object is an array?
You can utilize Array.isArray() method: Array.isArray(obj)
What is the spread operator (.) in JavaScript?
The spread operator enables iterables (such as arrays or objects) to be expanded into single elements or properties.
What is destructuring in JavaScript?
Destructuring enables you to unpack values from arrays or objects into separate variables.
What is a JavaScript module?
A module is a JavaScript file and can export values, objects, or functions that can be imported into other files.
What is localStorage in JavaScript?
localStorage permits storing data locally in the browser that remains when the browser closes.
What is the difference between localStorage and sessionStorage?
- localStorage: Data remains after the browser closes.
- sessionStorage: Stores data for the lifetime of the page session and deletes it when the page is closed.
what are IIFE (Immediately Invoked Function Expressions)?
IIFE is an expression that functions immediately after being defined.
Example: (function() { console.log(‘Hello!’); })();
What is the typeof operator used for?
typeof is employed to get the data type of a variable or an expression.
What is a JavaScript class?
A class is a template to create objects with common properties and methods. It was added in ES6.
What is the difference between a function and a method in JavaScript?
A method is an object-bound function, whereas a function stands alone.
What are template literals in JavaScript?
Template literals provide the ability to insert expressions into string literals using backticks.
Example: `Hello, ${name}`.
Describe the setTimeout() function?
setTimeout() schedules a function or code block to be called after a given number of milliseconds.
What is JavaScript’s map() method?
map() returns a new array filled with the return value of calling a given function on each element in the calling array.
What is the difference between map() and forEach()?
- forEach(): Calls a function for each element, but doesn’t return anything.
- map(): Calls a function for each element and returns a new array.
What is JavaScript’s reduce()?
reduce() takes a function to apply to the accumulator and each element in the array (left to right) to condense it into a single value.
What is a promise chain?
A promise chain is when there are several promises chained together, with each one being called after the last one.
What is the event loop in JavaScript?
The event loop is a system used to process asynchronous code execution by checking the call stack and message queue.
What are closures, and how do they function?
Closures are functions that maintain references to variables in their lexical scope, even after the outer function has already returned.
Describe the concept of hoisting in JavaScript?
Hoisting is JavaScript’s standard behavior of hoisting declarations to the top of the current scope prior to code execution.
What is currying in JavaScript?
Currying is a functional programming technique where a function is decomposed into a sequence of unary functions.
Describe the meaning of this in the global scope?
In the global scope, this is the global object (window in browsers, global in Node.js).
What is a higher-order function in JavaScript?
Higher-order function is a function that either accepts another function as an argument or returns a function, or both.
Why is Object.freeze() used in JavaScript?
Object.freeze() prohibits modification of the properties and values of an object.
What is the Proxy object in JavaScript?
Proxy is an object that can wrap another object or function and enable you to intercept and control its behavior.
What is Symbol used for in JavaScript?
Symbol is an immutable and unique data type, commonly used to generate unique property keys.
How are call(), apply(), and bind() different?
call(): Calls a function with a particular this value and arguments provided individually.
apply(): Like call(), but arguments are provided as an array.
bind(): Returns a new function with a particular this value and arguments.
Why do loops act differently in relation to closures than I’d intuitively anticipate?
Prior to ES6, closures in loops tended to cause surprising behavior since variables that were declared with var are function-scoped, not block-scoped.
As a result, closures that were generated within loops would end up with the same reference to the loop variable, and all the closures would have references to the last value of that variable.
To correct this, you can use let (block-scoped) within the loop or define an IIFE (Immediately Invoked Function Expression) to establish a new scope for every loop.
for (let i = 0; i < 3; i++) {
setTimeout(function() {
console.log(i); // Logs correctly 0, 1, and 2
}, 1000);
}How do closures assist in the construction of callback functions?
Closures are commonly employed when working with asynchronous code, for example, with callbacks in event handling or setTimeout.
Closures enable the callback functions to keep access to the variables they require, even after the execution context has altered.
What are some performance considerations when using closures?
Use closures judiciously since every closure has its own environment, and that has overhead.
Avoid unnecessary closures in performance-sensitive applications where possible, like in object constructor methods or function definitions inside loops.
How do I prevent common closure-related errors?
Use let or const rather than var to define block-scoped variables, particularly in loops.
Be mindful of memory usage and avoid unnecessary closures when not needed.
For event handling or when passing data to callbacks, consider using function factories or using bind to preserve context.
What is a generator function in JavaScript?
A generator function is a special kind of function that returns multiple values over time with the help of the yield keyword.
What is an async function in JavaScript?
An async function always returns a promise. It enables the usage of await within to wait for asynchronous operations.
What are WeakMap and WeakSet in JavaScript?
- WeakMap: Set of key-value pairs with objects as keys, and values may be any type. The keys are weakly referenced.
- WeakSet: Set of unique objects, weakly referenced.
What does setImmediate() do in Node.js?
SetImmediate() is called to run a function on the next event loop iteration, after the current one.
Describe the async/await pattern?
Async/await is a syntax to work with promises, making the asynchronous code simpler to read and write.
What is a service worker in JavaScript?
A service worker is a background script, independent of the main browser thread, where you can cache content and manage network requests.
What is the difference between new and new Object()?
new: Creates a new object instance from a constructor.
new Object(): Creates an empty object directly.
What are set and map in JavaScript?
- set: An array of unique values.
- map: An unordered collection of key-value pairs, with keys of any value.
What is requestAnimationFrame() used for in JavaScript?
requestAnimationFrame() calls a function before the next repaint, commonly used for animations.
How does JavaScript manage memory?
JavaScript manages memory through automatic garbage collection, freeing up memory no longer in use or accessible.
Write a function to flatten a nested array?
for (let i = 0; i < 3; i++) {
setTimeout(function() {
console.log(i); // Logs correctly 0, 1, and 2
}, 1000);
}Write a function to merge two sorted arrays into one sorted array.
function mergeSortedArrays(arr1, arr2) {
let result = [];
let i = 0, j = 0;
while (i < arr1.length && j < arr2.length) {
if (arr1[i] < arr2[j]) {
result.push(arr1[i]);
i++;
} else {
result.push(arr2[j]);
j++;
}
}
return result.concat(arr1.slice(i), arr2.slice(j));
}Write a function to sort an array of objects based on a key.
function sortArrayByKey(arr, key) {
return arr.sort((a, b) => a[key] > b[key] ? 1 : -1);
}Write a function to find the missing number in an array containing numbers from 1 to N.
function sortArrayByKey(arr, key) {
return arr.sort((a, b) => a[key] > b[key] ? 1 : -1);
}Write a function to get the most frequent element in an array.
function mostFrequentElement(arr) {
const frequencyMap = {};
let maxCount = 0;
let mostFrequent = null;
for (let num of arr) {
frequencyMap[num] = (frequencyMap[num] || 0) + 1;
if (frequencyMap[num] > maxCount) {
maxCount = frequencyMap[num];
mostFrequent = num;
}
}
return mostFrequent;
}Write a function to calculate the Fibonacci sequence up to the Nth number.
function fibonacci(n) {
const sequence = [0, 1];
for (let i = 2; i < n; i++) {
sequence.push(sequence[i - 1] + sequence[i - 2]);
}
return sequence;
}Write a function to find the intersection of two arrays.
function arrayIntersection(arr1, arr2) {
return arr1.filter(value => arr2.includes(value));
}Write a function to get the longest word in a string.
function longestWord(str) {
const words = str.split(' ');
return words.reduce((longest, word) => word.length > longest.length ? word : longest, '');
}Write a function that implements bind() without using a built-in function.
function longestWord(str) {
const words = str.split(' ');
return words.reduce((longest, word) => word.length > longest.length ? word : longest, '');
}Write a function that implements debounce?
function debounce(func, delay) {
let timer;
return function(...args) {
clearTimeout(timer);
timer = setTimeout(() => func.apply(this, args), delay);
};
}Write a function to implement a deep cloning of the object?
function deepClone(obj) {
if (null === obj || typeof obj !== 'object') return obj;
let clone = Array.isArray(obj) ? [] : {};
for (let key in obj) {
if (obj.hasOwnProperty(key)) {
clone[key] = deepClone(obj[key]);
}
}
return clone;
}Write a function to implement throttle() functionality.
function throttle(func, limit) {
let inThrottle;
return function(...args) {
if (!inThrottle) {
func.apply(this, args);
inThrottle = true;
setTimeout(() => inThrottle = false, limit);
}
};
}Write a function to generate all permutations of a string.
function throttle(func, limit) {
let inThrottle;
return function(...args) {
if (!inThrottle) {
func.apply(this, args);
inThrottle = true;
setTimeout(() => inThrottle = false, limit);
}
};
}Write a function to detect a cycle in a linked list?
function throttle(func, limit) {
let inThrottle;
return function(...args) {
if (!inThrottle) {
func.apply(this, args);
inThrottle = true;
setTimeout(() => inThrottle = false, limit);
}
};
}Write a function to implement the bind() method using closures?
Function.prototype.myBind = function(context) {
const fn = this;
const args = Array.from(arguments).slice(1);
return function() {
return fn.apply(context, args.concat(Array.from(arguments)));
};
};How to Prepare for a JavaScript Interview
JavaScript Basics
- Get to know variables, types of variables, kinds of data in JavaScript with symbols, number, strings, boolean, null, undefined, and symbol.
- Control Constructs: Study switch statements, branching or if-else statements.
- Functions: Think of function declarations, expressions, arrow functions, and closures.
- Scope and Closure: Make clear the difference between local and global scopes. Understand lexical scoping and closures.
- Asynchronous Programming: Get their heads around callbacks, promises, async/await, and the way to handle asynchronous operations.
- Get aquaintedwith: JavaScript ES6+ Features:
- Arrow functions: Their syntax and Bac before you know it sich methods creatures; work whenever you have to.
- Destructuring: Object destructuring, array destructuring, and making working with data much easier.
- Spread and rest operators: How to use each and What’s the difference between them.
- Modules: Using import and export.
- Promises & async/await: Handling asynchronous code in conjunction with chaining.
Walk deep into JavaScript objects and arrays
- Objects: Know about creating, manipulating, and iterating low on objects.
- Arrays: Review methods like map, filter, etc.
- Prototype and inheritance: Prototype chains work in JavaScript.
- Soft and Contextual Skills
- Communication: Work on clearly and methodically describing your thought process.
- Problem-Solving Method: Dissect issues into manageable chunks and explain your strategy.
Get Ready for Typical JavaScript Interview questions
- Explain closures in JavaScript.
- What are promises, and how do they work?
- How does JavaScript handle asynchronous code?
- What is event delegation?
- What is the difference between null and undefined?
Key Takeaways
- Master the primary concepts of JavaScript, including variables, methods, and data structures.
- Toward being well-acquainted with modern ES6 features such as arrow functions, destructuring, and async/await.
- Solve coding problems based mainly on algorithms and data structures to be well-practiced.
- Knowledge of time complexity (Big O notation) analysis is also necessary.
- Proficiency in document object manipulation, event-handling, and browser APIs.
- All this would converge to optimal communication skills to best articulate your thought process.
- Mock interviews with a realistic interviewing background would be good practice.
- Stay current with JavaScript trends and new features to be well-rounded readers.
- Understand most problem-solving approaches applicable to real-life situations.
- Be confident and ready to demonstrate your JavaScript knowledge.