50 Simple JavaScript Programming Answers

#	Question	Answer
1	How do you declare a variable?	var, let, or const
2	What is the difference between let and var?	let is block-scoped, can be reassigned; var is function-scoped, can be reassigned
3	What is the difference between const and let?	const is block-scoped, cannot be reassigned; let is block-scoped, can be reassigned
4	How do you create an array?	const arr = [1, 2, 3];
5	How do you create an object?	const obj = { name: 'Alice', age: 30 };
6	How do you access an element in an array?	arr[0]
7	How do you access a property in an object?	obj.name or obj['name']
8	How do you define a function?	function myFunction() { ... } or const myFunction = () => { ... };
9	How do you call a function?	myFunction();
10	What is an if-else statement?	if (condition) { ... } else { ... }
11	What is a for loop?	for (let i = 0; i < 5; i++) { ... }
12	What is a while loop?	while (condition) { ... }
13	How do you add an element to an array?	arr.push(element);
14	How do you remove an element from an array?	arr.pop(); (last), arr.shift(); (first), arr.splice(index, 1);
15	How do you iterate over an array?	arr.forEach(item => { ... }); or for (const item of arr) { ... }

#	Question	Answer	Example
16	What is the map() method?	Creates a new array by calling a function on every element in the original array	const newArr = arr.map(x => x * 2);
17	What is the filter() method?	Creates a new array with all elements that pass a test	const filteredArr = arr.filter(x => x > 5);
18	What is the reduce() method?	Executes a reducer function on each element of the array, resulting in a single value	const sum = arr.reduce((acc, curr) => acc + curr, 0);
19	What is an arrow function?	A concise syntax for writing functions	const add = (a, b) => a + b;
20	What is the difference between == and ===?	== is loose equality (type coercion); === is strict equality (no type coercion)	5 == '5' (true), 5 === '5' (false)
21	What is undefined?	A variable has been declared but not yet assigned a value	let x; console.log(x); // undefined
22	What is null?	Represents the intentional absence of any object value	let y = null; (explicitly set to null)
23	What is a Promise?	An object representing the eventual completion (or failure) of an asynchronous operation	new Promise((resolve, reject) => { ... });
24	What is async/await?	Syntax sugar over Promises, making asynchronous code look and feel more synchronous	async function fetchData() { const response = await fetch('/api'); ... }
25	What is a try...catch block?	Used for error handling in synchronous code	try { ... } catch (error) { ... }
26	What is a Closure?	A function that has access to variables from its outer scope, even after the outer function has finished	function outer() { let x = 10; function inner() { console.log(x); } return inner; }
27	What is the this keyword?	Refers to the object that owns the currently executing code (context depends on how it's called)	const obj = { method: function() { console.log(this); } };
28	What is the DOM (Document Object Model)?	A tree-like structure representing HTML and XML documents	document.getElementById('myId');
29	How do you select an HTML element?	Using document.getElementById(), document.querySelector(), document.querySelectorAll()	document.querySelector('.myClass');
30	How do you add an event listener?	Using element.addEventListener()	button.addEventListener('click', () => { ... });
31	What is JSON (JavaScript Object Notation)?	A lightweight data-interchange format	{ "name": "Bob", "age": 25 }
32	How do you convert a JS object to a JSON string?	JSON.stringify(obj)	const jsonString = JSON.stringify(obj);
33	How do you convert a JSON string to a JS object?	JSON.parse(jsonString)	const obj = JSON.parse(jsonString);
34	What are Template Literals?	String literals using backticks (`) allowing embedded expressions	const message = Hello, ${name}!;
35	What is Destructuring Assignment?	Extracting values from arrays or properties from objects into distinct variables	const { name, age } = { name: 'Alice', age: 30 }; or const [a, b] = [1, 2];
36	What is the Spread Syntax (...)?	Expands an iterable (like an array or string) into individual elements	const newArr = [...arr1, ...arr2];, const newObj = { ...obj1, c: 3 };
37	What are Rest Parameters (...)?	Collects an indefinite number of arguments into an array	function sum(...args) { return args.reduce((a, b) => a + b, 0); }
38	What is a Callback Function?	A function passed as an argument to another function to be executed later	setTimeout(callback, 1000);
39	What is Callback Hell?	Nested callbacks that make code difficult to read and maintain	Promises offer a cleaner alternative.
40	What is the purpose of super?	Calls the constructor or methods of the parent class	class Child extends Parent { constructor() { super(); ... } }
41	What is the instanceof operator?	Checks if an object is an instance of a specific class or constructor function	if (obj instanceof Array) { ... }
42	What is the in operator?	Checks if a property exists in an object (either directly or in its prototype chain)	if ('name' in obj) { ... }
43	What is setTimeout()?	Executes a function after a specified delay	setTimeout(callback, 1000);
44	What is setInterval()?	Executes a function repeatedly at a specified time interval	setInterval(callback, 1000);
45	What is clearTimeout()?	Stops a timer created with setTimeout()	clearTimeout(timerId);
46	What is clearInterval()?	Stops a timer created with setInterval()	clearInterval(intervalId);
47	What is the difference between call(), apply(), and bind()?	All set the this value for a function. call takes arguments individually; apply takes arguments as an array; bind returns a new function with fixed this.	func.call(thisArg, arg1, arg2); func.apply(thisArg, [arg1, arg2]); const boundFunc = func.bind(thisArg);
48	What is a Shallow Copy?	Creates a new object/array with references to the same nested objects/arrays as the original	const newObj = { ...obj }; or const newArr = [...arr];
49	What is a Deep Copy?	Creates a completely new object/array, including copies of all nested objects/arrays	JSON.parse(JSON.stringify(obj)) (simple, but has limitations) or libraries like _.cloneDeep
50	What is the typeof operator?	Returns a string indicating the type of the operand	typeof 'hello' ('string'), typeof 10 ('number'), typeof {} ('object'), typeof [] ('object'), typeof function(){} ('function')