JavaScript Complete Course: final quiz

Fifteen questions covering the whole JavaScript Complete Course.

Beginner
15 questions

15 questions. Pass mark 70%.

Question 1 of 15 What does this code log?
JavaScript
```
const scores = [10, 20];
scores.push(30);
console.log(scores.length);
```
- 3
- A TypeError, because scores is a constant
- 2
Question 2 of 15 What does typeof null return?
- "object"
- "null"
- "undefined"
Question 3 of 15 Which of these expressions evaluate to true?
Select all that apply.
- null == undefined
- 0 == ""
- "5" === 5
- NaN === NaN
Question 4 of 15 Why does 0.1 + 0.2 === 0.3 evaluate to false?
- Numbers are stored in binary floating point, which cannot represent 0.1 or 0.2 exactly, so the sum is 0.30000000000000004
- The + operator joins the numbers as strings, giving “0.10.2”
- === compares types first, and the result of an addition has a different type from a number literal
Question 5 of 15 What does this loop log?
JavaScript
```
const letters = ["a", "b"];
for (const i in letters) {
  console.log(typeof i);
}
```
- "string" twice
- "number" twice
- "a" then "b"
Question 6 of 15 Which statement about arrow functions is true?
- They do not have their own this; they use the this of the surrounding code
- They are hoisted, so they can be called before the line that defines them
- They always need a return statement

Question 7 of 15 What does this code log?

JavaScript

function makeCounter() {
  let count = 0;
  return () => ++count;
}

const a = makeCounter();
const b = makeCounter();
a();
a();
console.log(a(), b());

3 1
3 3
1 1
2 1

Question 8 of 15 What is the value of result?
JavaScript
```
const result = [1, 2, 3, 4]
  .filter((n) => n % 2 === 0)
  .map((n) => n * 10);
```
- [20, 40]
- [10, 20, 30, 40]
- [false, true, false, true]
- 60
Question 9 of 15 What does this code log?
JavaScript
```
const volume = 0;
console.log(volume || 50, volume ?? 50);
```
- 50 0
- 0 0
- 50 50
- 0 50
Question 10 of 15 You need to show a name that a visitor typed into a form inside a paragraph. What should you use?
- paragraph.textContent = name, because the text is never parsed as HTML
- paragraph.innerHTML = name, because it is faster
- paragraph.innerHTML = name, as long as the input has a maxlength

Question 11 of 15 In what order are the letters logged?

JavaScript

console.log("A");
setTimeout(() => console.log("B"), 0);
Promise.resolve().then(() => console.log("C"));
console.log("D");

A, D, C, B
A, B, C, D
A, D, B, C
A, C, D, B

Question 12 of 15 getUser() and getCourses() both return promises and do not depend on each other. Which code inside an async function starts both at once and waits for both?
- const [user, courses] = await Promise.all([getUser(), getCourses()]);
- const user = await getUser(); const courses = await getCourses();
- const [user, courses] = [await getUser(), await getCourses()];
Question 13 of 15 A fetch() request gets a 404 Not Found response. What happens?
- The promise fulfils with a response whose ok property is false, so you must check it
- The promise rejects, so the error goes straight to catch
- fetch() throws synchronously before returning a promise

Question 14 of 15 What does this code log?

JavaScript

const original = { tags: ["js"] };
const copy = { ...original };
copy.tags.push("css");
console.log(original.tags.length);

2
1
A TypeError

Question 15 of 15 You load three independent reports and want every result, even if one of the requests fails. Which method fits?
- Promise.allSettled()
- Promise.all()
- Promise.race()
- Promise.any()