Top Javascript Interview Questions and answer

1.) What is JavaScript?

JavaScript is a high-level, interpreted programming language primarily used for building interactive web pages and web applications. It provides dynamic functionality, enables client-side and server-side scripting, and can be used alongside HTML and CSS to create dynamic web content.

2.) What are the different data types in JavaScript?

JavaScript has six primitive data types: string, number, boolean, null, undefined, and symbol (added in ECMAScript 6). Additionally, there is one complex data type: object, which includes arrays, functions, and objects.

3.) What is the difference between null and undefined in JavaScript?

• null is an assignment value that represents no value or an empty value intentionally assigned by the developer. It is of the object type.
• undefined means a variable has been declared but has not been assigned a value. It is of an undefined type.

4.) What is the use of this keyword in JavaScript?

This keyword refers to the context in which a function is called. Its value depends on how the function is invoked.

5.) Explain the concept of closures in JavaScript.

 A closure is the combination of a function bundled together (enclosed) with references to its surrounding state (the lexical environment). In other Words, a closure gives you access to an outer function’s scope from an inner function. 

function init() {
  var name = "Mozilla"; // name is a local variable created by init
  function displayName() {
    // displayName() is the inner function, that forms the closure
    console.log(name); // use variable declared in the parent function
  }
  displayName();
}
init();

6.) What are the different ways to declare variables in JavaScript?

Variables can be declared using var, let, or const. var has function scope, let has block scope, and const is used to declare constants.

7.) What is hoisting in JavaScript?

Hoisting is a JavaScript behavior where variable and function declarations are moved to the top of their containing scope during the compilation phase, allowing them to be used before they are declared.

8.) Explain the concept of event bubbling and event capturing.

Event bubbling is the process where an event triggered on a nested element is first handled by the innermost element and then propagated to its parent elements. Event capturing is the opposite, where the event is first captured by the outermost element and then propagated inward.

9.) What are arrow functions in JavaScript?

Arrow functions are a shorthand syntax for writing function expressions in JavaScript. They have a concise syntax and do not bind their own this, arguments, super, or new.target. Arrow functions are particularly useful in scenarios where you want to preserve the lexical scope of this from the surrounding code.

10.) What is the difference between == and === operators?

• == is the equality operator and performs type coercion, meaning it converts the operands to a common type before making the comparison. For example, 5 == ‘5’ will return true.
• === is the strict equality operator and does not perform type coercion. It checks for both value and type equality. For example, 5 === ‘5’ will return false.

11.) What are the different data types in JavaScript?

1. Number: represents numeric values.
2. String: represents textual data.
3. Boolean: represents true or false values.
4. Object: represents a collection of key-value pairs.
5. Array: represents an ordered list of values.
6. Null: represents the absence of any value.
7. Undefined: represents a variable that has been declared but not assigned a value.

12.) What is event delegation in JavaScript?

Event delegation is a technique in JavaScript where you attach a single event listener to a parent element, instead of attaching multiple event listeners to individual child elements. When an event occurs on a child element, the event bubbles up to the parent element, where the single event listener handles it. This approach is useful when you have a large number of dynamically created elements or when efficiency is a concern.

13.) What are the different ways to handle asynchronous operations in JavaScript?

There are several ways to handle asynchronous operations in JavaScript:

1. Callbacks: Functions that are passed as arguments to another function and are executed when the asynchronous operation completes.
2. Promises: Objects that represent the eventual completion (or failure) of an asynchronous operation, allowing you to attach callbacks to handle the results.
3. Async/await: A modern syntax that allows writing asynchronous code using a synchronous style, making it easier to read and maintain.
4. Observables: A pattern used to handle asynchronous operations and data streams. It provides powerful operators for manipulating and transforming data.

14.) Explain the concept of prototypal inheritance in JavaScript.

 JavaScript, objects can inherit properties and methods from other objects. This is achieved through prototypal inheritance. Each object has an internal link to another object called its prototype. If a property or method is not found on the object itself, JavaScript looks up the prototype chain until it finds the property or reaches the end of the chain.

15.) What are the differences between “call”, “apply”, and “bind” in JavaScript?

“call” and “apply” are used to invoke a function with a specified context (the value of “this”) and arguments. The difference is how arguments are passed. With “call”, arguments are passed individually, while with “apply”, arguments are passed as an array.

“bind” is used to create a new function with a specified context (“this”) and any initial arguments. It does not immediately invoke the function but returns a new function that can be called later.

16.)What is the “this” keyword in JavaScript?

The “this” keyword refers to the context in which a function is called. It can have different values depending on how the function is invoked. In a method, “this” refers to the object that the method belongs to. In a regular function, “this” refers to the global object (window in the browser, global in Node.js) or can be undefined in strict mode.

17.) Explain the concept of promises in JavaScript.

Promises are objects that represent the eventual completion (or failure) of an asynchronous operation. They allow you to attach callbacks to handle the results once the operation is completed. Promises provide a more structured way to handle asynchronous code compared to callbacks, making it easier to chain and manage asynchronous tasks.

Example:

// Function that returns a promise to simulate fetching data
function fetchData() {
  return new Promise((resolve, reject) => {
    // Simulating an asynchronous operation (e.g., fetching data from an API)
    setTimeout(() => {
      const data = { message: "Data fetched successfully!" };
      // Resolve the promise with the data
      resolve(data);
      // Or reject the promise if an error occurs
      // reject(new Error("Failed to fetch data"));
    }, 2000); // Simulating a delay of 2 seconds
  });
}

// Using the fetchData function that returns a promise
fetchData()
  .then((result) => {
    // Handle the resolved data
    console.log(result.message);
  })
  .catch((error) => {
    // Handle any errors that occurred during the operation
    console.error("Error fetching data:", error);
  });

18.) What is the difference between synchronous and asynchronous code?

• Synchronous code is executed in sequence, blocking further execution until the current operation completes. It waits for each operation to finish before moving to the next one.
• Asynchronous code allows multiple operations to be performed concurrently. It doesn’t wait for a single operation to finish and instead continues executing other tasks. Completion of asynchronous operations is typically handled through callbacks, promises, or async/await syntax.

19.) What are generators in JavaScript?

Generators are functions that can be paused and resumed. They use the function* syntax and yield values using the yield keyword. When a generator is called, it returns an iterator that can be used to control the execution of the generator function.

20.) What is the event loop in JavaScript?

• The Event Loop is a mechanism in JavaScript that handles asynchronous callbacks and ensures they are executed in a non-blocking manner. It consists of two main components: the call stack and the task queue.
• When an asynchronous operation is completed, its callback is placed in the task queue.
• The Event Loop continuously checks if the call stack is empty. If so, it takes the first callback from the task queue and pushes it onto the call stack for execution.

21.) How do you declare variables in JavaScript?

Variables in JavaScript are declared using the var, let, or const keywords. var and let allow variable reassignment, while const creates a read-only reference to a value that cannot be reassigned.

22.) What is the difference between let, const, and var?

var has function scope and can be accessed globally or locally within a function. It can be redeclared and reassigned.

let and const have block scope and are limited to the block in which they are declared. They have a block-level scope and are not hoisted. let allows variable reassignment, while const creates a read-only reference that cannot be reassigned.

23.) How do you check the data type of a variable in JavaScript?

You can use the typeof operator to check the data type of a variable. For example: typeof myVariable will return a string representing the data type of myVariable.

24.) How do you create an array in JavaScript?

An array in JavaScript can be created using square brackets [] and comma-separated values. For example: let myArray = [1, 2, 3];

25.) What is the difference between call(), apply(), and bind() methods?

The call() method invokes a function with a specified value and arguments provided individually.
The apply() method is similar to call(), but it accepts arguments as an array or an array-like object.
The bind() method creates a new function that, when called, has this value set to a specific value, and any arguments are prepended to the function call.

26.) What are the differences between JavaScript map(), filter(), and reduce() methods?

• The map() method creates a new array by applying a function to each element of an existing array.
• The filter() method creates a new array by filtering out elements from an existing array based on a condition specified in a function.
• The reduce() method reduces an array to a single value by applying a function to each element and accumulating the result.

27.) What are Promises in JavaScript, and how do they work?

• Promises are objects that represent the eventual completion or failure of an asynchronous operation and allow the handling of asynchronous code in a more organized manner.
• A Promise has three states: pending, fulfilled, or rejected. It transitions from pending to either fulfilled or rejected based on the result of the asynchronous operation.
• Promises can be chained using the .then() method and handled using .catch() for error handling. They also support the newer async/await syntax for a more synchronous-like code flow.

28.) How do you create a promise?

To create a promise, you can use the Promise constructor, which takes a callback function as an argument. The callback function, often referred to as the executor, takes two parameters: resolve and reject. Inside the executor, you perform the asynchronous operation and call either resolve(value) when the operation succeeds or reject(error) when it fails.

const promise = new Promise((resolve, reject) => {
  // Asynchronous operation
  // Call resolve(value) when successful
  // Call reject(error) when failed
});

29.) How do you handle the result of a promise?

You can handle the result of a promise using the then() method. It takes two optional callbacks as arguments: onFulfilled and onRejected. The onFulfilled callback is executed when the promise is fulfilled (resolved), and the onRejected callback is executed when the promise is rejected.

promise.then(
  (result) => {
    // Handle the fulfillment (success) case
  },
  (error) => {
    // Handle the rejection (error) case
  }
);

30.) What is the purpose of the catch() method in promises?

The catch() method is used to handle any errors that occur during the execution of a promise or in any preceding then() callback. It allows you to specify a callback function that will be called when an error occurs, simplifying error handling in promise chains.

promise
  .then((result) => {
    // Handle the fulfillment (success) case
  })
  .catch((error) => {
    // Handle any errors that occur in the promise chain
  });

31.) How can you handle multiple promises concurrently?

To handle multiple promises concurrently, you can use Promise.all(). It takes an array of promises as an argument and returns a new promise. The new promise is fulfilled when all the input promises are fulfilled, and the resulting values are collected in an array. If any of the input promises are rejected, the new promise is immediately rejected with the corresponding error.

const promise1 = someAsyncOperation();
const promise2 = anotherAsyncOperation();

Promise.all([promise1, promise2])
  .then(([result1, result2]) => {
    // Handle the fulfillment (success) case for both promises
  })
  .catch((error) => {
    // Handle any error that occurs in any of the promises
  });

32.) What is ES6?

ES6, also known as ECMAScript 2015, is the sixth major edition of the ECMAScript standard for JavaScript. It introduced significant enhancements and new features to the language, improving developer productivity and code maintainability.

33.) What are some key features introduced in ES6?

• Arrow functions: A concise syntax for writing function expressions.
• Block-scoped variables: let and const for block-level variable scoping.
• Template literals: A new way to define strings with interpolation and multiline support.
• Enhanced object literals: Shorthand syntax for defining objects and defining methods.
• Destructuring assignment: An efficient way to extract values from arrays or objects.
• Default function parameters: Ability to set default values for function parameters.
• Rest and spread operators: … syntax to gather or spread elements in arrays or function arguments.
• Modules: A standardized module system for organizing and sharing code.
• Promises: A built-in mechanism for handling asynchronous operations.
• Classes: Syntactic sugar for defining classes and inheritance.
• Arrow functions: Concise syntax for writing function expressions.
• Iterators and generators: A way to define custom iteration behavior.
• Map and Set data structures: New built-in data structures for handling collections.

34.) How can you transpile ES6 code to ES5 for browser compatibility?

To transpile ES6 code to ES5, you can use tools like Babel. Babel is a popular JavaScript compiler that can convert ES6 (and newer) code into ES5 code, ensuring compatibility with older browsers. It allows you to write modern JavaScript syntax while targeting older environments.

35.) What is a Proxy in ES6?

A Proxy is an object introduced in ES6 that allows you to intercept and customize fundamental operations on other objects, such as property access, assignment, function invocation, and more. By defining custom behavior for these operations, you can create powerful mechanisms like data validation, logging, and more. Proxies provide a high level of control and flexibility over object interactions.

36.) How does the spread operator (…) work in ES6?

The spread operator (…) in ES6 is used to expand an iterable (e.g., an array or a string) into individual elements. It can be used in function calls, array literals, object literals, and more. When used in function calls, it allows passing individual elements of an array as separate arguments. In array literals, it can concatenate arrays or clone them. In object literals, it can merge multiple objects together.

// Spread operator in function calls
const numbers = [1, 2, 3];
console.log(...numbers); // Outputs: 1 2 3

// Spread operator in array literals
const array1 = [1, 2, 3];
const array2 = [4, 5, 6];
const mergedArray = [...array1, ...array2];
console.log(mergedArray); // Outputs: [1, 2, 3, 4, 5, 6]

// Spread operator in object literals
const obj1 = { a: 1, b: 2 };
const obj2 = { c: 3, d: 4 };
const mergedObject = { ...obj1, ...obj2 };
console.log(mergedObject); // Outputs: { a: 1, b: 2, c: 3, d: 4 }

37.) What is a JavaScript function?

A JavaScript function is a block of reusable code designed to perform a specific task. It can be defined using the function keyword and can take input parameters and return a value. Functions are used to organize code, make it more modular, and promote code reuse.

38.) How do you define a JavaScript function?

A JavaScript function can be defined using the function keyword followed by the function name, a pair of parentheses for parameters (optional), and curly braces to enclose the function body. Here’s an example of a function that adds two numbers:

function addNumbers(a, b) {
  return a + b;
}

39.) What is the difference between function declarations and function expressions?

A function declaration is defined using the function keyword followed by the function name, while a function expression assigns a function to a variable. The main difference is those function declarations are hoisted, meaning they can be called before they are declared, whereas function expressions must be defined before they are called.

// Function declaration
function greet() {
  console.log("Hello!");
}

// Function expression
var sayHello = function() {
  console.log("Hello!");
};

40.) What is the purpose of the return statement in a function?

The return statement is used to specify the value that a function should return. When a return statement is encountered in a function, the function stops executing and returns the specified value to the caller. If no return statement is used or if it doesn’t specify a value, the function returns undefined.

41.) How do you pass arguments to a JavaScript function?

Arguments can be passed to a JavaScript function by including them inside the parentheses when the function is called. These arguments can then be accessed within the function through the function’s parameter names.

function greet(name) {
  console.log("Hello, " + name + "!");
}

greet("Alice");  // Output: Hello, Alice!

42.) What is the difference between parameters and arguments in a function?

Parameters are the named variables defined in a function’s declaration, while arguments are the actual values passed to the function when it is called. Parameters act as placeholders for the values that will be passed as arguments.

function addNumbers(a, b) {  // a and b are parameters
  return a + b;
}

addNumbers(5, 3);  // 5 and 3 are arguments

43.) What are the different types of functions in JavaScript?

In JavaScript, there are several types of functions, including:

1. Named Functions: Functions declared with a name and can be called using that name.
2. Anonymous Functions: Functions without a name and are typically assigned to variables or used as callback functions.
3. Arrow Functions: A concise syntax introduced in ES6, using the => arrow notation, commonly used for shorter function expressions.
4. IIFE (Immediately Invoked Function Expressions): Functions that are immediately executed after they are defined, often used for creating private scopes.
5. Generator Functions: Functions that can be paused and resumed, allowing multiple values to be generated over time using the yield keyword.
6. Constructor Functions: Functions used as templates for creating objects using the new keyword.
7. Callback Functions: Functions passed as arguments to other functions, to be invoked at a later time.
8. Higher-Order Functions: Functions that can accept other functions as arguments or return functions as their result.

44.) Explain the difference between regular functions and arrow functions in JavaScript.

The main differences between regular functions and arrow functions are as follows:

1. Syntax: Arrow functions have a more concise syntax using the => arrow notation, while regular functions use the function keyword.
2. Context (this): Regular functions have their own this context, determined by how they are called, while arrow functions inherit the this context from their surrounding scope.
3. Arguments object: Regular functions have their own arguments object, which contains all the arguments passed to the function, while arrow functions do not have their own arguments object.
4. Binding: Regular functions can be bound to a specific context using methods like bind(), call(), or apply(), while arrow functions cannot be bound explicitly.

45.) What is the purpose of Immediately Invoked Function Expressions (IIFE)?

An IIFE is a function that is executed immediately after it is defined. It is commonly used to create a private scope and avoid polluting the global namespace and is often used to encapsulate code and variables, allowing for modular and reusable code.

(function() {
  var message = "Hello!";
  console.log(message);
})();

// Output: Hello!

46.) What is a regular function in JavaScript?

A regular function in JavaScript is defined using the function keyword. It can be invoked using the function name followed by parentheses.

function greet(name) {
  console.log("Hello, " + name + "!");
}

greet("John"); // Output: Hello, John!

47.) What is an anonymous function in JavaScript?

An anonymous function in JavaScript is a function without a specified name. It is typically used as a callback function or assigned to a variable for later use.

var sum = function(a, b) {
  return a + b;
};

console.log(sum(5, 3)); // Output: 8

48.) What is an arrow function in JavaScript?

An arrow function in JavaScript is a concise syntax for writing function expressions. It provides a shorter syntax compared to regular functions and lexically binds the this value.

var multiply = (a, b) => a * b;

console.log(multiply(4, 5)); // Output: 20

49.) What is a generator function in JavaScript?

A generator function is a special type of function that can be paused and resumed. It uses the yield keyword to produce a sequence of values over time.

function* countDown(from) {
  while (from > 0) {
    yield from;
    from--;
  }
}

var countdownIterator = countDown(5);
console.log(countdownIterator.next().value); // Output: 5
console.log(countdownIterator.next().value); // Output: 4
console.log(countdownIterator.next().value); // Output: 3
// ...

Some basic Javascript code Question and answer

1.) Write a JavaScript function to find the largest of three numbers.

function findLargestNumber(num1, num2, num3) {
  return Math.max(num1, num2, num3);
}

console.log(findLargestNumber(5, 9, 3));  // Output: 9

2.) Write a JavaScript program to check if a number is prime or not.

function isPrime(number) {
  if (number <= 1) {
    return false;
  }

  for (let i = 2; i <= Math.sqrt(number); i++) {
    if (number % i === 0) {
      return false;
    }
  }

  return true;
}

console.log(isPrime(7));  // Output: true
console.log(isPrime(4));  // Output: false

3.) Write a JavaScript program to check if a string is a palindrome.

function isPalindrome(str) {
  const reversedStr = str.split('').reverse().join('');
  return str === reversedStr;
}

console.log(isPalindrome('level'));   // Output: true
console.log(isPalindrome('hello'));   // Output: false

4.) Write a JavaScript function to calculate the factorial of a number.

function factorial(number) {
  if (number === 0 || number === 1) {
    return 1;
  }

  let result = 1;
  for (let i = 2; i <= number; i++) {
    result *= i;
  }

  return result;
}

console.log(factorial(5));   // Output: 120
console.log(factorial(0));   // Output: 1

5.) Write a JavaScript program to reverse a string.

function reverseString(str) {
  return str.split('').reverse().join('');
}

console.log(reverseString('hello'));   // Output: olleh

6.) Write a JavaScript function to remove duplicate elements from an array.

function removeDuplicates(array) {
  return [...new Set(array)];
}

const numbers = [1, 2, 3, 3, 4, 4, 5];
console.log(removeDuplicates(numbers));   // Output: [1, 2, 3, 4, 5]

7.) Write a function to reverse a string in JavaScript.

function reverseString(str) {
  return str.split('').reverse().join('');
}

// Example usage:
console.log(reverseString('hello')); // Output: "olleh"

8.) Write a function to check if a given string is a palindrome in JavaScript.

function isPalindrome(str) {
  const reversedStr = str.split('').reverse().join('');
  return str === reversedStr;
}

// Example usage:
console.log(isPalindrome('madam')); // Output: true

9.) Write a function to find the maximum number in an array in JavaScript.

function findMaxNumber(arr) {
  return Math.max(...arr);
}

// Example usage:
console.log(findMaxNumber([2, 5, 1, 9, 3])); // Output: 9

10.) Write a function to remove duplicate values from an array in JavaScript.

function removeDuplicates(arr) {
  return [...new Set(arr)];
}

// Example usage:
console.log(removeDuplicates([1, 2, 3, 2, 4, 1])); // Output: [1, 2, 3, 4]

11.) Write a function to remove falsy values (false, null, 0, “”, undefined, and NaN) from an array in JavaScript.

function removeFalsyValues(arr) {
  return arr.filter(Boolean);
}

// Example usage:
console.log(removeFalsyValues([0, false, null, 1, 2, "", "hello"])); // Output: [1, 2, "hello"]

12.) Write a function to flatten a nested array in JavaScript.

function flattenArray(arr) {
  return arr.reduce((flat, current) => {
    return flat.concat(Array.isArray(current) ? flattenArray(current) : current);
  }, []);
}

// Example usage:
console.log(flattenArray([1, [2, [3, 4], 5], 6])); // Output: [1, 2, 3, 4, 5, 6]

13.) Implement a function to check if a given string is an anagram in JavaScript.

function isAnagram(str1, str2) {
  const sortedStr1 = str1.toLowerCase().split('').sort().join('');
  const sortedStr2 = str2.toLowerCase().split('').sort().join('');
  return sortedStr1 === sortedStr2;
}

// Example usage:
console.log(isAnagram('listen', 'silent')); // Output: true

14.) Write a function to reverse the order of words in a given sentence in JavaScript

function reverseWords(sentence) {
  return sentence.split(' ').reverse().join(' ');
}

// Example usage:
console.log(reverseWords('Hello World')); // Output: "World Hello"

15.) Implement a function to find the Fibonacci sequence up to a given number in JavaScript.

function fibonacciSequence(num) {
  const sequence = [0, 1];
  while (sequence[sequence.length - 1] < num) {
    const nextNum = sequence[sequence.length - 1] + sequence[sequence.length - 2];
    sequence.push(nextNum);
  }
  return sequence.slice(0, -1);
}

// Example usage:
console.log(fibonacciSequence(50)); // Output: [0, 1, 1, 2, 3, 5, 8, 13, 21, 34]

16.) Implement a function to count the occurrence of each character in a string in JavaScript.

function countCharacters(str) {
  const charCount = {};
  for (let char of str) {
    charCount[char] = (charCount[char] || 0) + 1;
  }
  return charCount;
}

// Example usage:
console.log(countCharacters('hello')); // Output: { h: 1, e: 1, l: 2, o: 1 }