Real interview questions from Ola company. Includes theoretical concepts and coding problems.
What are the key features of JavaScript?
JavaScript is a high-level, dynamic, and interpreted programming language that supports object-oriented, imperative, and functional programming styles. It has key features such as first-class functions, closures, and a built-in support for asynchronous programming.
What is the difference between null and undefined in JavaScript?
In JavaScript, null and undefined are two distinct primitive values. Null represents the absence of a value, whereas undefined represents an uninitialized or non-existent variable. Null is a primitive value that can be assigned to a variable, whereas undefined is a type that represents an uninitialized variable.
What is a closure in JavaScript?
A closure in JavaScript is a function that has access to its outer scope's variables, even when the outer function has returned. This allows the inner function to use and manipulate the variables of the outer function, creating a persistent reference to the outer scope.
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 the properties and methods of an object, and its value depends on how the function is called. The 'this' keyword is often used to create methods that operate on an object's properties.
What is the difference between a for loop and a while loop in JavaScript?
A for loop in JavaScript is used to iterate over a block of code for a specified number of times, whereas 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 the number of iterations is known, whereas while loops are used when the number of iterations is unknown.
What is the purpose of the 'let' and 'const' keywords in JavaScript?
The 'let' and 'const' keywords in JavaScript are used to declare variables. The 'let' keyword declares a variable that can be reassigned, whereas the 'const' keyword declares a constant that cannot be reassigned. Both 'let' and 'const' have block scope, meaning they are only accessible within the block they are declared in.
What is the difference between an array and an object in JavaScript?
In JavaScript, an array is a collection of values of any data type, including strings, numbers, and objects, that are stored in a single variable. An object, on the other hand, is a collection of key-value pairs, where each key is a string and each value can be of any data type. Arrays are typically used to store collections of values, whereas objects are used to store collections of key-value pairs.
What is the purpose of the 'map' method in JavaScript?
The 'map' method in JavaScript is used to create a new array by applying a given function to each element of an existing array. It returns a new array with the results of applying the function to each element, without modifying the original array.
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 a provided function. It returns a new array with the elements that satisfy the condition, without modifying the original array.
What is the purpose of the 'reduce' method in JavaScript?
The 'reduce' method in JavaScript is used to apply a function against an accumulator and each element in the array (from left to right) to reduce it to a single value. It returns the accumulated value, which can be of any data type, including numbers, strings, and objects.
What is the difference between synchronous and asynchronous programming in JavaScript?
Synchronous programming in JavaScript refers to the execution of code in a sequential manner, where each statement is executed one after the other. Asynchronous programming, on the other hand, refers to the execution of code in a non-sequential manner, where multiple tasks can be executed concurrently, without blocking each other.
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, using the 'async' keyword to declare asynchronous functions and the 'await' keyword to pause the execution of an asynchronous function until a promise is resolved or rejected.
What is the difference between a promise and an observable in JavaScript?
A promise in JavaScript is a result object that is used to handle asynchronous operations, representing a value that may not be available yet, but will be resolved at some point in the future. An observable, on the other hand, is a stream of values that can be subscribed to, allowing developers to handle asynchronous data streams in a more efficient and scalable way.
What is the purpose of the 'try-catch' block in JavaScript?
The 'try-catch' block in JavaScript is used to handle errors and exceptions that occur during the execution of code. The 'try' block contains the code that may throw an error, and the 'catch' block contains the code that will be executed if an error occurs, allowing developers to handle errors and exceptions in a more robust and reliable way.
What is the difference between a class and an object in JavaScript?
In JavaScript, a class is a blueprint or a template that defines the properties and methods of an object, whereas an object is an instance of a class, representing a real-world entity with its own set of properties and methods. Classes are used to define the structure and behavior of objects, whereas objects are used to represent specific instances of a class.
What is the purpose of the 'extends' keyword in JavaScript?
The 'extends' keyword in JavaScript is used to create a new class that inherits the properties and methods of an existing class. It allows developers to create a hierarchy of classes, where a subclass inherits the behavior of a parent class and can also add new properties and methods or override the ones inherited from the parent class.
What is the difference between a constructor and a method in JavaScript?
In JavaScript, a constructor is a special method that is used to initialize objects when they are created, whereas a method is a function that is associated with an object or a class and is used to perform a specific task. Constructors are used to set the initial state of an object, whereas methods are used to perform actions on an object or return values.
What is the purpose of the 'super' keyword in JavaScript?
The 'super' keyword in JavaScript is used to access the properties and methods of a parent class from a child class. It allows developers to call the constructor or methods of a parent class from a child class, enabling them to reuse the behavior of the parent class and add new functionality to the child class.
What is the difference between a static method and an instance method in JavaScript?
In JavaScript, a static method is a method that belongs to a class, rather than an instance of the class, and can be called without creating an instance of the class. An instance method, on the other hand, is a method that belongs to an instance of a class and can only be called on an instance of the class. Static methods are used to perform actions that do not depend on the state of an instance, whereas instance methods are used to perform actions that depend on the state of an instance.
What is the purpose of the 'bind' method in JavaScript?
The 'bind' method in JavaScript is used to create a new function that has its 'this' keyword set to a specified value, with a given sequence of arguments preceding any provided when the new function is called. It allows developers to create functions that can be used as event handlers or callbacks, with a specific context and set of arguments.
What is the difference between a function expression and a function declaration in JavaScript?
In JavaScript, a function expression is a function that is defined as an expression, whereas a function declaration is a function that is defined using the 'function' keyword. Function expressions are used to create functions that can be assigned to variables or passed as arguments to other functions, whereas function declarations are used to create functions that can be called by their name.
What is the purpose of the 'call' method in JavaScript?
The 'call' method in JavaScript is used to call a function with a given 'this' value and arguments. It allows developers to call a function with a specific context and set of arguments, enabling them to reuse functions and create more flexible and modular code.
What is the difference between a callback function and a promise in JavaScript?
In JavaScript, a callback function is a function that is passed as an argument to another function, and is executed when a specific operation is completed. A promise, on the other hand, is a result object that is used to handle asynchronous operations, representing a value that may not be available yet, but will be resolved at some point in the future. Callback functions are used to handle asynchronous operations in a more traditional way, whereas promises provide a more modern and efficient way to handle asynchronous operations.
What is the purpose of the 'apply' method in JavaScript?
The 'apply' method in JavaScript is used to call a function with a given 'this' value and arguments. It allows developers to call a function with a specific context and set of arguments, enabling them to reuse functions and create more flexible and modular code.
What is the difference between a JavaScript object and a JSON object?
In JavaScript, an object is a collection of key-value pairs, where each key is a string and each value can be of any data type. A JSON object, on the other hand, is a string representation of a JavaScript object, using a specific syntax to represent the key-value pairs. JavaScript objects are used to represent data in a more flexible and dynamic way, whereas JSON objects are used to represent data in a more structured and standardized way.
What is the purpose of the 'JSON.parse' method in JavaScript?
The 'JSON.parse' method in JavaScript is used to parse a JSON string and convert it into a JavaScript object. It allows developers to convert JSON data into a more usable and flexible format, enabling them to work with the data in a more efficient and effective way.
What is the difference between a JavaScript array and a JavaScript object?
In JavaScript, an array is a collection of values of any data type, including strings, numbers, and objects, that are stored in a single variable. An object, on the other hand, is a collection of key-value pairs, where each key is a string and each value can be of any data type. Arrays are typically used to store collections of values, whereas objects are used to store collections of key-value pairs.
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
functionlongestCommonPrefix(strs) {
if (!strs.length) {
return'';
}
let prefix = strs[0];
for (let i = 1; i < strs.length; i++) {
while (strs[i].indexOf(prefix) !== 0) {
prefix = prefix.substring(0, prefix.length - 1);
if (prefix === '') {
return'';
}
}
}
return prefix;
}
Write a JavaScript function to find the first non-repeating character in a string
functionfirstNonRepeatingChar(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
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 that contains all elements of another array
functionminWindow(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) {
if (map[num] === 0) {
formed--;
}
map[num]++;
}
left++;
}
}
return minWindow;
}
Write a JavaScript function to find the maximum frequency of a character in a string
functionmaxCharFrequency(str) {
let charCount = {};
for (let char of str) {
if (charCount[char]) {
charCount[char]++;
} else {
charCount[char] = 1;
}
}
let maxFrequency = 0;
for (let char in charCount) {
if (charCount[char] > maxFrequency) {
maxFrequency = charCount[char];
}
}
return maxFrequency;
}
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