Question 1

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

Accepted Answer

3 3 3 var is function scoped, so all three callbacks share one i, which is 3 once the loop ends. let creates a fresh binding per iteration and logs 0 1 2.

Question 2

What is the log order?console.log(1);
setTimeout(() => console.log(2));
Promise.resolve().then(() => console.log(3));
console.log(4);

Accepted Answer

1 4 3 2 Synchronous code runs first. Promise callbacks are microtasks and run as soon as the stack is empty, before any timer task.

Question 3

In JavaScript, which expression is true?

Accepted Answer

null == undefined Loose equality treats null and undefined as equal to each other only. NaN is not equal to itself, use Number.isNaN. Two object literals are different references.

Question 4

In JavaScript, what does [10, 9, 1].sort() return?

Accepted Answer

[1, 10, 9] Without a comparator, sort converts elements to strings and compares UTF-16 code units, so "10" comes before "9". Pass (a, b) => a - b for numbers.

Question 5

In JavaScript, what does ['1', '2', '3'].map(parseInt) return?

Accepted Answer

[1, NaN, NaN] map passes (value, index), so parseInt gets radix 0, 1 and 2. Radix 0 means default, radix 1 is invalid, and 3 is not a digit in base 2. Use map(Number) or a lambda.

Question 6

What does this log?const a = { n: { v: 1 } };
const b = { ...a };
b.n.v = 2;
console.log(a.n.v);

Accepted Answer

2 Spread copies only the top level, so b.n is the same object as a.n. Use structuredClone for a deep copy.

Question 7

In an ES module, what happens on the last line?const obj = { n: 1, get() { return this.n; } };
const g = obj.get;
g();

Accepted Answer

It throws a TypeError this is set by how a function is called. A bare call in strict mode, which modules always use, gives this = undefined, so reading this.n throws. In sloppy scripts it would read the global object and return undefined. Fix with obj.get.bind(obj).

Question 8

What is wrong here?items.forEach(async (item) => {
 await save(item);
});
console.log("done");

Accepted Answer

"done" logs before the saves finish, and failures go unhandled forEach ignores the promises its callback returns. Use for...of with await for sequential work, or await Promise.all(items.map(save)) for parallel work.

Question 9

In JavaScript, you fire 5 independent requests and need every result, even if some fail. Which do you await?

Accepted Answer

Promise.allSettled allSettled waits for all and reports each as fulfilled or rejected. all rejects on the first failure. race settles with the first to finish. any resolves with the first success.

Question 10

Three independent fetches each take about 1 second. How long does this take?for (const url of urls) {
 results.push(await fetch(url));
}

Accepted Answer

About 3 seconds Each await waits before the next fetch starts, so they run one after another. Start them all first: await Promise.all(urls.map(fetch)).

Question 11

In JavaScript, what does 0.1 + 0.2 === 0.3 evaluate to?

Accepted Answer

false JavaScript numbers are IEEE 754 doubles, and 0.1 + 0.2 is 0.30000000000000004. Compare with a tolerance, or use integers such as cents for money.

Question 12

What does this log?const user = { name: "Ana" };
user.name = "Bo";
console.log(user.name);

Accepted Answer

Bo const stops the variable being reassigned, not the object being mutated. Object.freeze would block the change, and only at the top level.

Question 13

In TypeScript, const user = JSON.parse(body) as User. The body has no email field. What happens at runtime?

Accepted Answer

Nothing is checked, user.email is just undefined Types are erased when compiling to JavaScript, and as is only an assertion to the compiler. Validate untrusted input with a runtime check or a schema library.

Question 14

Which assignment compiles?interface P { x: number }
const a = { x: 1, y: 2 };
const p1: P = a;
const p2: P = { x: 1, y: 2 };

Accepted Answer

Only p1 Typing is structural, so a with an extra y is still a P. Excess property checks apply only to fresh object literals, which is why p2 is an error.

Question 15

In TypeScript, a function receives data from an untrusted source. Which parameter type forces the caller to narrow before use?

Accepted Answer

unknown unknown accepts anything but allows no operations until narrowed. any switches off checking. object only rules out primitives, and never accepts no value at all.

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