Real interview questions from Cred 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 after the outer functions have finished executing.
What is the purpose of the 'this' keyword in JavaScript?
The 'this' keyword in JavaScript refers to the current execution context of a function. It can be used to access properties and methods of an object, and to distinguish between global and local variables.
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.
What is the purpose of the 'let' and 'const' keywords in JavaScript?
The 'let' and 'const' keywords in JavaScript are used to declare variables. 'Let' is used to declare a variable that can be reassigned, while 'const' is used to declare a constant that cannot be reassigned.
What is the difference between a function declaration and a function expression in JavaScript?
A function declaration is a statement that defines a function, while a function expression is an expression that defines a function. Function declarations are hoisted to the top of their scope, while function expressions are not.
What is the purpose of the 'bind' method in JavaScript?
The 'bind' method in JavaScript is used to set the 'this' keyword of a function to a specific value. It returns a new function that has the same behavior as the original function, but with the 'this' keyword set to the specified value.
What is the difference between a synchronous and an asynchronous function in JavaScript?
A synchronous function is a function that executes immediately and returns a value, while an asynchronous function is a function that executes at a later time and returns a promise or a callback.
What is the purpose of the 'Promise' object in JavaScript?
The 'Promise' object in JavaScript is used to represent a value that may not be available yet, but will be available at some point in the future. It provides a way to handle asynchronous operations in a more manageable way.
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, while a 'for...in' loop is used to iterate over the keys of an object.
What is the purpose of the 'Map' object in JavaScript?
The 'Map' object in JavaScript is a data structure that stores key-value pairs. It provides a way to store and retrieve values using a unique key.
What is the difference between a 'Set' and a 'Map' in JavaScript?
A 'Set' is a data structure that stores unique values, while a 'Map' is a data structure that stores key-value pairs.
What is the purpose of the 'filter' method in JavaScript?
The 'filter' method in JavaScript 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 'map' and 'forEach' methods in JavaScript?
The 'map' method is used to create a new array with the results of applying a provided function to every element in the original array, while the 'forEach' method is used to execute a provided function once for each element in the array.
What is the purpose of the 'reduce' method in JavaScript?
The 'reduce' method in JavaScript is used to apply a function to each element in an array and reduce it to a single value.
What is the difference between a 'callback' and a 'promise' in JavaScript?
A 'callback' is a function that is passed as an argument to another function, while a 'promise' is an object that represents a value that may not be available yet, but will be available at some point in the future.
What is the purpose of the 'async/await' syntax in JavaScript?
The 'async/await' syntax in JavaScript is used to write asynchronous code that is easier to read and maintain. It allows developers to write asynchronous code that looks and feels like synchronous code.
What is the difference between a 'class' and a 'function' in JavaScript?
A 'class' is a blueprint for creating objects, while a 'function' is a block of code that can be executed multiple times.
What is the purpose of the 'extends' keyword in JavaScript?
The 'extends' keyword in JavaScript is used to create a new class that inherits properties and methods from an existing class.
What is the difference between 'super' and 'this' in JavaScript?
The 'super' keyword in JavaScript is used to access the parent class, while the 'this' keyword is used to access the current object.
What is the purpose of the 'static' keyword in JavaScript?
The 'static' keyword in JavaScript is used to define a static method or property that belongs to a class, rather than an instance of the class.
What is the difference between a 'static' method and an 'instance' method in JavaScript?
A 'static' method is a method that belongs to a class, while an 'instance' method is a method that belongs to an instance of a class.
What is the purpose of the 'get' and 'set' keywords in JavaScript?
The 'get' and 'set' keywords in JavaScript are used to define getter and setter functions for a property.
What is the difference between a 'getter' and a 'setter' in JavaScript?
A 'getter' is a function that returns the value of a property, while a 'setter' is a function that sets the value of a property.
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 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 numbers
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
functionfindLongestCommonPrefix(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 first non-repeating character in a string
functionfindFirstNonRepeatingChar(str) {
let charCount = {};
for (let char of str) {
if (charCount[char]) {
charCount[char]++;
} else {
charCount[char] = 1;
}
}
for (let char of str) {
if (charCount[char] === 1) {
return char;
}
}
returnnull;
}
Write a JavaScript function to find the maximum sum of a subarray
functionfindMaxSubarraySum(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 that contains all elements of another array
functionfindMinWindow(arr1, arr2) {
let map = {};
for (let num of arr2) {
map[num] = (map[num] || 0) + 1;
}
let required = Object.keys(map).length;
let left = 0;
let minLen = Infinity;
let minWindow = '';
let formed = 0;
for (let right = 0; right < arr1.length; right++) {
let num = arr1[right];
if (num in map) {
map[num]--;
if (map[num] === 0) {
formed++;
}
}
while (left <= right && formed === required) {
num = arr1[left];
if (right - left + 1 < minLen) {
minLen = right - left + 1;
minWindow = arr1.slice(left, right + 1).join('');
}
if (num in map) {
map[num]++;
if (map[num] > 0) {
formed--;
}
}
left++;
}
}
return minWindow;
}
Write a JavaScript function to find the maximum length of a subarray with a given sum
functionfindMaxLengthSubarray(arr, targetSum) {
let maxLen = 0;
let sum = 0;
let map = { 0: -1 };
for (let i = 0; i < arr.length; i++) {
sum += arr[i];
if (sum - targetSum in map) {
maxLen = Math.max(maxLen, i - map[sum - targetSum]);
}
if (!(sum in map)) {
map[sum] = i;
}
}
return maxLen;
}
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