JavaScript MCQs - ES6+ Features: Destructuring, Spread, Map, Set and Modules (Set 4)

Q1. What is the value of b?

const { a, b = 2 } = { a: 1, b: undefined };

a) undefined

b) TypeError

c) 2

d) null

Q2. What is the value of b?

const { a, b = 2 } = { a: 1, b: null };

a) 2

b) 0

c) undefined

d) null

Q3. What is printed?

const x = 0 ?? 5;
const y = 0 || 5;
console.log(x, y);

a) 0 5

b) 0 0

c) 5 5

d) 5 0

Q4. What is printed?

const user = {};
console.log(user?.address?.city);

a) null

b) ''

c) TypeError

d) undefined

Q5. What is printed?

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

a) 1 2

b) 2 2

c) SyntaxError

d) 2 1

Q6. What does f(1, 2, 3) return?

function f(a, ...rest) {
  return rest.length;
}

a) 2

b) 3

c) 1

d) undefined

Q7. What is printed?

console.log([...new Set([1, 1, 2, '2'])].length);

a) 3

b) 1

c) 2

d) 4

Q8. Which is an advantage of Map over a plain object for key-value data?

a) Map is always serialised by JSON.stringify

b) Map keys can only be strings

c) Map keys can be any value, including objects, and Map has a size property and keeps insertion order

d) Map has no iteration support

Q9. Which statement about ES module exports is correct?

a) A module can have one default export and any number of named exports

b) Modules cannot export functions

c) A module can have many default exports

d) Named exports must be imported with any name you like

Q10. What happens if a module does import { count } from './a.js' and then runs count = 5?

a) The value in a.js is updated

b) A new local variable is created

c) Nothing happens

d) A TypeError is thrown because imported bindings are read-only

Q11. What is printed?

const o = Object.freeze({ a: { b: 1 } });
o.a.b = 2;
console.log(o.a.b);

a) TypeError always

b) 1

c) undefined

d) 2

Q12. What is printed?

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

a) { value: 2, done: true }

b) { value: 2, done: false }

c) { value: undefined, done: true }

d) { value: 1, done: false }

Q13. What does [10, 20, 30].at(-1) return?

a) 10

b) undefined

c) TypeError

d) 30

Q14. You need a deep copy of an object that contains a Date and a Map. Which built-in works best?

a) Object.assign({}, obj)

b) { ...obj }

c) JSON.parse(JSON.stringify(obj))

d) structuredClone(obj)

Q15. Which statement about WeakMap is correct?

a) It has a size property

b) It is iterable with for...of

c) Its keys can be strings

d) Its keys must be objects (or non-registered symbols) and it does not stop them from being garbage collected

Q16. What is printed?

const prices = { a: 1, b: 2 };
const doubled = Object.fromEntries(
  Object.entries(prices).map(([k, v]) => [k, v * 2])
);
console.log(doubled);

a) [2, 4]

b) { a: 2, b: 4 }

c) [['a', 2], ['b', 4]]

d) { a: 1, b: 2 }