JS basics

Question 1

Q: What is JavaScript?

Answer 1

A: A core web technology enabling interactivity in browsers; also used in Node.js, Electron, React Native.

Question 2

Q: Who created JavaScript and when?

Answer 2

A: Brendan Eich in 1995; originally named Mocha, then LiveScript, and finally JavaScript.

Question 3

Q: What is ECMAScript?

Answer 3

A: The standardized version of JavaScript maintained by ECMA. Major versions include ES1, ES5, ES6.

Question 4

Q: How can JavaScript be run?

Answer 4

A: Via <.script> in HTML, browser console, Node.js, or REPL environments.

Question 5

Q: What are the keywords to declare variables in JavaScript?

Answer 5

A: var, let, and const.

Question 6

Q: Differences between var, let, and const?

Answer 6

A: var is function-scoped; let/const are block-scoped. const can’t be reassigned.

Question 7

Q: Can const variables be mutated?

Answer 7

A: Yes, for objects/arrays – only reassignment is disallowed.

Question 8

Q: What happens when using var inside a loop?

Answer 8

A: It leaks outside the block due to function scope.

Question 9

Q: What is hoisting in JavaScript?

Answer 9

A: JavaScript lifts declarations (not initializations) to the top of their scope.

Question 10

Q: What gets hoisted?

Answer 10

A: var declarations and function declarations; let/const hoisted but uninitialized (TDZ).

Question 11

Q: What is the temporal dead zone (TDZ)?

Answer 11

A: The phase between scope entry and variable declaration where access throws an error.

Question 12

Q: What are best practices for naming variables?

Answer 12

A: Use camelCase, meaningful names, constants in UPPER_SNAKE_CASE, avoid reserved words.

Question 13

Q: Should variable names reflect type or purpose?

Answer 13

A: Purpose – e.g., userList over arrUsers.

Question 14

Q: What are the types of scopes in JavaScript?

Answer 14

A: Global, Function, Block, Module.

Question 15

Q: Difference between block and function scope?

Answer 15

A: Block scope is within {}; function scope applies to function body.

Question 16

Q: What is lexical scope?

Answer 16

A: Scope defined by code structure – inner functions can access outer variables.

Question 17

Q: What are the 7 primitive data types in JavaScript?

Answer 17

A: Number, String, Boolean, Null, Undefined, Symbol, BigInt.

Question 18

Q: What is typeof null?

Answer 18

A: ‘object’ – due to a long-standing JavaScript bug.

Question 19

Q: Is typeof [] equal to ‘array’?

Answer 19

A: No – use Array.isArray() or Object.prototype.toString.call([]).

Question 20

Q: Example of different number notations?

Answer 20

A: 255 == 0xff == 0b11111111 == 0.255e3; all evaluate to same number.

Question 21

Q: Difference between type casting and type coercion?

Answer 21

A: Casting is explicit (e.g., Number(‘5’)); coercion is implicit (e.g., ‘5’ - 1).

Question 22

Q: What is implicit coercion?

Answer 22

A: JavaScript automatically converts data types during operations (e.g., ‘5’ + 1 = ‘51’).

Question 23

Q: What is explicit type casting?

Answer 23

A: Manual conversion using String(), Number(), Boolean().

Question 24

Q: When does coercion happen implicitly?

Answer 24

A: During arithmetic, comparisons, string ops, template literals.

Question 25

Q: Difference between == and ===?

Answer 25

A: == allows coercion; === checks type and value strictly.

Question 26

Q: What happens during object-to-primitive coercion?

Answer 26

A: JavaScript calls valueOf(), then toString().

Question 27

Q: What are built-in objects in JS?

Answer 27

A: Global language-provided objects: Number, String, Object, Math, Date, Error, etc.

Question 28

Q: Difference between new Number(5) and Number(5)?

Answer 28

A: new Number(5) creates object; Number(5) returns primitive number.

Question 29

Q: Can Math be instantiated?

Answer 29

A: No – it's a utility object with static methods.

Question 30

Q: What does the Error object do?

Answer 30

A: Handles exceptions with properties like name, message, and stack.

Question 31

Q: What is typeof in JavaScript?

Answer 31

A: An operator that returns a string indicating the operand's data type.

Question 32

Q: What does typeof function() {} return?

Answer 32

A: 'function' – a special case.

Question 33

Q: How to differentiate arrays and objects?

Answer 33

A: Use Array.isArray() or Object.prototype.toString.call(val).

Question 34

Q: What is a prototype in JavaScript?

Answer 34

A: An object from which other objects inherit properties and methods.

Question 35

Q: How to access or set a prototype?

Answer 35

A: Use Object.getPrototypeOf(obj), Object.setPrototypeOf(obj), or Object.create(proto).

Question 36

Q: What is prototypal inheritance?

Answer 36

A: Objects inherit from other objects via the prototype chain.

Question 37

Q: Difference from classical inheritance?

Answer 37

A: Classical uses classes; prototypal uses direct object-to-object delegation.