JavaScript Quiz (Advanced – Extreme Hard) - All Questions

This JavaScript Extreme Hard Quiz is crafted for senior and expert-level developers. It focuses on deep JavaScript internals such as execution context, closures, prototypes, event loop, memory management, async behavior, edge cases, and real-world interview scenarios.

Question 1: What creates a new execution context in JavaScript?

Block scope
Function invocation
Variable declaration
Object creation

Explanation: Each function invocation creates a new execution context.

Question 2: Which statement about closures is correct?

They copy variables
They capture lexical scope
They work only with var
They destroy memory automatically

Explanation: Closures retain access to their lexical scope.

Question 3: What is the output type of typeof null?

null
undefined
object
number

Explanation: This is a historical JavaScript bug.

Question 4: Which queue has higher priority in the event loop?

Callback queue
Microtask queue
Render queue
Macro queue

Explanation: Microtasks are executed before macrotasks.

Question 5: Which method schedules a microtask?

setTimeout()
setInterval()
Promise.then()
requestAnimationFrame()

Explanation: Promise.then() is queued in the microtask queue.

Question 6: What does Object.create(null) produce?

Empty object
Object without prototype
Frozen object
Sealed object

Explanation: It creates an object without Object.prototype.

Question 7: Which statement about arrow functions is true?

They have their own this
They bind arguments object
They inherit this lexically
They can be used as constructors

Explanation: Arrow functions inherit this from surrounding scope.

Question 8: What happens when you use new with an arrow function?

Creates object
Throws error
Returns undefined
Ignores new

Explanation: Arrow functions cannot be used as constructors.

Question 9: Which operation causes garbage collection eligibility?

Setting variable to null
Removing event listener
Losing all references
Calling delete

Explanation: Objects are collected when no references exist.

Question 10: Which keyword prevents prototype modification?

const
seal
freeze
lock

Explanation: Object.freeze() prevents modifications.

Question 11: What is the result of [] + {}?

[object Object]
{}
NaN
undefined

Explanation: Array converts to empty string, object to string.

Question 12: What is the result of {} + [] in non-strict mode?

0
[object Object]
NaN
SyntaxError

Explanation: {} is treated as block, +[] evaluates to 0.

Question 13: Which feature enables tail-call optimization?

Arrow functions
Strict mode
Recursion
Generators

Explanation: TCO is only specified in strict mode.

Question 14: Which statement about hoisting is correct?

let is hoisted and initialized
const is hoisted and initialized
var is hoisted and initialized
let is hoisted but uninitialized

Explanation: let exists in TDZ until initialized.

Question 15: What is Temporal Dead Zone?

Garbage memory
Unreachable scope
Time before initialization
Async delay

Explanation: TDZ exists before variable initialization.

Question 16: Which operator short-circuits on null or undefined only?

Explanation: Nullish coalescing ignores falsy values like 0.

Question 17: Which method preserves property descriptors?

Object.assign()
Spread operator
Object.create()
JSON.parse()

Explanation: Object.create() allows descriptor control.

Question 18: What is the prototype of a function?

null
Object.prototype
Function.prototype
undefined

Explanation: Functions inherit from Function.prototype.

Question 19: Which API blocks the main thread?

setTimeout()
fetch()
alert()
Promise

Explanation: alert() blocks execution.

Question 20: Which structure ensures immutability?

Map
Set
Object.freeze()
WeakMap

Explanation: freeze prevents modifications.

Question 21: Which object allows weakly held keys?

Map
Set
WeakMap
Array

Explanation: WeakMap keys are weakly referenced.

Question 22: Which value is falsy?

{}
[]
"0"
NaN

Explanation: NaN is falsy.

Question 23: Which method schedules rendering callback?

setTimeout()
Promise.then()
requestAnimationFrame()
queueMicrotask()

Explanation: requestAnimationFrame syncs with repaint.

Question 24: Which mechanism prevents memory leaks in DOM?

Garbage collector
Weak references
Event delegation
Strict mode

Explanation: Event delegation reduces detached listeners.

Question 25: Which object is global in Node.js?

window
global
this
self

Explanation: Node.js uses global instead of window.

Question 26: Which statement about promises is true?

They are synchronous
They execute immediately
They resolve once
They block threads

Explanation: Promises settle only once.

Question 27: Which method flattens nested arrays?

flat()
reduce()
concat()
spread

Explanation: flat() flattens nested arrays.

Question 28: Which feature enables lazy evaluation?

Promises
Generators
Async functions
Closures

Explanation: Generators pause and resume execution.

Question 29: Which syntax pauses generator execution?

return
yield
await
break

Explanation: yield pauses generator execution.

Question 30: Which object converts iterable to iterator?

Symbol.iterator
Iterator()
forEach()
map()

Explanation: Symbol.iterator defines iteration behavior.

Question 31: Which operator checks prototype chain?

hasOwn
instanceof
typeof
in

Explanation: instanceof checks prototype chain.

Question 32: Which operation mutates array length?

slice()
map()
length = 0
filter()

Explanation: Setting length mutates the array.

Question 33: Which API enables shared memory?

SharedArrayBuffer
ArrayBuffer
Buffer
TypedArray

Explanation: SharedArrayBuffer enables shared memory.

Question 34: Which keyword prevents variable hoisting misuse?

var
const
let
static

Explanation: let introduces block scoping.

Question 35: Which feature ensures referential transparency?

Closures
Pure functions
Promises
Callbacks

Explanation: Pure functions always return same output.

Question 36: Which API allows atomic operations?

Atomics
Mutex
Locks
Threads

Explanation: Atomics provide atomic operations.

Question 37: Which value is returned by Promise.resolve()?

Resolved value
Promise
Callback
Undefined

Explanation: Promise.resolve() returns a resolved Promise.

Question 38: Which structure preserves insertion order?

Object
Map
Set
WeakMap

Explanation: Map preserves insertion order.

Question 39: Which operator prevents optional chaining errors?

Explanation: Optional chaining prevents access errors.

Question 40: Which pattern avoids callback hell?

Event listeners
Promises
Loops
Closures

Explanation: Promises flatten async chains.

Question 41: Which API handles streaming data?

Streams
Buffers
Promises
Workers

Explanation: Streams process data incrementally.

Question 42: Which method freezes nested objects automatically?

Object.freeze()
Deep freeze
JSON.stringify()
None

Explanation: freeze is shallow by default.

Question 43: Which statement about this in strict mode is correct?

Defaults to window
Defaults to global
Is undefined
Is null

Explanation: In strict mode, this is undefined.

Question 44: Which construct enables cooperative multitasking?

Promises
Generators
Async/await
Workers

Explanation: Generators yield control cooperatively.

Question 45: Which API enables true parallelism?

setTimeout()
Promises
Web Workers
Event loop

Explanation: Web Workers run in separate threads.

Question 46: Which operation forces synchronous layout?

Reading offsetHeight
Changing opacity
Using transform
Using will-change

Explanation: Reading layout properties forces reflow.

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JavaScript Quiz (Advanced – Extreme Hard)

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JavaScript Quiz (Advanced – Extreme Hard)