JavaScript fundamentals practice set 2: hoisting, closures, coercion, async order and modern syntax

Question 1

What does this print?
for (var i = 0; i < 3; i++) {
  setTimeout(() => console.log(i), 0);
}

Accepted Answer

3 3 3

Question 2

What does this print?
console.log(typeof ([] + []));

Accepted Answer

string

Question 3

What does this print?
console.log(typeof NaN);

Accepted Answer

number

Question 4

What does this print?
console.log([1, 2, 3].reduce((acc, x) => acc + x, 10));

Accepted Answer

16

Question 5

What does this print?
console.log(Boolean('0'), Boolean(''));

Accepted Answer

true false

Question 6

What does this print?
console.log(x);
var x = 5;

Accepted Answer

undefined

Question 7

What happens when this runs?
console.log(y);
let y = 5;

Accepted Answer

ReferenceError is thrown

Question 8

What does this print?
const obj = { a: 1 };
const copy = { ...obj };
copy.a = 2;
console.log(obj.a);

Accepted Answer

1

Question 9

What does this print?
console.log([1, [2, [3, [4]]]].flat(2));

Accepted Answer

[1, 2, 3, [4]]

Question 10

What does this print?
console.log('abcd'.at(-1));

Accepted Answer

d

Question 11

What does this print?
console.log(parseInt('42px'), Number('42px'));

Accepted Answer

42 NaN

Question 12

What does this print?
console.log([...new Set([1, 2, 2, 3, 3, 3])].length);

Accepted Answer

3

Question 13

What does this print?
function f(a, b = 2) {
  return a + b;
}
console.log(f(1), f(1, undefined), f(1, null));

Accepted Answer

3 3 1

Question 14

What does this print? console.log(1 < 2 < 3, 3 > 2 > 1);

Accepted Answer

true false

Question 15

What does this print?
const counter = (() => {
  let c = 0;
  return () => ++c;
})();
counter();
counter();
console.log(counter());

Accepted Answer

3

Question 16

In what order are the numbers printed?
async function f() {
  console.log(1);
  await null;
  console.log(2);
}
f();
console.log(3);

Accepted Answer

1 3 2

Question 17

Which array method returns the first element that satisfies a condition (or undefined if none does)?

Accepted Answer

find()

Question 18

Which expression creates a shallow copy of the array arr?

Accepted Answer

[...arr]

Question 19

What does this print?
console.log(Object.keys({ b: 1, a: 2, 1: 'x' }));

Accepted Answer

['1', 'b', 'a']

Question 20

What does this print?
console.log(0 || 'a', 0 ?? 'a');

Accepted Answer

a 0

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