Sean Freeman

Posted on Jul 22, 2025

See if you can answer these weird questions related to JS

Here are 100 extremely tricky JavaScript questions covering various concepts that confuse developers:

HOISTING & TEMPORAL DEAD ZONE

1. What's the output?

console.log(a);
console.log(b);
console.log(c);
var a = 1;
let b = 2;
const c = 3;

2. What happens here?

function test() {
  console.log(x);
  if (false) {
    var x = 1;
  }
}
test();

3. What's logged?

console.log(typeof foo);
console.log(typeof bar);
var foo = function () {};
function bar() {}

CLOSURES & SCOPE

4. What's the output after 1 second?

for (var i = 0; i < 3; i++) {
  setTimeout(() => console.log(i), 1000);
}

5. What's different here?

for (let i = 0; i < 3; i++) {
  setTimeout(() => console.log(i), 1000);
}

6. What's the final value of counter?

function createCounter() {
  var counter = 0;
  return {
    increment: () => ++counter,
    decrement: () => --counter,
    value: counter,
  };
}
const c = createCounter();
c.increment();
console.log(c.value);

TYPE COERCION

7. What's the result?

console.log([] + []);
console.log([] + {});
console.log({} + []);
console.log({} + {});

8. What's logged?

console.log(0.1 + 0.2 === 0.3);
console.log(0.1 + 0.2);
console.log((0.1 * 10 + 0.2 * 10) / 10);

9. What's the output?

console.log(null == undefined);
console.log(null === undefined);
console.log(null == 0);
console.log(null >= 0);
console.log(null > 0);

WEIRD OPERATORS

10. What's the result?

console.log(+"");
console.log(+[]);
console.log(+{});
console.log(+null);
console.log(+undefined);

11. What happens?

const a = [1, 2, 3];
const b = [1, 2, 3];
console.log(a == b);
console.log(a === b);
console.log(a.toString() == b.toString());

12. What's logged?

console.log(1 < 2 < 3);
console.log(3 > 2 > 1);

FUNCTION WEIRDNESS

13. What's the output?

function foo() {
  return {
    bar: "hello",
  };
}

function baz() {
  return;
  {
    bar: "hello";
  }
}

console.log(foo());
console.log(baz());

14. What happens?

(function () {
  var a = (b = 3);
})();
console.log(typeof a);
console.log(typeof b);

15. What's the result?

console.log(foo);
var foo = 1;
function foo() {}
console.log(foo);

ARRAY WEIRDNESS

16. What's logged?

const arr = [1, 2, 3];
arr[10] = 99;
console.log(arr.length);
console.log(arr[5]);

17. What's the output?

console.log([1, 2, 3] + [4, 5, 6]);
console.log([1, 2, 3] - [4, 5, 6]);

18. What happens?

const arr = new Array(3);
console.log(arr);
console.log(arr.map((x) => "a"));
console.log(arr.fill().map((x) => "a"));

OBJECT PROPERTY WEIRDNESS

19. What's the output?

const obj = {};
obj[true] = "boolean";
obj[1] = "number";
obj["1"] = "string";
obj["true"] = "string boolean";
console.log(Object.keys(obj));

20. What happens?

const a = {};
const b = { key: "b" };
const c = { key: "c" };
a[b] = 123;
a[c] = 456;
console.log(a[b]);

PARSING & NUMBERS

21. What's logged?

console.log(parseInt("123abc"));
console.log(parseInt("abc123"));
console.log(parseInt(""));
console.log(parseInt("0x10"));
console.log(parseInt("10", 2));

22. What's the result?

console.log(Math.max());
console.log(Math.min());
console.log(Math.max() < Math.min());

23. What happens?

console.log((0.1).toFixed(20));
console.log(0.1 + 0.2 - 0.3);
console.log((0.1 + 0.2 - 0.3).toFixed(20));

REFERENCE VS VALUE

24. What's the final array?

function modify(arr) {
  arr.push(4);
  arr = [1, 2, 3, 4, 5];
  arr.push(6);
}
const myArray = [1, 2, 3];
modify(myArray);
console.log(myArray);

25. What's logged?

let a = { x: 1 };
let b = a;
a.x = 2;
a = { x: 3 };
console.log(b.x);

PROTOTYPES

26. What's the output?

function Parent() {}
Parent.prototype.name = "parent";
function Child() {}
Child.prototype = new Parent();
const child = new Child();
console.log(child.name);
delete Child.prototype.name;
console.log(child.name);

27. What happens?

const obj = Object.create(null);
obj.toString = function () {
  return "custom";
};
console.log(obj + "");
console.log(String(obj));

ASYNC WEIRDNESS

28. What's the execution order?

console.log("1");
setTimeout(() => console.log("2"), 0);
Promise.resolve().then(() => console.log("3"));
console.log("4");

29. What's logged?

async function test() {
  console.log("start");
  await Promise.resolve();
  console.log("end");
}
console.log("before");
test();
console.log("after");

30. What happens?

Promise.resolve(1)
  .then((x) => x + 1)
  .then((x) => {
    throw new Error("oops");
  })
  .then((x) => x + 1)
  .catch((err) => 5)
  .then((x) => x + 1)
  .then(console.log);

DESTRUCTURING WEIRDNESS

31. What's the output?

const [a, b] = [1, 2, 3];
const [c, d, e, f] = [4, 5];
console.log(a, b, c, d, e, f);

32. What happens?

const obj = { a: 1, b: 2 };
const { a, b, c = 3, d } = obj;
console.log(a, b, c, d);

33. What's logged?

let a = 1,
  b = 2;
[a, b] = [b, a];
console.log(a, b);

OPERATOR PRECEDENCE

34. What's the result?

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

35. What happens?

console.log(false == "0");
console.log(false === "0");
console.log(0 == "");
console.log(0 === "");

VARIABLE DECLARATIONS

36. What's the output?

console.log(x);
var x = 5;
var x = 10;
console.log(x);

37. What happens?

function test() {
  if (true) {
    var a = 1;
    let b = 2;
    const c = 3;
  }
  console.log(a);
  console.log(b);
  console.log(c);
}
test();

REGEX WEIRDNESS

38. What's logged?

const regex = /test/g;
console.log(regex.test("test"));
console.log(regex.test("test"));
console.log(regex.test("test"));

39. What's the result?

console.log("123".match(/\d/));
console.log("123".match(/\d/g));
console.log("abc".match(/\d/g));

DATE WEIRDNESS

40. What's the output?

console.log(new Date("2023-02-30"));
console.log(new Date("2023/02/30"));
console.log(new Date(2023, 1, 30));

EVAL & WITH

41. What happens?

const code = "var x = 1; x + 1";
console.log(eval(code));
console.log(x);

42. What's logged?

function test() {
  const obj = { a: 1 };
  with (obj) {
    var a = 2;
    var b = 3;
  }
  console.log(obj.a);
  console.log(b);
}
test();

ERROR HANDLING

43. What's the output?

try {
  throw "string error";
} catch (e) {
  console.log(e instanceof Error);
  console.log(typeof e);
}

44. What happens?

function test() {
  try {
    return "try";
  } catch (e) {
    return "catch";
  } finally {
    return "finally";
  }
}
console.log(test());

STRICT MODE

45. What's the difference?

function normal() {
  x = 1;
  return x;
}

function strict() {
  "use strict";
  x = 1;
  return x;
}

console.log(normal());
// console.log(strict()); // What happens?

SYMBOL WEIRDNESS

46. What's logged?

const s1 = Symbol("test");
const s2 = Symbol("test");
console.log(s1 == s2);
console.log(s1 === s2);
console.log(s1.toString() === s2.toString());

47. What happens?

const obj = {};
const sym = Symbol("key");
obj[sym] = "symbol value";
obj["sym"] = "string value";
console.log(Object.keys(obj));
console.log(Object.getOwnPropertySymbols(obj));

PROXY WEIRDNESS

48. What's the output?

const target = { a: 1 };
const proxy = new Proxy(target, {
  get: (target, prop) => target[prop] * 2,
});
console.log(proxy.a);
console.log(target.a);
target.a = 5;
console.log(proxy.a);

UNICODE & STRINGS

49. What's logged?

console.log("👨‍👩‍👧‍👦".length);
console.log([..."👨‍👩‍👧‍👦"].length);
console.log("café".length);
console.log("café".normalize().length);

MEMORY & PERFORMANCE

50. What's the final state?

let obj = { data: new Array(1000000).fill("data") };
let ref = obj;
obj = null;
console.log(typeof obj);
console.log(typeof ref);
// Is the array garbage collected?

Absolutely! Here are 50 more extremely tricky JavaScript questions covering even more edge cases and concepts:

GENERATOR WEIRDNESS

51. What's the output?

function* gen() {
  yield 1;
  yield 2;
  return 3;
}
const g = gen();
console.log(g.next());
console.log(g.next());
console.log(g.next());
console.log(g.next());

52. What happens?

function* gen() {
  const x = yield 1;
  yield x + 10;
}
const g = gen();
console.log(g.next());
console.log(g.next(5));
console.log(g.next());

53. What's logged?

function* gen() {
  try {
    yield 1;
    yield 2;
  } catch (e) {
    yield e;
  }
}
const g = gen();
console.log(g.next());
console.log(g.throw("error"));
console.log(g.next());

ITERATION PROTOCOL

54. What's the result?

const obj = {
  [Symbol.iterator]: function () {
    let count = 0;
    return {
      next: () => ({ value: count++, done: count > 3 }),
    };
  },
};
console.log([...obj]);

55. What happens?

const arr = [1, 2, 3];
const iterator = arr[Symbol.iterator]();
arr.push(4);
console.log([...iterator]);

MICROTASKS & EVENT LOOP

56. What's the execution order?

console.log("1");
setTimeout(() => console.log("2"), 0);
Promise.resolve().then(() => {
  console.log("3");
  setTimeout(() => console.log("4"), 0);
});
queueMicrotask(() => console.log("5"));
console.log("6");

57. What's logged?

Promise.resolve().then(() => console.log("A"));
queueMicrotask(() => console.log("B"));
setTimeout(() => console.log("C"), 0);
queueMicrotask(() => console.log("D"));
Promise.resolve().then(() => console.log("E"));

WEAK REFERENCES

58. What happens over time?

let obj = { data: "important" };
const weakRef = new WeakRef(obj);
const finalizationRegistry = new FinalizationRegistry((heldValue) => {
  console.log("Object was garbage collected:", heldValue);
});
finalizationRegistry.register(obj, "my-object");
obj = null;
// What happens to weakRef.deref()?

BIGINT WEIRDNESS

59. What's the output?

console.log(1n + 2);
console.log(1n == 1);
console.log(1n === 1);
console.log(typeof 1n);
console.log(1n > 0);

60. What happens?

const big = BigInt(Number.MAX_SAFE_INTEGER) + 1n;
console.log(big);
console.log(Number(big));
console.log(big.toString());

ARRAY BUFFER & TYPED ARRAYS

61. What's logged?

const buffer = new ArrayBuffer(8);
const view1 = new Int32Array(buffer);
const view2 = new Uint8Array(buffer);
view1[0] = 0x12345678;
console.log(view2[0], view2[1], view2[2], view2[3]);

62. What happens?

const arr = new Int8Array([128, 255, 300]);
console.log(arr);
console.log(arr[0] + arr[1] + arr[2]);

SHARED ARRAY BUFFER

63. What's the result?

const shared = new SharedArrayBuffer(4);
const view1 = new Int32Array(shared);
const view2 = new Int32Array(shared);
view1[0] = 42;
console.log(view2[0]);
Atomics.add(view1, 0, 8);
console.log(view2[0]);

PROPERTY DESCRIPTORS EDGE CASES

64. What's the output?

const obj = {};
Object.defineProperty(obj, "x", {
  get: function () {
    return this._x;
  },
  set: function (val) {
    this._x = val * 2;
  },
});
obj.x = 5;
console.log(obj.x);
console.log(obj._x);
console.log(Object.keys(obj));

65. What happens?

const obj = { a: 1 };
Object.defineProperty(obj, "b", { value: 2 });
obj.b = 3;
console.log(obj.b);
delete obj.b;
console.log(obj.b);

FROZEN/SEALED OBJECTS

66. What's logged?

const obj = { a: 1, b: { c: 2 } };
Object.freeze(obj);
obj.a = 3;
obj.b.c = 4;
console.log(obj.a);
console.log(obj.b.c);

67. What's the difference?

const obj1 = { a: 1 };
const obj2 = { a: 1 };
Object.seal(obj1);
Object.freeze(obj2);
obj1.a = 2;
obj2.a = 2;
obj1.b = 3;
obj2.b = 3;
console.log(obj1);
console.log(obj2);

MODULE BEHAVIOR

68. What happens with circular imports?

// file a.js
import { b } from "./b.js";
export const a = "a";
console.log("a.js:", b);

// file b.js
import { a } from "./a.js";
export const b = "b";
console.log("b.js:", a);

69. What's the execution order?

console.log("before import");
import("./module.js").then((m) => console.log("dynamic import:", m));
console.log("after import");

TEMPLATE LITERAL EDGE CASES

70. What's the output?

function tag(strings, ...values) {
  console.log(strings);
  console.log(values);
  return "result";
}
const x = 1,
  y = 2;
console.log(tag`a${x}b${y}c`);

71. What happens?

const template = (strings, ...keys) => {
  return (values) => {
    const result = [strings[0]];
    keys.forEach((key, i) => {
      result.push(values[key], strings[i + 1]);
    });
    return result.join("");
  };
};
const t = template`Hello ${"name"}!`;
console.log(t({ name: "World" }));

REFLECT API WEIRDNESS

72. What's the result?

const obj = { a: 1 };
console.log(Reflect.has(obj, "a"));
console.log(Reflect.has(obj, "toString"));
console.log(Reflect.ownKeys(obj));
console.log(Reflect.getPrototypeOf(obj) === Object.prototype);

73. What happens?

function test(a, b, c) {
  return a + b + c;
}
console.log(Reflect.apply(test, null, [1, 2, 3]));
console.log(Reflect.construct(Array, [1, 2, 3]));

IMPORT MAPS & DYNAMIC IMPORTS

74. What's logged?

async function loadModule() {
  try {
    const module = await import("nonexistent-module");
    console.log(module);
  } catch (e) {
    console.log("Error:", e.message);
    return await import("./fallback.js");
  }
}

OBSERVABLE PATTERN

75. What's the execution pattern?

class Observable {
  constructor(subscriber) {
    this.subscriber = subscriber;
  }
  subscribe(observer) {
    return this.subscriber(observer);
  }
}
const obs = new Observable((observer) => {
  observer.next(1);
  setTimeout(() => observer.next(2), 100);
  return () => console.log("unsubscribed");
});
const subscription = obs.subscribe({
  next: (x) => console.log(x),
});

PRIVATE FIELDS EDGE CASES

76. What happens?

class Test {
  #private = "secret";
  static check(obj) {
    try {
      return obj.#private;
    } catch (e) {
      return "no access";
    }
  }
}
const t1 = new Test();
const t2 = { "#private": "fake" };
console.log(Test.check(t1));
console.log(Test.check(t2));

77. What's the output?

class Parent {
  #value = "parent";
  getValue() {
    return this.#value;
  }
}
class Child extends Parent {
  #value = "child";
  getChildValue() {
    return this.#value;
  }
}
const child = new Child();
console.log(child.getValue());
console.log(child.getChildValue());

TOP-LEVEL AWAIT

78. What's the execution order?

console.log("start");
const data = await fetch("/api/data").catch(() => ({ json: () => ({}) }));
console.log("fetched");
const result = await data.json();
console.log("parsed", result);

OPTIONAL CHAINING EDGE CASES

79. What's logged?

const obj = null;
console.log(obj?.a?.b?.c);
console.log(obj?.[Symbol.iterator]?.());
console.log(obj?.method?.());
console.log(obj?.method?.());

80. What happens?

const obj = { a: null };
console.log(obj.a?.b);
console.log(obj.a?.b?.c);
console.log(obj.a?.["b"]);
console.log(obj.a?.b?.["c"]?.d);

NULLISH COALESCING

81. What's the output?

console.log(null ?? "default");
console.log(undefined ?? "default");
console.log(0 ?? "default");
console.log("" ?? "default");
console.log(false ?? "default");

82. What's the difference?

const a = 0,
  b = null;
console.log(a || "default");
console.log(a ?? "default");
console.log(b || "default");
console.log(b ?? "default");

LOGICAL ASSIGNMENT

83. What's the final value?

let a = null;
let b = 0;
let c = "hello";
a ||= "default";
b ||= "default";
c ||= "default";
console.log(a, b, c);

84. What happens?

const obj = { x: null, y: 0 };
obj.x ??= "assigned";
obj.y ??= "assigned";
obj.z ??= "assigned";
console.log(obj);

NUMERIC SEPARATORS & LITERALS

85. What's the output?

console.log(1_000_000);
console.log(0b1010_1010);
console.log(0o755);
console.log(0xff_ec_de_5e);
console.log(1_000_000 === 1000000);

INTL API WEIRDNESS

86. What's logged?

const formatter = new Intl.NumberFormat("en-US", {
  style: "currency",
  currency: "USD",
});
console.log(formatter.format(1234.56));
console.log(formatter.format(-1234.56));
console.log(formatter.formatToParts(1234.56));

87. What happens with dates?

const date = new Date("2023-12-25");
const formatter = new Intl.DateTimeFormat("en-US", {
  timeZone: "UTC",
  year: "numeric",
  month: "long",
  day: "numeric",
});
console.log(formatter.format(date));

WEB APIS IN NODE/BROWSER

88. What's different?

console.log(typeof window);
console.log(typeof global);
console.log(typeof globalThis);
console.log(typeof self);

PERFORMANCE API

89. What's measured?

performance.mark("start");
for (let i = 0; i < 1000000; i++) {
  Math.random();
}
performance.mark("end");
performance.measure("loop", "start", "end");
console.log(performance.getEntriesByName("loop"));

ERROR BOUNDARIES & STACK TRACES

90. What's in the stack?

function a() {
  b();
}
function b() {
  c();
}
function c() {
  throw new Error("oops");
}
try {
  a();
} catch (e) {
  console.log(e.stack);
  console.log(e.message);
  console.log(e.name);
}

91. What's the output?

Error.stackTraceLimit = 2;
function deep1() {
  deep2();
}
function deep2() {
  deep3();
}
function deep3() {
  deep4();
}
function deep4() {
  throw new Error("deep");
}
try {
  deep1();
} catch (e) {
  console.log(e.stack.split("\n").length);
}

CUSTOM ELEMENTS

92. What happens?

class MyElement extends HTMLElement {
  connectedCallback() {
    this.innerHTML = "<p>Hello World</p>";
  }
}
customElements.define("my-element", MyElement);
console.log(customElements.get("my-element"));

WORKER BEHAVIOR

93. What's the communication pattern?

const worker = new Worker('data:application/javascript,postMessage("hello")');
worker.onmessage = (e) => console.log("Received:", e.data);
worker.postMessage("test");

STREAMING & ASYNC ITERATION

94. What's the output pattern?

async function* asyncGenerator() {
  yield 1;
  await new Promise((r) => setTimeout(r, 100));
  yield 2;
  await new Promise((r) => setTimeout(r, 100));
  yield 3;
}

for await (const value of asyncGenerator()) {
  console.log(value);
}

95. What happens with async iteration?

const asyncIterable = {
  async *[Symbol.asyncIterator]() {
    yield "a";
    yield "b";
    yield "c";
  },
};
Promise.all([...asyncIterable]); // What happens?

ABORT CONTROLLER

96. What's the behavior?

const controller = new AbortController();
const signal = controller.signal;

setTimeout(() => controller.abort(), 1000);

try {
  await fetch("/api/data", { signal });
} catch (e) {
  console.log(e.name);
  console.log(signal.aborted);
}

STRUCTURED CLONING

97. What's cloned and what's not?

const original = {
  date: new Date(),
  regex: /test/g,
  func: () => "hello",
  symbol: Symbol("test"),
  undefined: undefined,
  map: new Map([["key", "value"]]),
};
const cloned = structuredClone(original);
console.log(cloned);

COMPRESSION STREAMS

98. What's the output?

const stream = new CompressionStream("gzip");
const writer = stream.writable.getWriter();
const reader = stream.readable.getReader();

writer.write(new TextEncoder().encode("Hello World"));
writer.close();

reader.read().then((result) => {
  console.log(result.value); // Compressed bytes
});

SCHEDULER API

99. What's the execution order?

scheduler.postTask(() => console.log("high"), { priority: "user-blocking" });
scheduler.postTask(() => console.log("low"), { priority: "background" });
scheduler.postTask(() => console.log("normal"));
console.log("sync");

TEMPORAL API (PROPOSAL)

100. What's different?

// If Temporal was available
const date1 = new Date("2023-01-15T10:30:00");
const date2 = new Date("2023-01-15T10:30:00Z");
console.log(date1.getTime() === date2.getTime()); // What's the difference?

// vs Temporal (when available)
const temporal1 = Temporal.PlainDateTime.from("2023-01-15T10:30:00");
const temporal2 = Temporal.ZonedDateTime.from("2023-01-15T10:30:00Z");

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See if you can answer these weird questions related to JS

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