JavaScript rounds test how well you understand the language itself, not just a framework. Expect questions on scope, closures, the event loop, promises and how values are compared and copied, often as "what does this code print?" puzzles.
Below are 60 JavaScript questions with the correct answer and a short explanation, grouped from fresher to experienced level. Try each one before you open the answer.
What does this print?
console.log(typeof null, typeof [], typeof undefined)
Answer: B. object object undefined
typeof null returns "object" because of an old bug in the first version of JavaScript that can never be fixed. Arrays are objects, so typeof [] is also "object". Use value === null to check for null and Array.isArray(value) to check for arrays.
What does this print?
const user = { name: 'Ana' }
user.name = 'Ben'
console.log(user.name)Answer: C. Ben
const only stops the variable from being reassigned to a different value. The object it points to can still be changed, so updating user.name works and prints Ben. Writing user = {} would throw, and Object.freeze is needed to make the object itself read-only.
What does this print?
const a = 0 || 10 const b = 0 ?? 10 console.log(a, b)
Answer: C. 10 0
|| falls back for any falsy value, and 0 is falsy, so a becomes 10. ?? only falls back when the left side is null or undefined, so b keeps 0. Use ?? for defaults when 0, an empty string or false are valid values.
What do these two lines print?
console.log("5" + 3)
console.log("5" - 3)Answer: A. "53" and 2
When either side of + is a string, + joins strings, so "5" + 3 is "53". The - operator only works on numbers, so "5" is converted to 5 and the result is 2. Convert input with Number() before adding to avoid this bug.
What does this code print?
function greet(name = "Guest") {
return "Hi " + name
}
console.log(greet(undefined), "|", greet(null))Answer: C. Hi Guest | Hi null
A default parameter is used only when the argument is undefined (or missing). null is a real value, so it is passed through and joined as "null". If null should also fall back, use name ?? "Guest" inside the function.
What does this code print?
const user = { name: "Asha" }
const { name: fullName, city = "Dubai" } = user
console.log(fullName, city)Answer: D. Asha Dubai
name: fullName reads the name property into a new variable called fullName. city = "Dubai" gives a default because user.city is undefined. Destructuring a missing property is not an error; it just gives undefined or the default.
In an ES module (which is always strict mode), a developer writes totl = 100 by mistake, where only total was declared. What happens?
Answer: D. A ReferenceError is thrown
In strict mode, assigning to a name that was never declared throws a ReferenceError. In old sloppy scripts the same typo silently created a global variable, which hid bugs. Modules and class bodies are strict by default.
In a classic non-module script (sloppy mode), what does this print?
let s = "cat" s[0] = "b" console.log(s)
Answer: C. cat
Strings in JavaScript are immutable, so assigning to an index does nothing (in strict mode it throws a TypeError). To change a letter, build a new string, for example "b" + s.slice(1).
Where should you hide a loading spinner so it hides whether the request succeeds or fails?
Answer: A. In .finally() or a finally block
finally runs after the promise settles either way and passes the original result or error through unchanged. Putting the cleanup there avoids repeating it in both the success and error paths.
A promise rejects and no code calls .catch or awaits it in try/catch. What happens in the browser?
Answer: C. An unhandledrejection event fires and it is logged
Browsers fire an unhandledrejection event on window and log the error to the console. Error trackers listen for this event. In Node.js, unhandled rejections crash the process by default.
What does data hold here?
const res = await fetch("/api/user")
const data = res.json()
console.log(data.name)Answer: C. A Promise, so data.name is undefined
res.json() reads and parses the body, which takes time, so it returns a promise. Without await, data is the promise and data.name is undefined. Write const data = await res.json().
An invoice page must parse a 40 MB CSV in the browser without freezing the UI. What is the best tool?
Answer: A. A Web Worker
An async function still runs its synchronous parts on the main thread, and timers only delay the work. A Web Worker runs on a separate thread, so heavy parsing does not block clicks or rendering. Results come back with postMessage.
Two declarations of interface User exist in the same scope, one with name and one with email. What is the result?
Answer: A. They merge into one User with both properties
Interfaces support declaration merging: same-named interfaces in one scope are combined. A type alias with the same name twice is an error instead. Merging is how libraries let you add fields to types such as Window.
Given const a = [1, 2, 3, 4, 5], which call changes a itself?
Answer: B. a.splice(1, 2)
splice removes or inserts elements in place and returns the removed items, so a becomes [1, 4, 5]. slice, concat and map all return new arrays and leave the original untouched.
What is the result of this expression?
new Array(3).map(() => 0)
Answer: A. An array of 3 empty slots
new Array(3) creates an array of length 3 with holes, not with undefined values. map skips holes, so the result is still three empty slots. Use new Array(3).fill(0) or Array.from({ length: 3 }, () => 0).
What does [1, null, 2, undefined].join('-') return?
Answer: A. '1--2-'
join converts null and undefined to empty strings, so their slots become empty fields between the separators. This can hide missing data in a CSV export, so filter or replace them first.
What does this print?
const o = {}
o[1] = 'a'
o['1'] = 'b'
console.log(Object.keys(o).length, o[1])Answer: B. 1 and 'b'
Plain object keys are strings (or symbols), so the number 1 is converted to '1' and both writes hit the same key. The second write wins. A Map keeps 1 and '1' as different keys.
A price parser returns NaN for bad input. Which check reliably detects it in JavaScript?
Answer: C. Number.isNaN(price)
NaN is the only value that is not equal to itself, so price === NaN is always false. Number.isNaN returns true only for the NaN value itself. The older global isNaN first converts its argument, so isNaN('abc') is also true.
What does this JavaScript code print?
const marks = [10, 9, 1, 100] marks.sort() console.log(marks)
Answer: B. [1, 10, 100, 9]
With no comparator, Array.prototype.sort converts items to strings and compares them by UTF-16 code units. As strings, "1" < "10" < "100" < "9". Use marks.sort((a, b) => a - b) to sort numbers by value.
How do you check whether two string memory values are equal in Solidity?
Answer: D. keccak256(bytes(a)) == keccak256(bytes(b))
Solidity has no == operator for strings. The common approach is comparing the keccak256 hashes of their bytes, which works for any length. Converting to bytes32 truncates longer strings. Newer helpers like OpenZeppelin's Strings.equal do the same under the hood.
This code runs as an ES module. What does it print?
const counter = {
count: 5,
show: () => console.log(this?.count),
print() { console.log(this.count) },
}
counter.show()
counter.print()Answer: B. undefined, then 5
Arrow functions do not get their own this; they use the this of the surrounding scope. At the top level of a module this is undefined, so this?.count gives undefined. print is a normal method, so when called as counter.print() its this is counter and it prints 5.
A dashboard loads three widgets with Promise.all. When one API fails, the whole page shows an error even though two widgets loaded fine. Which method lets each widget show its own result or error?
Answer: C. Promise.allSettled
Promise.all rejects as soon as any promise rejects, so you lose the successful results. Promise.allSettled waits for all of them and gives an array of { status, value } or { status, reason } objects. Promise.any resolves with only the first success, and Promise.race settles with whichever finishes first.
What happens when this script runs?
console.log(sayHi())
console.log(total)
function sayHi() { return 'hi' }
let total = 5Answer: D. It prints hi, then throws a ReferenceError
Function declarations are hoisted with their body, so sayHi can be called before the line where it is written. let variables are hoisted too, but they stay in the temporal dead zone until their declaration runs. Reading total before that throws "Cannot access total before initialization".
What does this print?
function makeCounter() {
let n = 0
return () => ++n
}
const a = makeCounter()
const b = makeCounter()
a(); a()
console.log(a(), b())Answer: D. 3 1
Each call to makeCounter creates a new n, and the returned arrow function keeps its own n alive through a closure. a has been called three times, so it returns 3, while b has its own n and returns 1 on its first call.
What does this print?
const cfg = Object.freeze({ tax: { gst: 18 } })
cfg.tax.gst = 5
console.log(cfg.tax.gst)Answer: D. 5
Object.freeze only freezes the object’s own properties. cfg.tax cannot be replaced, but the nested object it points to is not frozen, so its gst can change. To lock everything, freeze each nested object too (a deep freeze).
What do these return?
Number("12px")
parseInt("12px", 10)
Number("")Answer: B. NaN, 12 and 0
Number needs the whole string to be numeric, so "12px" gives NaN, but an empty or blank string becomes 0. parseInt reads digits from the start and stops at the first non-digit, so it returns 12. The empty-string case is a common form validation bug.
What does this function return?
function f() {
try {
return "try"
} finally {
console.log("cleanup")
}
}
console.log(f())Answer: A. It logs "cleanup", then "try"
finally always runs, even when try returns. The return value "try" is held while finally runs, so "cleanup" is logged first, then the caller logs "try". If finally itself had a return, it would replace the try value.
A payment status call retries up to 3 times after the first failure, waiting 500 ms before the first retry and doubling each time. If every attempt fails, how long does the code spend waiting in total (ignoring request time)?
Answer: C. 3,500 ms
The waits are 500 ms, then 1,000 ms, then 2,000 ms before retries 1, 2 and 3. 500 + 1,000 + 2,000 = 3,500 ms. Adding random jitter to each wait stops many clients from retrying at the same moment.
In Node.js (CommonJS script), what is the log order?
Promise.resolve().then(() => console.log("promise"))
process.nextTick(() => console.log("tick"))
console.log("sync")Answer: D. sync, tick, promise
Synchronous code runs first. After it, Node drains the process.nextTick queue before the promise microtask queue, so "tick" comes before "promise". Heavy use of nextTick can starve I/O, so queueMicrotask or promises are usually preferred.
fetch has no timeout option. What is the simplest modern way to fail a request after 5 seconds?
Answer: C. fetch(url, { signal: AbortSignal.timeout(5000) })
AbortSignal.timeout(ms) returns a signal that aborts on its own after the given time, rejecting fetch with a TimeoutError. It is supported in all modern browsers and Node. Combine it with other signals using AbortSignal.any if the user can also cancel.
Inside case "card", which properties can you read on p without an error?
type Payment =
| { kind: "upi"; vpa: string }
| { kind: "card"; last4: string; network: string }
function label(p: Payment) {
switch (p.kind) {
case "card": /* here */
}
}Answer: B. kind, last4 and network
Each member of the union has a literal kind, which acts as a discriminant. Checking p.kind === "card" narrows p to the card member, so its last4 and network fields become available, while vpa does not.
With "noUncheckedIndexedAccess": true, what is the type of rate in const rate = rates["dubai"], where rates is Record<string, number>?
Answer: B. number | undefined
Without the flag, TypeScript assumes every index exists and gives number. With it, index reads on arrays and string-keyed records add undefined, because the key may be missing. This catches out-of-range and missing-key bugs.
Which code removes the order with id 42 from orders state without mutating it?
Answer: D. setOrders(orders.filter((o) => o.id !== 42))
filter returns a new array without the matching item, which is the immutable way to delete. splice and delete change the original array, and find returns a single item, not an array.
What does this print?
const orders = [{ id: 1, status: 'new' }]
const copy = [...orders]
copy[0].status = 'paid'
console.log(orders[0].status)Answer: C. 'paid'
Spread makes a new outer array but copies element references, so both arrays point to the same object. Changing a property through copy changes it for orders too. Use structuredClone or map with object spread for a deep enough copy.
a and b are Sets of strings. Why is a === b the wrong test for "same contents", and what works?
Answer: A. === compares references; check size and every item of a in b
Objects, including Sets, are equal under === only if they are the same object. For contents, check a.size === b.size and that every value of a is in b, which is O(n). Modern engines also offer a.isSubsetOf(b) with a size check.
In JavaScript, what is the value of `-1 >>> 0`?
Answer: C. 4294967295
-1 in 32-bit two's complement is all 32 bits set. The unsigned right shift >>> treats the result as an unsigned 32-bit integer, so a shift of 0 just reinterprets those bits as 2^32 - 1 = 4294967295. x >>> 0 is a common way to get an unsigned 32-bit view of a number.
A developer writes this to sort numbers. Why is it a bug?
scores.sort((a, b) => a > b)
Answer: D. It never returns a negative value, so the order is unreliable
The comparator must return negative, zero or positive. true and false become 1 and 0, so "a should come first" is never signalled. Results depend on the engine and input, and often look almost right. Use (a, b) => a - b.
A UAE app sorts customer names that include accents and mixed case, such as "élodie", "Zara" and "aisha". Which JavaScript approach gives a natural alphabetical order?
Answer: D. names.sort(new Intl.Collator('en').compare)
Default sort compares UTF-16 code units, so all uppercase letters come before lowercase and accented letters go to the end. Intl.Collator (or localeCompare) applies language rules. A shared Collator is faster than calling localeCompare on every comparison.
The backend renamed fullName to displayName. tsc still passes, but the frontend crashes in production. Why didn’t TypeScript catch it?
type User = { id: string; fullName: string }
const res = await fetch('/api/me')
const user = (await res.json()) as User
render(user.fullName.toUpperCase())Answer: A. Types are erased at runtime and "as" is unchecked; validate data at the boundary
res.json() returns unknown data and "as User" tells the compiler to believe you without any check. Nothing about types exists at runtime, so a changed API shape only shows up as a crash. Parsing responses with a schema, or generating types from a shared contract such as OpenAPI or tRPC, makes the mismatch visible.
Why must the completion parameter be marked @escaping here?
var handlers: [() -> Void] = []
func onFinish(_ completion: @escaping () -> Void) {
handlers.append(completion)
}Answer: B. It is stored and called after the function returns
Closures are non-escaping by default, meaning they cannot outlive the call. Storing one in a property or calling it later (for example in a network callback) needs @escaping, which also makes you think about self capture.
What does this print?
const o = { b: 1, 2: 1, a: 1, 1: 1 }
console.log(Object.keys(o))Answer: B. ['1', '2', 'b', 'a']
Property order is defined by the spec: integer-like keys come first in ascending numeric order, then string keys in insertion order. So 1 and 2 come first, followed by b and a. If you need a reliable insertion order for any key, including numeric IDs, use a Map.
Which line or lines does TypeScript report as an error?
type User = { name: string }
const data = { name: 'Ana', age: 30 }
const a: User = data // line A
const b: User = { name: 'Ana', age: 30 } // line BAnswer: B. Only line B
TypeScript is structural, so a value with extra properties is still assignable to User, which makes line A fine. But fresh object literals get an extra "excess property check" that flags unknown properties as likely typos, so line B errors. This is why a misspelled option in an inline object is caught, while the same object passed through a variable is not.
What does this print?
const id = Symbol("id")
const user = { name: "Ravi", [id]: 42 }
console.log(Object.keys(user), JSON.stringify(user))Answer: A. [ "name" ] {"name":"Ravi"}
Symbol-keyed properties are skipped by Object.keys, for...in and JSON.stringify. They can be read with Object.getOwnPropertySymbols or Reflect.ownKeys. This makes symbols useful for metadata that should not leak into serialised data.
What does this print?
console.log(-0 === 0, Object.is(-0, 0), Object.is(NaN, NaN))
Answer: D. true false true
Strict equality treats -0 and +0 as equal and NaN as unequal to itself. Object.is uses the SameValue rule, which separates -0 from +0 and treats NaN as equal to NaN. React uses Object.is to decide whether state has changed.
In Node.js, slowTask takes 2 s and failingTask rejects after 100 ms. Why can the process crash with an unhandled rejection?
const a = slowTask() const b = failingTask() await a await b // inside try/catch
Answer: C. b rejects while the code still waits on a, with no handler yet
A rejection is checked for a handler soon after it happens. b rejects at 100 ms, but nothing has attached to b yet because the code is still waiting on a. Node treats it as unhandled and exits. Use await Promise.all([a, b]) so both have handlers at once.
An async generator opens a database cursor and closes it in a finally block. What happens when the loop breaks early?
async function* rows() {
const cur = await db.openCursor()
try {
for await (const r of cur) yield r
} finally {
await cur.close()
}
}
for await (const r of rows()) {
if (r.id === 10) break
}Answer: A. break calls return() on the generator, so finally runs
When a for...of or for await...of loop exits early with break, return or throw, it calls the iterator’s return() method. For a generator this resumes it as if a return happened at the yield, so finally blocks run and resources are released.
This code updates a progress bar 100 times while processing items, but the user only sees 100% at the end. Why?
for (let i = 1; i <= 100; i++) {
processItem(i) // 30 ms of sync work
bar.style.width = i + "%"
}Answer: D. The browser paints only after the whole task finishes
Rendering happens between tasks, not during one. The loop is one long 3-second task, so the browser cannot paint until it ends. Process chunks and yield with await scheduler.yield() or setTimeout so the bar can repaint, or use a Worker.
In Node.js, what is the order inside a file read callback?
fs.readFile("a.txt", () => {
setTimeout(() => console.log("timeout"), 0)
setImmediate(() => console.log("immediate"))
})Answer: A. immediate, then timeout, every time
After the poll (I/O) phase, Node’s event loop goes to the check phase, where setImmediate callbacks run, before looping back to the timers phase. So inside an I/O callback, immediate always comes first. At the main module top level, the order can vary.
You upload 100 invoices. Each upload takes 200 ms, and a pool runs at most 5 at a time. Roughly how long does the whole batch take?
Answer: C. 4 seconds
With 5 running at once, 100 uploads need 100 ÷ 5 = 20 rounds. Each round takes about 200 ms, so 20 × 200 ms = 4,000 ms. Unlimited Promise.all would start all 100 at once and may hit rate limits or browser connection caps.
p is a native promise. What does Promise.resolve(p) === p print?
Answer: B. true
If the argument is already a native promise created by the same Promise constructor, Promise.resolve returns it unchanged. For other thenables or values it creates a new promise. new Promise(r => r(p)) would always make a new one.
What type does Item resolve to?
type ElementOf<T> = T extends readonly (infer U)[] ? U : never
const cart = [{ sku: "A1", qty: 2 }]
type Item = ElementOf<typeof cart>Answer: A. { sku: string; qty: number }
infer U declares a type variable that TypeScript fills by matching T against the pattern. typeof cart is { sku: string; qty: number }[], which matches an array of U, so U is the object type. Non-array types would give never.
What does this mapped type produce for { readonly id?: string }?
type Mutable<T> = { -readonly [K in keyof T]-?: T[K] }Answer: C. { id: string }
In a mapped type, -readonly removes the readonly modifier and -? removes optionality from every property. The built-in Required<T> uses -?, and a + prefix (or none) adds modifiers. The result is a plain required, writable id.
Which assignment is a compile error?
let a: {} = 42
let b: {} = "hi"
let c: object = 42
let d: unknown = nullAnswer: A. Only c
The {} type means "any value except null and undefined", so numbers and strings fit. object means non-primitive values only, so 42 is rejected. unknown accepts everything, including null.
The user types more while save() is running. What does the alert show?
async function handleSave() {
await save(text)
alert("Saved: " + text)
}Answer: A. The text from the render when the click happened
The handler closes over text from the render it belongs to. Later renders create new handlers with new text, but this running function still sees its own snapshot. Read the latest value from a ref if you really need it.
useCart returns addItem, but addItem always adds to the cart as it was on the first render. What is the bug?
function useCart() {
const [items, setItems] = useState([])
const addItem = useCallback((it) => {
setItems([...items, it])
}, [])
return { items, addItem }
}Answer: C. It keeps the first items; use setItems((prev) => [...prev, it])
With an empty dependency array, the callback is created once and keeps the items from the first render. Using the functional update form reads the latest state at update time, so the callback can stay stable without listing items.
This code should remove every cancelled order but leaves one behind for [c, c, ok] (c = cancelled). Why?
for (let i = 0; i < orders.length; i++) {
if (orders[i].cancelled) orders.splice(i, 1)
}Answer: C. After splice the next item shifts into index i and is skipped
Removing index 0 moves the second cancelled order into index 0, but the loop then increments i to 1 and never checks it. Loop backwards, decrement i after a splice, or better, use filter to build a new array in O(n).
After m.set(NaN, 'a') and m.set(0, 'b'), what do m.get(NaN) and m.get(-0) return?
Answer: D. 'a' and 'b'
Map keys use SameValueZero: NaN is treated as equal to NaN, and +0 and -0 count as the same key. This differs from ===, where NaN !== NaN.
This search finds the last index where ok(i) is true. When does it loop forever?
while (lo < hi) {
const mid = Math.floor((lo + hi) / 2)
if (ok(mid)) lo = mid
else hi = mid - 1
}Answer: C. When hi = lo + 1 and ok(lo) is true
With hi = lo + 1, mid rounds down to lo. If ok(lo) is true, lo = mid changes nothing, and the loop repeats. When the update is lo = mid, round the midpoint up: mid = lo + Math.ceil((hi - lo) / 2).
You fan out three LLM calls in Node.js. What does this code log?
async function call(id) {
if (id === 2) throw new Error('rate limited')
return id * 10
}
const out = []
try {
const r = await Promise.all([1, 2, 3].map(call))
out.push(...r)
} catch (e) {
out.push(e.message)
}
console.log(out)Answer: D. [ 'rate limited' ]
Promise.all rejects as soon as any promise rejects, so r is never assigned and the successful results 10 and 30 are lost; only the error message is pushed. For batch LLM calls, use Promise.allSettled or per-call try/catch with retries so one rate-limit error does not discard the others.
This test sometimes times out even though the request succeeds. Why, and what is the fix?
await page.getByRole('button', { name: 'Search' }).click()
const res = await page.waitForResponse('**/api/search*')
expect(res.ok()).toBeTruthy()Answer: D. The response can arrive before waitForResponse starts listening; start waiting before the click, for example const p = page.waitForResponse(...) then click, then await p
If the API is fast, the response finishes during the click and the listener registered afterwards never sees it. Creating the wait promise first, or using Promise.all with the click, removes the race.
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