Engine-Level Behaviour

Preview — 3 of 10 questions

What is the output?

javascript

Promise.resolve()
  .then(() => {
    console.log(1);
    return Promise.resolve(2);
  })
  .then((v) => console.log(v));

Promise.resolve()
  .then(() => console.log(3))
  .then(() => console.log(4));

A1 2 3 4

B1 3 4 2

C1 3 2 4

D3 1 4 2

What is the output?

javascript

const obj = {
  [Symbol.toPrimitive](hint) {
    if (hint === "number") return 42;
    if (hint === "string") return "hello";
    return true;
  },
};

console.log(+obj);
console.log(`${obj}`);
console.log(obj + "");

A42 "hello" "true"

B42 "hello" "42"

CNaN "hello" "true"

D42 "[object Object]" "true"

What is guaranteed about this code's output?

javascript

let obj = { name: "target" };
const ref = new WeakRef(obj);

console.log(ref.deref()?.name);

obj = null;

// GC may or may not have run here

console.log(ref.deref()?.name);

A"target" then "target" always

B"target" then undefined always

C"target" then either "target" or undefined

Dundefined then undefined

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