Real interview questions from Zepto 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 difference between the var, let, and const keywords in JavaScript?
The var keyword declares a variable that can be scoped to the nearest function block, while the let and const keywords declare variables that are scoped to the nearest block statement. Let and const are block-scoped, while var is function-scoped.
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, allowing for better error handling and code organization.
What is the difference between a synchronous and asynchronous function in JavaScript?
A synchronous function executes immediately and returns a value, while an asynchronous function returns a promise that resolves to a value at a later time. Asynchronous functions are used to handle operations that take time to complete, such as network requests or database queries.
What is a callback function in JavaScript?
A callback function is a function that is passed as an argument to another function, to be executed after a specific operation has been completed. Callbacks are often used to handle the response from an asynchronous operation, such as a network request or a database query.
What is the purpose of the this keyword in JavaScript?
The this keyword refers to the current execution context of a function, which can be the global object, a function, or an object. The this keyword is used to access properties and methods of the current object, and to distinguish between global and local variables.
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 used to abstract away low-level details and to create more reusable and modular code.
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 a recursive function in JavaScript?
A recursive function is a function that calls itself, either directly or indirectly, to solve a problem. Recursive functions are used to solve problems that can be broken down into smaller sub-problems of the same type.
What is memoization in JavaScript?
Memoization is an optimization technique that stores the results of expensive function calls and returns the cached result when the same inputs occur again. Memoization is used to improve the performance of recursive functions and other computationally expensive operations.
What is a design pattern in JavaScript?
A design pattern is a reusable solution to a common problem in software design. Design patterns provide a proven development paradigm to help developers create more maintainable, flexible, and scalable software systems.
What is the Singleton pattern in JavaScript?
The Singleton pattern is a design pattern that restricts a class to instantiate its multiple objects. It creates a single instance of a class and provides a global point of access to that instance.
What is the Factory pattern in JavaScript?
The Factory pattern is a design pattern that provides a way to create objects without specifying the exact class of object that will be created. It provides a way to decouple object creation from the specific class of object being created.
What is the Observer pattern in JavaScript?
The Observer pattern is a design pattern that defines a one-to-many dependency between objects, so that when one object changes state, all its dependents are notified and updated automatically.
What is the difference between a class and an object in JavaScript?
A class is a blueprint or a template that defines the properties and methods of an object, while an object is an instance of a class, which has its own set of attributes (data) and methods (functions).
What is inheritance in JavaScript?
Inheritance is a mechanism in JavaScript that allows one class to inherit the properties and methods of another class. The child class inherits all the properties and methods of the parent class and can also add new properties and methods or override the ones inherited from the parent class.
What is polymorphism in JavaScript?
Polymorphism is the ability of an object to take on multiple forms, depending on the context in which it is used. In JavaScript, polymorphism is achieved through method overriding or method overloading.
What is encapsulation in JavaScript?
Encapsulation is the concept of hiding the implementation details of an object from the outside world and only exposing the necessary information through public methods. In JavaScript, encapsulation is achieved through the use of closures and the module pattern.
What is abstraction in JavaScript?
Abstraction is the concept of showing only the necessary information to the outside world while hiding the internal details. In JavaScript, abstraction is achieved through the use of abstract classes, interfaces, and modules.
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 for 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 break statement in JavaScript?
The break statement is used to terminate a loop or a switch statement and transfer control to the statement that follows the loop or switch statement.
What is the purpose of the continue statement in JavaScript?
The continue statement is used to skip the rest of the code in the current iteration of a loop and move on to the next iteration.
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 number is prime
functionisPrime(num) {
if (num <= 1) {
returnfalse;
}
for (var i = 2; i <= Math.sqrt(num); i++) {
if (num % i === 0) {
returnfalse;
}
}
returntrue;
}
Write a JavaScript function to find the first duplicate in an array
functionfindFirstDuplicate(arr) {
var seen = {};
for (var i = 0; i < arr.length; i++) {
if (seen[arr[i]]) {
return arr[i];
}
seen[arr[i]] = true;
}
returnnull;
}
Write a JavaScript function to find the missing number in an array
functionfindMissingNumber(arr) {
var n = arr.length + 1;
var sum = (n * (n + 1)) / 2;
var actualSum = arr.reduce((a, b) => a + b, 0);
return sum - actualSum;
}
Write a JavaScript function to check if a binary tree is balanced
Write a JavaScript function to find the lowest common ancestor of two nodes in a binary tree
functionfindLCA(root, p, q) {
if (root === null) {
returnnull;
}
if (root === p || root === q) {
return root;
}
var left = findLCA(root.left, p, q);
var right = findLCA(root.right, p, q);
if (left && right) {
return root;
}
return left ? left : right;
}
Write a JavaScript function to check if a string is a palindrome
functionisPalindrome(s) {
var left = 0;
var right = s.length - 1;
while (left < right) {
if (s[left] !== s[right]) {
returnfalse;
}
left++;
right--;
}
returntrue;
}
Write a JavaScript function to find the maximum sum of a subarray
functionmaxSubArray(nums) {
var maxSoFar = nums[0];
var maxEndingHere = nums[0];
for (var i = 1; i < nums.length; i++) {
maxEndingHere = Math.max(nums[i], maxEndingHere + nums[i]);
maxSoFar = Math.max(maxSoFar, maxEndingHere);
}
return maxSoFar;
}
Write a JavaScript function to check if a number is a power of two
