Ace Your JavaScript Interview: 60 Expert Questions and Answers

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// Number
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let age = 25;
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let price = 9.99;
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 // String
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let name = "Alice";
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let greeting = 'Hello, World!';
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// Boolean
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let isTrue = true;
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let isFalse = false;
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// Undefined
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let uninitializedVar;
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// Null
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let emptyValue = null;
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// Symbol
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const uniqueSymbol = Symbol('description');
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// Big Int
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const bigIntValue = 1234567890123456789012345678901234567890n;

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let details = {name: 'Dane', age: '26'}

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var name = 'John'; // using var
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let age = 30; // using let
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const PI = 3.14; // using const

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// This is a single-line comment
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/*
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 This is a
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 multi-line comment
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*/

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let a;
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let b = null;
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console.log(a); // undefined
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console.log(b); // null

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// Function Declaration
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function sayHello(name) {
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  console.log(`Hello, ${name}!`);
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}
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 // Function Expression
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const add = function(x, y) {
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  return x + y;
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};

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// Normal Function Declaration
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const add = function(x, y) {
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  return x + y;
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};
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// Allow Function Syntax
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const add = (x, y) => x + y;

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const fruits = ['apple', 'banana'];
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const newLength = fruits.push('orange');
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// fruits now contains ['apple', 'banana', 'orange']
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// newLength is 3

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const fruits = ['apple', 'banana', 'orange'];
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const lastFruit = fruits.pop(); // lastFruit is 'orange'
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// fruits now contains ['apple', 'banana']

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const numbers = [1, 2, 3, 4, 5];
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numbers.splice(2, 2, 6, 7); // Removes 3 and 4, adds 6 and 7
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// numbers now contains [1, 2, 6, 7, 5]

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const colors = ['red', 'green', 'blue'];
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for (let i = 0; i < colors.length; i++) {
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  console.log(colors[i]);
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}

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colors.forEach(function(color) {
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  console.log(color);
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});

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for (const color of colors) {
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  console.log(color);
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}

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const uppercaseColors = colors.map(function(color) {
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  return color.toUpperCase();
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});

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const array = [1, 2, 3, 4, 5];
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const evenNumbers = array.filter(function (element) {
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  return element % 2 === 0;
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});
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console.log(evenNumbers);

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const array = [1, 2, 3, 4, 5];
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const sum = array.reduce(function (accumulator, currentValue) {
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  return accumulator + currentValue;
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}, 0);
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console.log(sum);

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const numbers = [1, 2, 3];
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const doubled = numbers.map(function(number) {
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  return number * 2;
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});
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// doubled is [2, 4, 6]

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const numbers = [1, 2, 3, 4, 5];
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const evenNumbers = numbers.filter(function(number) {
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  return number % 2 === 0;
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});
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// evenNumbers is [2, 4]

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const numbers = [1, 2, 3, 4, 5];
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const sum = numbers.reduce(function(accumulator, currentValue) {
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  return accumulator + currentValue;
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}, 0); // Initial value of accumulator is 0
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// sum is 15

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const fruits = ['apple', 'banana', 'cherry'];
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const slicedFruits = fruits.slice(1, 2); // Returns a new array with ['banana']
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// fruits remains ['apple', 'banana', 'cherry']

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const numbers = [1, 2, 3, 4, 5];
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numbers.splice(2, 2, 6, 7); // Removes 3 and 4, adds 6 and 7
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// numbers now contains [1, 2, 6, 7, 5]

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const person = {
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  firstName: 'John',
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  lastName: 'Doe',
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  age: 30,
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};

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function Person(firstName, lastName, age) {
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  this.firstName = firstName;
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  this.lastName = lastName;
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  this.age = age;
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}
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const person = new Person('John', 'Doe', 30);

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class Person {
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  constructor(firstName, lastName, age) {
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    this.firstName = firstName;
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    this.lastName = lastName;
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    this.age = age;
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  }
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}
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const person = new Person('John', 'Doe', 30);

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const obj1 = { name: 'John' };
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const obj2 = { name: 'John' };
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console.log(obj1 == obj2);  // false (different memory references)
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console.log(obj1 === obj2); // false (different memory references)

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const element = document.getElementById('elementId');

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const element = document.querySelector('.className');

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const elements = document.querySelectorAll('.className');

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const elements = document.getElementsByClassName('className');

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const elements = document.getElementsByTagName('tagname');

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fetchData
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  .then(function(data) {
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    console.log(data);
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  })
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  .catch(function(error) {
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    console.error(error);
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  });

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setTimeout(function() {
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  console.log('This will run after a delay.');
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}, 1000); // Executes after 1 second

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const intervalId = setInterval(function() {
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  console.log('This will run repeatedly at an interval.');
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}, 2000); // Executes every 2 seconds
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// To stop the interval after some time
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setTimeout(function() {
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  clearInterval(intervalId);
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}, 10000); // Stops after 10 seconds

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// For Arrays:
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const array1 = [1, 2, 3];
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const array2 = [4, 5, ...array1]; // Combining arrays
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// array2 is now [4, 5, 1, 2, 3]
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// Copying an array
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const copyOfArray1 = [...array1];
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// For Objects (shallow copy):
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const obj1 = { name: "Alice", age: 30 };
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const obj2 = { ...obj1, city: "New York" }; // Combining objects
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// obj2 is now { name: "Alice", age: 30, city: "New York" }

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function sum(...numbers) {
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  return numbers.reduce((total, num) => total + num, 0);
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}
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const result = sum(1, 2, 3, 4); // Result is 10