Expert Mastery — Series 3

Preview — 3 of 10 questions

What is the output?

javascript

const target = { a: 1 };
const p = new Proxy(target, {
  has(t, key) {
    return key.startsWith('_') ? false : Reflect.has(t, key);
  },
});

target._secret = 2;
console.log('a' in p, '_secret' in p, p._secret);

Atrue true 2

Btrue false undefined

Ctrue false 2

DTypeError: 'has' on proxy: trap returned falsish

Which statement about WeakMap is correct?

AA WeakMap accepts primitive keys, and its entries disappear when memory runs low.

BWeakMap keys must be objects (or non-registered symbols) and are held weakly, so an entry becomes unreachable once nothing else references its key.

CWeakMap behaves exactly like Map but exposes size and can be iterated with for...of.

DA WeakMap holds its values weakly, so a value can vanish while its key is still reachable.

What does the following print?

javascript

function* inner() {
  yield 1;
  return 'ret';
}

function* outer() {
  const r = yield* inner();
  yield r;
}

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

A[1]

B[1, undefined]

C['ret']

D[1, 'ret']

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