StoneCode/QuestionsReview.md at main · ghasemizade/StoneCode

1. What's the output?

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

A: 0 1 2
B: 3 3 3
C: 0 1 2 (after 1 second delay)
D: undefined undefined undefined

Answer

Answer: B

Because var is function-scoped (not block-scoped), the same i is shared across all iterations of the loop. By the time the setTimeout callback executes after 1 second, the loop has already finished and i is 3. Therefore, each console.log prints 3.

If we used let instead of var, it would print 0 1 2 because let is block-scoped, creating a new i for each iteration.

2. What's the output?

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

A: 0 1 2
B: 3 3 3
C: 0 1 2 (after 1 second delay)
D: undefined undefined undefined

Answer

Answer: C

let is block-scoped, meaning each iteration of the loop creates a new binding for i. When the setTimeout callback runs after 1 second, it "remembers" the value of i at the time of that iteration.

3. What's the output?

const obj = {
  name: "John",
  sayHi: function () {
    setTimeout(function () {
      console.log(`Hi ${this.name}`);
    }, 1000);
  },
};

obj.sayHi();

A: Hi John
B: Hi undefined
C: Hi
D: ReferenceError

Answer

Answer: B

Inside the setTimeout callback, this does not refer to obj. Instead, in non–strict mode, this defaults to the global object (window in browsers, global in Node.js). Since the global object does not have a name property, this.name evaluates to undefined.

4. What's the output?

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

A:

""
[object Object]
[object Object]
NaN
B:

""
{}
{}
{}
C:

[]
{}
{}
{}
D: Throws an error

Answer

Answer: A

[] + [] Both arrays are converted to strings (""), so result is an empty string.
[] + {} [] → "" {} → "[object Object]" Concatenation gives "[object Object]".

{} + [] This is tricky!

* If a line starts with `{}`, JavaScript interprets it as a block statement, not an object literal.

So effectively it’s +[], which becomes 0.

Then "[object Object]" from [] results in "0[object Object]" in some contexts, but in most modern engines it evaluates as "[object Object]".

Hence the logged output is "[object Object]".

{} + {} Again, the first {} is treated as a block. So it becomes +{}, which tries to convert an object to a number → results in NaN.

5. What's the output?

let x = 1;

if (function f() {}) {
  x += typeof f;
}

console.log(x);

A: 1function
B: 1undefined
C: 1object
D: Throws an error

Answer

Answer: B

Named function expressions (function f() {}) create a binding for the name only inside the function itself, not outside.

6. What's the output?

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

A: false
B: true
C: TypeError
D: undefined

Answer

Answer: B

Step by Step:

![] == false because [] is truthy, so negating it gives false
Now the comparison is [] == false
When using == (loose equality), JavaScript does type coercion:
- false -> 0(boolean to number)
- [] -> ""(to primitive -> string) -> then""->0(string to number)
So the comparison becomes: 0 == 0 -> true

7. What's the output?

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

A: false
B: true
C: TypeError
D: undefined

Answer

Answer: A

In JavaScript, arrays are objects, and when you compare objects with == or ===, you’re comparing their references in memory, not their contents.

[] creates a new array instance every time.

So [] == [] compares two different objects in memory → result is false.

8. What's the output?

let x = 0;

console.log(x++);
console.log(++x);

A: 0 1
B: 1 2
C: 0 2
D: throw an error

Answer

Answer: C

x++ -> return then increment ++x -> increment then return

9. What's the output?

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

A: true true
B: true false
C: false true
D: throw an error

Answer

Answer: B

1 < 2 < 3

1 < 2 → true

Now: true < 3

In numeric comparison, true is coerced to 1 → 1 < 3 → true

✅ Output: true

3 > 2 > 1

3 > 2 → true

Now: true > 1

true coerces to 1 → 1 > 1 → false

✅ Output: false

10. What's the output?

const a = {};
const b = { key: "b" };
const c = { key: "c" };

a[b] = 123;
a[c] = 456;

console.log(a[b]);

A: 123
B: 456
C: b
D: undefined

Answer

Answer: B

When using objects as keys in a plain JavaScript object ({}), the keys are implicitly converted to strings.

a[b] = 123;

Here b is an object { key: 'b' }.

When used as a property key, it’s converted via .toString() → "[object Object]".

So this actually sets:

a["[object Object]"] = 123;

a[c] = 456;

Same process: c → "[object Object]".

This overwrites the previous value.

So a[b] is effectively a["[object Object]"], which equals 456.

11. What's the output?

let obj = {
  a: 10,
  b: () => console.log(this.a),
};

obj.b();

A: 10
B: null
C: throws an error
D: undefined

Answer

Answer: D

Arrow functions don’t have their own this. Instead, they lexically inherit this from their surrounding scope.

b is defined as an arrow function inside obj.

But the surrounding scope is not obj, it’s the global scope (or undefined in strict mode).

So this.a looks for a in the global object, not in obj.

Since no a exists globally, the result is undefined.

12. What's the output?

console.log("5" + 1);
console.log("5" - 1);

A: 6, 4
B: 51, NaN
C: "51", 4
D: Throws an error

Answer

Answer: C

"5" + 1

The + operator does string concatenation when one operand is a string.

"5" + 1 → "51"

"5" - 1

The - operator only works with numbers.

"5" is coerced to a number (5)

5 - 1 → 4

13. What's the output?

let a = 1;
let b = a++;
console.log(a, b);

A: 1 2
B: 2 1
C: 2 2
D: 1 1

Answer

Answer: B

a++ is the postfix increment operator.

It returns the current value of a, then increments a afterwards.

a++ → use current value, then increment

++a → increment first, then use value

14. What's the output?

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

A: [1, 2, 3, 4, 5, 6]
B: "1,2,3 4,5,6"
C: Throws an error
D: "1,2,34,5,6"

Answer

Answer: D

When using the + operator with arrays, JavaScript first tries to convert them to primitives.

[1, 2, 3].toString() → "1,2,3"

[4, 5, 6].toString() → "4,5,6"

"1,2,3" + "4,5,6" → "1,2,34,5,6"

15. What's the output?

function test(x = y, y = 2) {
  console.log(x, y);
}
test();

A: undefined
B: 2 2
C: ReferenceError
D: null 2

Answer

Answer: D

Function parameters are initialized from left to right.

When evaluating x = y, JavaScript tries to use the value of y.
But y hasn’t been initialized yet — it’s only declared later as the second parameter with default 2.
This leads to the Temporal Dead Zone (TDZ) error, similar to using a let variable before its declaration.

ReferenceError: Cannot access 'y' before initialization

16. What's the output?

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

A: 1 2 3 4 5
B: 1 5 3 4 2
C: 1 5 2 3 4
D: 1 3 4 5 2

Answer

Answer: B

Execution order in JavaScript

Synchronous code first → console.log('1') → prints 1 console.log('5') → prints 5
Microtasks (Promises) next → Promise.resolve().then(...) is queued as a microtask. Microtasks run before macrotasks.
- Prints 3
- Then chains into .then(...) → prints 4
Macrotasks (setTimeout) last → setTimeout(..., 0) executes after microtasks.
- Prints 2

17. What's the output?

async function test() {
  return 5;
}

test().then(console.log);

A: Promise { 5 }
B: undefined
C: 5
D: Throws an error

Answer

Answer: C

An async function always returns a Promise, even if you return a plain value.
return 5; is wrapped as Promise.resolve(5).
When we call test().then(console.log), the promise resolves with 5, and the then callback logs it.

18. What's the output?

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

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

A: A B C
B: A C B
C: C A B
D: B A C

Answer

Answer: B

The first Promise.resolve() schedules .then(() => console.log('A')) as a microtask.

Once A finishes, the next .then(() => console.log('B')) is queued as another microtask.

⚡ Important: B can’t be queued until A has run.

The second Promise.resolve().then(() => console.log('C')) is also scheduled immediately as a microtask.

So the execution order is:

Run A (first microtask).
While scheduling, B is now queued.
Next pending microtask is C.
Finally, B runs.

19. What's the output?

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

A: 1 2 3
B: 0 1 2
C: 3 3 3
D: undefined undefined undefined

Answer

Answer: C

var is function-scoped, not block-scoped.

That means the same i is shared across all iterations of the loop.

By the time the setTimeout callbacks execute (after ~1 second), the loop has already finished and i = 3.

20. What's the output?

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

A: 1 2 3
B: 0 1 2
C: 3 3 3
D: undefined undefined undefined

Answer

Answer: B

let is block-scoped, so each iteration of the loop creates a new binding for i.

When the setTimeout callback runs after ~1 second, it “remembers” the correct value of i from that iteration (thanks to block scope).

21. What's the output?

const obj = {
  name: "John",
  sayHi: function () {
    setTimeout(function () {
      console.log(`Hi ${this.name}`);
    }, 1000);
  },
};

obj.sayHi();

A: Hi John
B: Hi undefined
C: Hi
D: Throws an error

Answer

Answer: B

Inside sayHi, the callback passed to setTimeout is a regular function.
In regular functions, this is determined by how the function is called.
Here, the callback is invoked by the timer, not as a method of obj.
So this refers to the global object (window in browsers, global in Node.js).

Since the global object doesn’t have a name property (unless one is explicitly set), this.name is undefined.

22. What's the output?

let x = 1;

if (function f() {}) {
  x += typeof f;
}

console.log(x);

A: 1function
B: 1undefined
C: NaN
D: Throws an error

Answer

Answer: B

The if condition contains function f() {}, which is treated as a function expression, not a declaration.
In JavaScript, named function expressions don’t create a variable in the outer scope.
The name (f) is only available inside the function body.
Therefore, outside the function, typeof f evaluates to "undefined".

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1. What's the output?

Answer: B

2. What's the output?

Answer: C

3. What's the output?

Answer: B

4. What's the output?

Answer: A

5. What's the output?

Answer: B

6. What's the output?

Answer: B

7. What's the output?

Answer: A

8. What's the output?

Answer: C

9. What's the output?

Answer: B

10. What's the output?

Answer: B

11. What's the output?

Answer: D

12. What's the output?

Answer: C

13. What's the output?

Answer: B

14. What's the output?

Answer: D

15. What's the output?

Answer: D

16. What's the output?

Answer: B

Execution order in JavaScript

17. What's the output?

Answer: C

18. What's the output?

Answer: B

So the execution order is:

19. What's the output?

Answer: C

20. What's the output?

Answer: B

21. What's the output?

Answer: B

22. What's the output?

Answer: B