Can You Guess These 50 JavaScript Outputs? (React Native Interview Edition)

React Native interviews rarely ask you to recite API docs.


They drop a short snippet and ask:

Guess the output?

Same language. Same traps. Whether the code runs in a browser, Metro, or Hermes — JavaScript rules don’t change.

These 50 challenges cover what interviewers actually probe:

hoisting · scope · closures · this · coercion · references · prototypes · Promises · async/await · event loop

How to play (same as the Instagram / LinkedIn challenges)

1. Read the snippet.


2. Pick A / B / C / D before you scroll.


3. Open 👀 Reveal answer only after you commit.


4. Say why out loud — interviewers care about the explanation as much as the letter.

Challenge rule: Do not run the code until you have made your prediction.

React Native note: Timers, Promises, closures, and shared object references show up as real bugs — stale state, double renders, wrong list keys, race conditions. The UI layer does not rewrite the language.

🔥 Warm-up: coercion, scope, and hoisting

⚡ Challenge 1 — Guess the output?

console.log(1 + "2" + 3 - 4);

A. 119


B. 1234


C. 1199


D. NaN

1 + "2" + 3 → "123" (string concat).


"123" - 4 coerces to number → 119.

⚡ Challenge 2 — Guess the output?

console.log(score);

var score = 10;

A. 10


B. undefined


C. ReferenceError


D. null

var is hoisted and initialized as undefined. Only the assignment stays in place.

⚡ Challenge 3 — Guess the output?

console.log(score);

let score = 10;

A. 10


B. undefined


C. ReferenceError


D. null

let is in the Temporal Dead Zone until its line runs. Classic RN interview follow-up after Challenge 2.

⚡ Challenge 4 — Guess the output?

console.log(add(2, 3));

function add(a, b) {

  return a + b;

}

A. 5


B. undefined


C. TypeError


D. ReferenceError

Function declarations are hoisted with their full body.

⚡ Challenge 5 — Guess the output?

console.log(add(2, 3));

var add = function (a, b) {

  return a + b;

};

A. 5


B. undefined


C. TypeError


D. ReferenceError

add is hoisted as undefined, then you call undefined(...).

⚡ Challenge 6 — Guess the output?

console.log(name);

const name = "Amit";

A. "Amit"


B. undefined


C. ReferenceError


D. TypeError

Same TDZ rules as let.

⚡ Challenge 7 — Guess the output?

var a = 1;

if (true) {

  var a = 2;

  let b = 3;

}

console.log(a, typeof b);

A. 1 "undefined"


B. 2 "undefined"


C. 2 "number"


D. Error

var is function-scoped (rewrites outer a).


b is block-scoped; typeof safely returns "undefined" for an undeclared identifier.

🔁 Closures & loops (stale-timer territory)

⚡ Challenge 8 — Guess the output?

for (var i = 0; i < 3; i++) {

  setTimeout(() => console.log(i), 0);

}

A. 0 1 2


B. 3 3 3


C. 0 0 0


D. Error

One shared var i. Loop finishes (i === 3) before timers fire.


Same class of bug as firing three API calls from a loop with the wrong index.

⚡ Challenge 9 — Guess the output?

for (let i = 0; i < 3; i++) {

  setTimeout(() => console.log(i), 0);

}

A. 0 1 2


B. 3 3 3


C. 0 0 0


D. Error

let creates a new binding per iteration.

⚡ Challenge 10 — Guess the output?

function makeCounter() {

  let count = 0;

  return () => ++count;

}

const counter = makeCounter();

console.log(counter(), counter());

A. 1 1


B. 0 1


C. 1 2


D. Error

The returned function closes over the same count. Think custom hooks / factory helpers.

⚡ Challenge 11 — Guess the output?

function createCounter() {

  let count = 0;

  return () => ++count;

}

const a = createCounter();

const b = createCounter();

console.log(a(), a(), b());

A. 1 2 1


B. 1 2 3


C. 1 1 1


D. 0 1 0

Each factory call gets its own lexical environment.

⚡ Challenge 12 — Guess the output?

let x = 5;

console.log(x++);

console.log(++x);

console.log(x);

A. 5, 6, 6


B. 5, 7, 7


C. 6, 6, 7


D. 6, 7, 7

x++ returns then increments (5, now 6).


++x increments then returns (7).


Final x is 7.

🎯 this binding (methods, bind, arrows)

⚡ Challenge 13 — Guess the output?

const user = {

  name: "Amit",

  getName() {

    return this.name;

  },

};

console.log(user.getName());

A. "Amit"


B. undefined


C. window.name


D. Error

Call site is user.getName() → this is user.

⚡ Challenge 14 — Guess the output?

"use strict";

const user = {

  name: "Amit",

  getName() {

    return this.name;

  },

};

const getName = user.getName;

console.log(getName());

A. "Amit"


B. undefined


C. TypeError


D. ReferenceError

Detached method → this === undefined in strict mode → reading .name throws.


Same pitfall as passing obj.method into a callback without binding.

⚡ Challenge 15 — Guess the output?

const user = {

  name: "Amit",

  getName: () => this.name,

};

console.log(user.getName());

A. "Amit"


B. undefined


C. TypeError


D. Always "global"

Arrows do not take this from the object. They capture outer this.

⚡ Challenge 16 — Guess the output?

const user = { name: "Amit" };

function greet() {

  return `Hi ${this.name}`;

}

const boundGreet = greet.bind(user);

console.log(boundGreet());

A. "Hi Amit"


B. "Hi undefined"


C. Error


D. undefined

bind locks this permanently.

🧪 Equality, types, and weird JS

⚡ Challenge 17 — Guess the output?

console.log([] == false, [] === false);

A. true true


B. true false


C. false true


D. false false

== coerces both sides toward numbers (0 == 0).


=== also checks type → false.

⚡ Challenge 18 — Guess the output?

console.log(null == undefined, null === undefined);

A. true true


B. true false


C. false true


D. false false

Special == rule: null and undefined are loosely equal.

⚡ Challenge 19 — Guess the output?

console.log(NaN === NaN, Object.is(NaN, NaN));

A. true true


B. true false


C. false true


D. false false

NaN !== NaN with ===.


Object.is treats two NaNs as equal — useful when validating parsed numbers from APIs.

⚡ Challenge 20 — Guess the output?

console.log(typeof null, typeof []);

A. "null" "array"


B. "object" "object"


C. "undefined" "object"


D. "null" "object"

Historic quirk. Use Array.isArray() for arrays.

📦 References (React state landmines)

⚡ Challenge 21 — Guess the output?

console.log([] === [], [] == []);

A. true true


B. true false


C. false true


D. false false

Each [] is a new object. References never match.

⚡ Challenge 22 — Guess the output?

const a = [1, 2];

const b = a;

b.push(3);

console.log(a);

A. [1, 2]


B. [1, 2, 3]


C. [3]


D. Error

a and b point to the same array. Mutating “copy” mutates state if you shared the reference.

⚡ Challenge 23 — Guess the output?

const a = { profile: { city: "Delhi" } };

const b = { ...a };

b.profile.city = "Pune";

console.log(a.profile.city);

A. "Delhi"


B. "Pune"


C. undefined


D. Error

Spread is shallow. Nested profile is still shared — classic “I updated state but nested object mutated” interview moment.

⚡ Challenge 24 — Guess the output?

const obj = {};

const a = {};

const b = {};

obj[a] = "first";

obj[b] = "second";

console.log(obj[a]);

A. "first"


B. "second"


C. undefined


D. Error

Object keys become strings → both are "[object Object]". Use Map for object identity keys.

📚 Arrays & helpers interviewers love

⚡ Challenge 25 — Guess the output?

console.log([1, 30, 4, 21].sort());

A. [1, 4, 21, 30]


B. [1, 21, 30, 4]


C. [30, 21, 4, 1]


D. Error

Default sort compares as strings. Use (a, b) => a - b for numbers.

⚡ Challenge 26 — Guess the output?

console.log(["1", "2", "3"].map(parseInt));

A. [1, 2, 3]


B. [1, NaN, NaN]


C. [1, 1, 1]


D. Error

map passes (value, index).


parseInt("2", 1) → NaN (bad radix). Prefer .map(Number) or .map((x) => parseInt(x, 10)).

⚡ Challenge 27 — Guess the output?

const items = [1, , 3];

console.log(items.map((x) => x * 2), items.length);

A. [2, 0, 6] 3


B. [2, undefined, 6] 3


C. [2, empty, 6] 3


D. Error

map skips holes and keeps the hole in the result. Length stays 3.

⚡ Challenge 28 — Guess the output?

console.log([0, 1, false, 2, "", 3, null].filter(Boolean));

A. [0, 1, 2, 3]


B. [1, 2, 3]


C. [false, null]


D. Error

filter(Boolean) drops all falsy values — including legitimate 0 (e.g. quantity / score).

⚡ Challenge 29 — Guess the output?

const values = [1, 2, 3];

const removed = values.splice(1, 1);

console.log(values, removed);

A. [1,2,3] [2]


B. [1,3] [2]


C. [1,3] [1,2]


D. Error

splice mutates and returns removed items. Prefer immutable updates in React state.

⚡ Challenge 30 — Guess the output?

const [a = 1, b = 2] = [undefined, null];

console.log(a, b);

A. 1 2


B. undefined null


C. 1 null


D. null 2

Defaults apply only for undefined, not null — important when APIs return null.

🧩 Parameters, prototypes, classes

⚡ Challenge 31 — Guess the output?

function greet(name = "Guest") {

  return name;

}

console.log(greet(), greet(undefined), greet(null));

A. Guest Guest Guest


B. Guest Guest null


C. undefined undefined null


D. Error

Defaults fire for missing args / undefined, not for null.

⚡ Challenge 32 — Guess the output?

function total(...nums) {

  return nums.reduce((sum, n) => sum + n, 0);

}

console.log(total(1, 2, 3));

A. 6


B. [1,2,3]


C. 123


D. Error

Rest params → real array → reduce.

⚡ Challenge 33 — Guess the output?

function login(email, password = "secret", remember) {}

console.log(login.length);

A. 3


B. 2


C. 1


D. 0

function.length counts parameters before the first defaulted one.

⚡ Challenge 34 — Guess the output?

const parent = { role: "admin" };

const user = Object.create(parent);

user.name = "Amit";

console.log(user.role, user.hasOwnProperty("role"));

A. "admin" true


B. "admin" false


C. undefined false


D. Error

role comes from the prototype chain, not an own property.

⚡ Challenge 35 — Guess the output?

const parent = { theme: "dark" };

const settings = Object.create(parent);

settings.theme = "light";

delete settings.theme;

console.log(settings.theme);

A. "light"


B. "dark"


C. undefined


D. Error

Deleting the own property reveals the inherited one again.

⚡ Challenge 36 — Guess the output?

const config = Object.freeze({

  api: { timeout: 1000 },

});

config.api.timeout = 2000;

console.log(config.api.timeout);

A. 1000


B. 2000


C. TypeError


D. undefined

Object.freeze is shallow. Nested objects stay mutable.

⚡ Challenge 37 — Guess the output?

const user = new User();

class User {}

A. A User instance


B. undefined


C. ReferenceError


D. TypeError

Classes are hoisted into the TDZ — not usable before the declaration line.

⏱️ Promises, async/await & the event loop

⚡ Challenge 38 — Guess the output?

console.log("start");

Promise.resolve().then(() => console.log("promise"));

console.log("end");

A. start promise end


B. promise start end


C. start end promise


D. Error

Sync first → then microtasks.

⚡ Challenge 39 — Guess the output?

console.log("start");

setTimeout(() => console.log("timer"), 0);

Promise.resolve().then(() => console.log("promise"));

console.log("end");

A. start end promise timer


B. start promise end timer


C. start end timer promise


D. timer promise start end

All microtasks before the next macrotask — even setTimeout(..., 0).


Core RN interview question for loading indicators + API race timing.

⚡ Challenge 40 — Guess the output?

Promise.resolve().then(() => {

  console.log(1);

  Promise.resolve().then(() => console.log(2));

});

Promise.resolve().then(() => console.log(3));

A. 1 2 3


B. 1 3 2


C. 3 1 2


D. 1 3

First wave queues 1 and 3. Nested 2 is appended after 1 runs.

⚡ Challenge 41 — Guess the output?

async function getValue() {

  return 42;

}

console.log(getValue());

A. 42


B. Promise { 42 }


C. undefined


D. Error

Formatting varies by engine, but an async function always returns a Promise.

⚡ Challenge 42 — Guess the output?

async function run() {

  console.log("inside");

  await 0;

  console.log("after await");

}

console.log("before");

run();

console.log("after");

A. before inside after await after


B. before inside after after await


C. before after inside after await


D. Error

await yields; the continuation runs as a microtask after sync work finishes.

⚡ Challenge 43 — Guess the output?

const slow = new Promise((resolve) => setTimeout(() => resolve("slow"), 10));

const fast = Promise.resolve("fast");

Promise.all([slow, fast]).then(console.log);

A. ["fast", "slow"]


B. ["slow", "fast"]


C. "slowfast"


D. Rejection

Promise.all keeps input order, not finish order — useful when joining multiple RN API calls.

⚡ Challenge 44 — Guess the output?

Promise.all([

  Promise.resolve("ok"),

  Promise.reject("failed"),

]).then(console.log).catch(console.log);

A. ["ok", "failed"]


B. "failed"


C. "ok"


D. Nothing

One rejection fails the whole Promise.all. Prefer Promise.allSettled when you need every result.

⚡ Challenge 45 — Guess the output?

Promise.resolve("data")

  .finally(() => "cleanup")

  .then(console.log);

A. "cleanup"


B. "data"


C. undefined


D. Rejection

A normal return from finally does not replace the resolved value (throwing would).

⚡ Challenge 46 — Guess the output?

async function run() {

  [1, 2].forEach(async (n) => {

    await Promise.resolve();

    console.log(n);

  });

  console.log("done");

}

run();

A. 1 2 done


B. done 1 2


C. done 2 1


D. Nothing

forEach does not await async callbacks. Use for...of + await for sequential RN work (uploads, chained requests).

🗺️ Sets, Maps, modern operators

⚡ Challenge 47 — Guess the output?

const first = { id: 1 };

const second = { id: 1 };

console.log(new Set([first, second]).size);

A. 1


B. 2


C. 0


D. Error

Objects compared by reference, not deep equality — same idea as deduping list items incorrectly.

⚡ Challenge 48 — Guess the output?

const map = new Map();

map.set({}, "value");

console.log(map.get({}));

A. "value"


B. undefined


C. null


D. Error

get({}) uses a new object key, not the stored one.

⚡ Challenge 49 — Guess the output?

const user = {};

console.log(user.profile?.address?.city);

A. null


B. undefined


C. ReferenceError


D. TypeError

Optional chaining stops safely — great for messy API payloads in RN.

⚡ Challenge 50 — Guess the output?

console.log(0 || 10, 0 ?? 10, "" || "guest", "" ?? "guest");

A. 10 0 "guest" ""


B. 0 0 "" ""


C. 10 10 "guest" "guest"


D. Error

|| falls back on any falsy value.


?? falls back only on null / undefined.


Use ?? when 0 or "" are valid (counts, empty search text).

Fast React Native interview revision

Comment challenge

Drop your score in the comments:

X / 50 — and the one question that still feels unfair 👇

If this helped your next React Native / JavaScript interview, bookmark it and practice explaining each answer out loud — that’s what interviewers actually hire for.

Happy coding 🚀