JavaScript Top Questions that every Developer should know

1. What is the use of isNaN function?

					// Example 1: Checking a string that's not a number
console.log(isNaN("John Bear")); // Output: true

// Example 2: Checking a string representing a valid number
console.log(isNaN("123")); // Output: false

// Example 3: Checking a real NaN value
console.log(isNaN(NaN)); // Output: true

// Example 4: Checking an actual number
console.log(isNaN(42)); // Output: false

// Example 5: Checking an object (which is not a number)
console.log(isNaN({})); // Output: true

// Example 6: Checking undefined
console.log(isNaN(undefined)); // Output: true

// Example 7: Checking null
console.log(isNaN(null)); // Output: false

						 

2. What are JavaScript Data Types?

// Integer
let age = 30;
console.log(age); // Output: 30

// Floating-point
let height = 5.8;
console.log(height); // Output: 5.8

					

//String: A sequence of characters written in single or double quotes.
	let greeting = "Hello, world!";
console.log(greeting); // Output: Hello, world!

let name = 'Alice';
console.log(name); // Output: Alice

	

	//Boolean: Represents logical values, either true or false
	let isLoggedIn = true;
console.log(isLoggedIn); // Output: true

let hasAccess = false;
console.log(hasAccess); // Output: false

	

//Object: Collections of key-value pairs, allowing structured data.
	// Creating a simple object
let person = {
    name: "John Doe",
    age: 25,
    isStudent: true
};
console.log(person); // Output: { name: 'John Doe', age: 25, isStudent: true }

// Accessing object properties
console.log(person.name); // Output: John Doe
console.log(person.age); // Output: 25

	

	//Undefined: A variable that hasn't been assigned a value is of the undefined type.
	let result; // Variable declared but not initialized
console.log(result); // Output: undefined

// When a variable is not explicitly assigned, its type is 'undefined'
let car = {};
console.log(car.model); // Output: undefined

	

3. Can you break JavaScript Code into several lines ?

document. Write ("This is my first\JS Code \and am Loving it");

4.Wrie a Code to Dynamically add an element into a JS program

//HTML Code
	const parentDiv = document.getElementById('parent-div');

// Create a new paragraph element
const newParagraph = document.createElement('p');
       

//JS Code
					// Select the parent element where the new element will be added
const parentDiv = document.getElementById('parent-div');

// Create a new paragraph element
const newParagraph = document.createElement('p');

// Add some text content to the new paragraph
newParagraph.textContent = 'This is a dynamically added paragraph.';

// Append the new paragraph to the parent div
parentDiv.appendChild(newParagraph);

					

5.Can you tell us about undefined and undeclared variables?

// Example 1
					// Declare a variable without initializing it
let declaredButUndefined;
console.log(declaredButUndefined); // Output: "undefined"

// Variable declared and later assigned a value
var initializedLater;
initializedLater = 10;
console.log(initializedLater); // Output: 10

// A function returning undefined
function noReturnValue() {}
console.log(noReturnValue()); // Output: "undefined"

//Example 2 
try {
    // Attempt to log an undeclared variable
    console.log(nonExistentVariable);
} catch (error) {
    console.log(error.message); // Output: "nonExistentVariable is not defined"
}

6.What do you know about Global variables in JS ?

//Example 1:
					// No declaration keyword used, becomes global implicitly
function implicitGlobalExample() {
    implicitGlobalVar = "I'm global!"; // Implicit global
}

implicitGlobalExample();
console.log(implicitGlobalVar); // Output: "I'm global!"
					

//Example 2:
					// Declared with `var` outside any function, explicitly global
var explicitGlobalVar = "I'm global too!";

function logGlobalVar() {
    console.log(explicitGlobalVar); // Accessing global variable inside a function
}

logGlobalVar(); // Output: "I'm global too!"
console.log(window.explicitGlobalVar); // Output: "I'm global too!"

					

7.Do you know how to create a Prompt box ?

					// Display a prompt box asking for the user's name
let userName = prompt("Please enter your name:", "Guest");

// Check if the user clicked "Cancel" (returns null)
if (userName !== null) {
    // Display a welcome message using the user's input
    console.log(`Hello, ${userName}! Welcome to our site.`);
} else {
    // Handle the case where the user canceled the prompt
    console.log("No name entered. Welcome, Guest!");
}

					

8. Do you know the What is the === operator ?

//Try this code

					// Example 1: Using `===` (Strict Equality)

let num = 5;
let str = "5";

// Strict equality - false, because `num` is a number and `str` is a string
console.log(num === str); // Output: false

// Strict equality - true, because both are numbers
console.log(num === 5); // Output: true

// Example 2: Using `==` (Loose Equality)
let boolTrue = true;
let numOne = 1;

// Loose equality - true, because `true` is coerced to 1
console.log(boolTrue == numOne); // Output: true

// Loose equality - true, because string "5" is coerced to the number 5
console.log(num == str); // Output: true

// Example 3: Comparing objects with `===`
let obj1 = {};
let obj2 = {};

// Even though they appear similar, different objects have different references
console.log(obj1 === obj2); // Output: false

// Identical references - pointing to the same object
let obj3 = obj1;
console.log(obj1 === obj3); // Output: true

					

9. Iterate Over an Array

fruits.forEach(function(item, index) {
                console.log(index, item);
            });

10. Filter an Array

const result = fruits.filter(fruit => fruit.startsWith('B'));
        console.log(result);

11. Merging Two Arrays without Duplicates

const array1 = [1, 2, 3];
const array2 = [2, 3, 4];
const mergedArray = [...new Set([...array1, ...array2])];
console.log(mergedArray);

12. Finding the Intersection of Two Arrays

const firstArray = [1, 2, 3, 4];
const secondArray = [2, 4, 6, 8];
const intersection = firstArray.filter(value => secondArray.includes(value));
console.log(intersection); // Output: [2, 4]

13.Removing Falsy Values from an Array

const mixedArray = [0, "text", false, 4, "", null, undefined, "0"];
const truthyArray = mixedArray.filter(Boolean);
console.log(truthyArray); // Output: ["text", 4, "0"]

14.Flattening an Array of Arrays

const nestedArray = [1, [2, 3], [4, [5, 6]]];
const flatArray = nestedArray.flat(2); // Level of depth is 2
console.log(flatArray); // Output: [1, 2, 3, 4, 5, 6]

15. Grouping Objects by a Property

const people = [
{ name: "John", age: 25 },
{ name: "Anna", age: 30 },
{ name: "John", age: 28 },
];
const groupedByNames = people.reduce((acc, current) => {
  (acc[current.name] = acc[current.name] || []).push(current);
  return acc;
}, {});
console.log(groupedByNames);

16. Grouping Objects by a Property

const original = [[1], [2], [3]];
const deepCopy = JSON.parse(JSON.stringify(original));
original[0].push(2);
console.log(original); // Output: [[1, 2], [2], [3]]
console.log(deepCopy); // Output: [[1], [2], [3]]

17.Sorting an Array of Objects by Property

const items = [
{ name: "Banana", price: 1 },
{ name: "Apple", price: 2 },
{ name: "Orange", price: 1.5 },
];
items.sort((a, b) => a.price - b.price);
console.log(items);

18.Finding the Difference Between Two Arrays

const arr1 = [1, 2, 3, 4, 5];
const arr2 = [4, 5, 6, 7];
const difference = arr1.filter(x => !arr2.includes(x));
console.log(difference); // Output: [1, 2, 3]

19. Converting an Array of Objects into a Map

const products = [
{ id: 1, name: "Pencil", price: 1.50 },
{ id: 2, name: "Notebook", price: 2.50 }
];
const productMap = new Map(products.map(product => [product.id, product]));
console.log(productMap);

20.Chunking an Array into Smaller Arrays of a Specific Length

const createChunks = (array, chunkSize) => {
const chunks = [];
  for (let i = 0; i < array.length; i += chunkSize) {
    chunks.push(array.slice(i, i + chunkSize));
  }
  return chunks;
};
const data = [1, 2, 3, 4, 5, 6, 7, 8, 9];
const chunkedData = createChunks(data, 3);
console.log(chunkedData); // Output: [[1, 2, 3], [4, 5, 6], [7, 8, 9]]

21. Find the result when this operation takes place

var num1 = 120;
var num2 = 30;
var num3 = num1 + num2+ "100";
console.log(num3);

22.What is NULL in JavaScript

//Example
let user = null;
if (user === null) {
  console.log("No user data available.");
}
//In this example, user is explicitly set to null to indicate that there is no user data available at this point in the script.

23.What is DELETE Operator

//Example
let userInfo = {
  name: "John Doe",
  age: 30
};
delete userInfo.age;
console.log(userInfo); // Output: { name: "John Doe" }
//Here, the delete operator removes the age property from the userInfo object. When we log userInfo, the age property is no longer part of the object.

24.Explain Pop-up Box in JavaScript-Alert

//Example
alert("Welcome to our website!");

//This alert simply informs the user with a message and an OK button.

25.Explain the Pop-up Box in JavaScript-Confirm

//Example
if (confirm("Are you sure you want to delete your profile?")) {
  console.log("Profile deleted.");
} else {
  console.log("Delete canceled.");
}

//This confirm box asks users if they are sure about deleting their profile, handling the response accordingly..

26.Explain the Pop-up Box in JavaScript-Prompt

//Example
let userAge = prompt("Please enter your age:", "18");
if (userAge) {
  console.log("User age is: " + userAge);
} else {
  console.log("User did not enter age.");
}

//This prompt box requests the user's age and provides a default value (18). The script then handles the response based on whether the user provides their age or not.

27.Explain the Use of void(0)

//Example
Click Me!


//In this example, clicking the "Click Me!" link will log a message to the console but will not cause the browser to navigate to a different URL or reload the page.

28.How do you Force a Page to Load Another Page

//Example
function redirectToHomePage() {
location.href = 'https://www.example.com';
}


//In this script, calling the redirectToHomePage() function will change the current window's location to 'https://www.example.com', thereby navigating to that URL.

29.Can you define the Data Types of Variables in JavaScript

//try these Examples:
let name = "John"; // String
let age = 30; // Number
let isVerified = false; // Boolean
let bigNumber = 1234567890123456789012345678901234567890n; // BigInt
let user = { firstName: "Jane", lastName: "Doe" }; // Object
let scores = [90, 85, 88, 92]; // Array
let uniqueID = Symbol('id'); // Symbol
let somethingUndefined; // Undefined
let action = null; // Null

console.log(typeof name); // "string"
console.log(typeof age); // "number"
console.log(typeof isVerified); // "boolean"
console.log(typeof bigNumber); // "bigint"
console.log(typeof user); // "object"
console.log(typeof scores); // "object" because Arrays in JavaScript are objects
console.log(typeof uniqueID); // "symbol"
console.log(typeof somethingUndefined); // "undefined"
console.log(typeof action); // "null" but `typeof` incorrectly returns "object"

30.Can you state the Difference Between an Alert Box and a Confirmation Box

//Example
				// Alert box example
alert("Welcome to our website!");

// Confirmation box example
if (confirm("Do you want to continue?")) {
    console.log("User clicked OK.");
} else {
    console.log("User clicked Cancel.");
}

31. What are Escape Characters

//Example
let message = "Hello, she said \"Welcome to JavaScript programming!\"\nLet's learn about escape characters.";
console.log(message);

//This example demonstrates using escape characters to include quotes in a string and to insert a new line.

31.Can you tell us about JavaScript Cookies

//Example
// Set a cookie
document.cookie = "username=John Doe; expires=Thu, 18 Dec 2023 12:00:00 UTC; path=/";

// Get a cookie
function getCookie(name) {
    let cookies = document.cookie.split('; ');
    for (let i = 0; i < cookies.length; i++) {
        let cookiePair = cookies[i].split('=');
        if (cookiePair[0] === name) {
            return cookiePair[1];
        }
    }
    return null;
}
console.log(getCookie("username"));

//This example shows how to set and get a cookie storing a username.