Real interview questions from Meesho company. Includes theoretical concepts and coding problems.
What is the difference between null and undefined in JavaScript?
Null and undefined are both primitive values in JavaScript, but they have different meanings. Null represents the absence of a value, while undefined represents an uninitialized variable or a variable that has not been declared.
What is a closure in JavaScript?
A closure is a function that has access to its own scope and the scope of its outer functions, even when the outer functions have returned. This allows the inner function to use variables from the outer functions, even when the outer functions are no longer executing.
What is the difference between the '==' and '===' operators in JavaScript?
The '==' operator checks for loose equality, which means it checks if the values are equal, but not necessarily of the same type. The '===' operator checks for strict equality, which means it checks if the values are equal and of the same type.
What is a promise in JavaScript?
A promise is an object that represents the eventual completion (or failure) of an asynchronous operation, and its resulting value. Promises provide a way to handle asynchronous operations in a more manageable way, by allowing you to write code that is easier to read and maintain.
What is the difference between a for loop and a while loop in JavaScript?
A for loop is used to iterate over a block of code a specified number of times, while a while loop is used to iterate over a block of code as long as a certain condition is true. For loops are typically used when you know the number of iterations in advance, while while loops are used when you don't know the number of iterations in advance.
What is a callback function in JavaScript?
A callback function is a function that is passed as an argument to another function, and is executed by that function. Callback functions are often used to handle the response from an asynchronous operation, such as an AJAX request or a database query.
What is the difference between the 'var', 'let', and 'const' keywords in JavaScript?
The 'var' keyword is used to declare a variable that can be accessed throughout the entire function, while the 'let' and 'const' keywords are used to declare variables that can only be accessed within the block they are defined in. The 'const' keyword is used to declare a constant variable, which cannot be reassigned.
What is a higher-order function in JavaScript?
A higher-order function is a function that takes another function as an argument, or returns a function as a result. Higher-order functions are often used to abstract away low-level details, and to create more reusable code.
What is a recursive function in JavaScript?
A recursive function is a function that calls itself, either directly or indirectly. Recursive functions are often used to solve problems that can be broken down into smaller sub-problems, such as tree traversals or dynamic programming.
What is memoization in JavaScript?
Memoization is a technique used to optimize the performance of a function by caching its results, so that if the function is called again with the same arguments, the cached result can be returned instead of recalculating it.
What is a design pattern in JavaScript?
A design pattern is a reusable solution to a common problem, that provides a proven development paradigm to help developers create more maintainable, flexible, and scalable software systems. Design patterns can be used to solve problems related to the structure, behavior, and interaction of objects in a system.
What is the difference between a factory function and a constructor function in JavaScript?
A factory function is a function that returns an object, while a constructor function is a function that is used to create a new object, and is typically used with the 'new' keyword. Factory functions are often used to create objects without using the 'new' keyword, while constructor functions are used to create objects that inherit properties from a prototype chain.
What is the difference between the 'prototype' and '__proto__' properties in JavaScript?
The 'prototype' property is used to define the prototype of a function, which is used to create new objects that inherit properties from the prototype chain. The '__proto__' property is used to access the prototype of an object, which is the object that the current object inherits properties from.
What is the difference between the 'call' and 'apply' methods in JavaScript?
The 'call' method is used to call a function with a specified 'this' value, and an optional list of arguments. The 'apply' method is used to call a function with a specified 'this' value, and an array of arguments.
What is the difference between the 'bind' method and the 'arrow function' syntax in JavaScript?
The 'bind' method is used to create a new function that has the same behavior as the original function, but with a specified 'this' value. The 'arrow function' syntax is used to create a new function that has the same behavior as the original function, but with a lexically bound 'this' value.
What is a generator function in JavaScript?
A generator function is a function that returns an iterator, which is an object that produces a sequence of values over time. Generator functions are often used to create iterators that can be used to iterate over a sequence of values, such as an array or a database query.
What is the difference between a 'for...of' loop and a 'for...in' loop in JavaScript?
A 'for...of' loop is used to iterate over the values of an iterable object, such as an array or a string. A 'for...in' loop is used to iterate over the properties of an object, including inherited properties.
What is the difference between the 'Map' and 'Set' data structures in JavaScript?
The 'Map' data structure is used to store key-value pairs, where each key is unique and maps to a specific value. The 'Set' data structure is used to store a collection of unique values, without any particular order or key.
What is the difference between the 'filter' and 'reduce' methods in JavaScript?
The 'filter' method is used to create a new array with all elements that pass the test implemented by the provided function. The 'reduce' method is used to apply a function to each element in an array, and reduce the array to a single value.
What is the difference between the 'some' and 'every' methods in JavaScript?
The 'some' method is used to test whether at least one element in an array passes the test implemented by the provided function. The 'every' method is used to test whether all elements in an array pass the test implemented by the provided function.
What is the difference between the 'find' and 'findIndex' methods in JavaScript?
The 'find' method is used to find the first element in an array that satisfies the provided testing function. The 'findIndex' method is used to find the index of the first element in an array that satisfies the provided testing function.
What is the difference between the 'includes' and 'indexOf' methods in JavaScript?
The 'includes' method is used to determine whether an array includes a certain value, returning true or false as appropriate. The 'indexOf' method is used to find the index of the first occurrence of a specified value in an array, returning -1 if the value is not found.
What is the difference between the 'slice' and 'splice' methods in JavaScript?
The 'slice' method is used to extract a section of an array and return it as a new array. The 'splice' method is used to add or remove elements from an array, and return the deleted elements.
What is the difference between the 'sort' and 'reverse' methods in JavaScript?
The 'sort' method is used to sort the elements of an array in place, and return the array. The 'reverse' method is used to reverse the order of the elements in an array, and return the array.
What is the difference between the 'join' and 'split' methods in JavaScript?
The 'join' method is used to join all elements of an array into a string, with a specified separator. The 'split' method is used to split a string into an array of substrings, using a specified separator.
What is the difference between the 'concat' and 'push' methods in JavaScript?
The 'concat' method is used to merge two or more arrays into a new array. The 'push' method is used to add one or more elements to the end of an array.
What is the difference between the 'unshift' and 'shift' methods in JavaScript?
The 'unshift' method is used to add one or more elements to the beginning of an array. The 'shift' method is used to remove the first element from an array, and return the removed element.
What is the difference between the 'pop' and 'push' methods in JavaScript?
The 'pop' method is used to remove the last element from an array, and return the removed element. The 'push' method is used to add one or more elements to the end of an array.
What is the difference between the 'forEach' and 'map' methods in JavaScript?
The 'forEach' method is used to execute a provided function once for each array element. The 'map' method is used to create a new array with the results of applying a provided function on every element in the calling array.
What is the difference between the 'filter' and 'map' methods in JavaScript?
The 'filter' method is used to create a new array with all elements that pass the test implemented by the provided function. The 'map' method is used to create a new array with the results of applying a provided function on every element in the calling array.
What is the difference between the 'some' and 'filter' methods in JavaScript?
The 'some' method is used to test whether at least one element in an array passes the test implemented by the provided function. The 'filter' method is used to create a new array with all elements that pass the test implemented by the provided function.
What is the difference between the 'every' and 'filter' methods in JavaScript?
The 'every' method is used to test whether all elements in an array pass the test implemented by the provided function. The 'filter' method is used to create a new array with all elements that pass the test implemented by the provided function.
What is the difference between the 'find' and 'filter' methods in JavaScript?
The 'find' method is used to find the first element in an array that satisfies the provided testing function. The 'filter' method is used to create a new array with all elements that pass the test implemented by the provided function.
What is the difference between the 'includes' and 'filter' methods in JavaScript?
The 'includes' method is used to determine whether an array includes a certain value, returning true or false as appropriate. The 'filter' method is used to create a new array with all elements that pass the test implemented by the provided function.
What is the difference between the 'indexOf' and 'filter' methods in JavaScript?
The 'indexOf' method is used to find the index of the first occurrence of a specified value in an array, returning -1 if the value is not found. The 'filter' method is used to create a new array with all elements that pass the test implemented by the provided function.
Write a JavaScript function to find the maximum value in an array.
function reverseString(str) {
return str.split('').reverse().join('');
}
Write a JavaScript function to check if a string is a palindrome.
function isPalindrome(str) {
return str === str.split('').reverse().join('');
}
Write a JavaScript function to find the first duplicate in an array.
functionfindFirstDuplicate(arr) {
let seen = newSet();
for (let i = 0; i < arr.length; i++) {
if (seen.has(arr[i])) {
return arr[i];
}
seen.add(arr[i]);
}
returnnull;
}
Write a JavaScript function to find the missing number in an array of consecutive integers.
functionfindMissingNumber(arr) {
let n = arr.length;
let sum = (n * (n + 1)) / 2;
let actualSum = arr.reduce((a, b) => a + b, 0);
return sum - actualSum;
}
Write a JavaScript function to find the longest common prefix in an array of strings.
functionlongestCommonPrefix(strs) {
if (!strs.length) {
return'';
}
let shortest = strs.reduce((a, b) => (a.length < b.length ? a : b));
for (let i = 0; i < shortest.length; i++) {
for (let j = 0; j < strs.length; j++) {
if (strs[j][i] !== shortest[i]) {
return shortest.slice(0, i);
}
}
}
return shortest;
}
Write a JavaScript function to find the maximum sum of a subarray in an array.
functionmaxSubarraySum(arr) {
let maxSum = -Infinity;
let currentSum = 0;
for (let i = 0; i < arr.length; i++) {
currentSum = Math.max(arr[i], currentSum + arr[i]);
maxSum = Math.max(maxSum, currentSum);
}
return maxSum;
}
Write a JavaScript function to find the minimum window substring that contains all characters of another string.
function minWindowSubstring(s, t){
let tCount = {};
for (let char of t) {
if (!tCount[char]) {
tCount[char] = 0;
}
tCount[char]++;
}
let required = Object.keys(tCount).length;
let left = 0;
let right = 0;
let formed = 0;
let windowCounts = {};
let ans = [0, Infinity];
while (right < s.length) {
let char = s[right];
if (tCount[char]) {
if (!windowCounts[char]) {
windowCounts[char] = 0;
}
windowCounts[char]++;
if (windowCounts[char] === tCount[char]) {
formed++;
}
}
while (left <= right && formed === required) {
char = s[left];
if (right - left + 1 < ans[1] - ans[0]) {
ans = [left, right];
}
if (tCount[char]) {
windowCounts[char]--;
if (windowCounts[char] < tCount[char]) {
formed--;
}
}
left++;
}
right++;
}
return ans[1] === Infinity ? '' : s.slice(ans[0], ans[1] + 1);
}
Write a JavaScript function to find the first non-repeating character in a string.
functionfirstNonRepeatingChar(s) {
let count = {};
for (let char of s) {
if (!count[char]) {
count[char] = 0;
}
count[char]++;
}
for (let char of s) {
if (count[char] === 1) {
return char;
}
}
returnnull;
}
Write a JavaScript function to find the maximum frequency of a character in a string.
functionmaxFrequency(s) {
let count = {};
for (let char of s) {
if (!count[char]) {
count[char] = 0;
}
count[char]++;
}
let max = 0;
for (let char in count) {
max = Math.max(max, count[char]);
}
return max;
}
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