JavaScript Quiz

1. Where is the recommended place to load a script that works with the page?

• At the very top of <head> with no attributes
• In <head> with the defer attribute
• Inside an HTML comment
• After the closing </html> tag

defer downloads in parallel and runs after the HTML is parsed (lesson 02).

2. Which keyword should you use by default for variables?

• var
• let
• const
• static

Use const unless the value must be reassigned (lesson 05).

3. What does typeof null return?

• "null"
• "object"
• "undefined"
• "number"

A historic bug that can never be fixed (lesson 06).

4. What is "5" + 3?

• 8
• "53"
• NaN
• An error

If either side of + is a string, it concatenates (lesson 07).

5. What is "5" - 3?

• 2
• "53"
• "2"
• NaN

- always converts to numbers (lesson 18).

6. Which comparison is true?

• 5 === "5"
• NaN === NaN
• null == undefined
• [] === []

Loose equality treats null and undefined as equal; the others are all false (lessons 08 and 16).

7. What does 0 ?? 10 evaluate to?

• 0
• 10
• null
• true

?? only replaces null and undefined (lesson 08).

8. In a switch, what happens if you leave out break?

• A syntax error
• Only the matching case runs
• Execution falls through into the following cases
• The default case runs first

Fall-through continues until a break or return (lesson 10).

9. Which loop gives you the VALUES of an array?

• for…of
• for…in
• for…each
• while…in

for…in gives keys as strings (lesson 12).

10. What does "hello".at(-1) return?

• "h"
• "o"
• undefined
• An error

Negative indexes count from the end (lesson 13).

11. Why does 0.1 + 0.2 === 0.3 return false?

• A JavaScript bug that will be fixed
• Binary floating point can't represent these decimals exactly
• Because === compares types
• Because 0.1 is a string

Round for display, use integer cents for money (lesson 16).

12. Which of these values is truthy?

• 0
• ""
• "false"
• NaN

Every non-empty string is truthy, even "false" (lesson 19).

13. In new Date(2026, 0, 15), which month is it?

• January
• February
• December
• An invalid date

Months are zero-based in the Date constructor (lesson 20).

14. What does an arrow function NOT have of its own?

• Parameters
• A return value
• Its own this
• A body

Arrows inherit this from the surrounding code (lesson 23).

15. What happens if you use a let variable before its declaration line?

• It's undefined
• A ReferenceError (temporal dead zone)
• It's null
• It works normally

let and const are hoisted but not initialised (lesson 25).

16. What is a closure?

• A function with no parameters
• A function that remembers variables from where it was created
• A way to close the browser
• A class with private fields

Closures keep a live link to outer variables (lesson 26).

17. What does ["1", "2", "3"].map(parseInt) return?

• [1, 2, 3]
• [1, NaN, NaN]
• ["1", "2", "3"]
• An error

map passes the index as parseInt's radix argument (lesson 28).

18. Which method changes the original array?

• slice
• map
• splice
• toSorted

splice modifies in place; the others return new arrays (lesson 30).

19. What does find return when nothing matches?

• -1
• null
• undefined
• An empty array

findIndex returns -1; find returns undefined (lesson 31).

20. What does [10, 9, 1].sort() produce?

• [1, 9, 10]
• [1, 10, 9]
• [10, 9, 1]
• [9, 10, 1]

The default sort compares items as strings (lesson 32).

21. How do you make a copy of an array?

• const copy = original;
• const copy = [...original];
• const copy = original.copy();
• const copy = new original();

Assignment shares the same array; spread creates a new one (lesson 33).

22. What is the quickest way to remove duplicates from an array?

• arr.unique()
• [...new Set(arr)]
• arr.filter(Set)
• Object.keys(arr)

A Set only stores unique values (lesson 34).

23. What does Object.entries({ a: 1 }) return?

• ["a", 1]
• [["a", 1]]
• { a: 1 }
• [1]

An array of [key, value] pairs (lesson 37).

24. Where do class methods actually live?

• On each instance
• On the class's prototype, shared by all instances
• In the global scope
• In a hidden array

Classes are built on prototypes (lesson 38).

25. What makes a class field truly private?

• Naming it with an underscore
• Starting its name with #
• Declaring it with const
• Putting it in the constructor

#private fields can't be accessed outside the class (lesson 40).

26. What does JSON.stringify do with a function property?

• Converts it to a string of code
• Leaves it out
• Throws an error
• Converts it to null

Functions and undefined values are omitted (lesson 41).

27. Which property safely sets text that came from a user?

• textContent
• innerHTML
• outerHTML
• document.write

innerHTML would interpret markup and allow XSS (lesson 43).

28. What is event delegation?

• Adding a listener to every element individually
• One listener on a parent that handles events from its children
• Passing events to a web worker
• Removing listeners automatically

It works because events bubble up (lesson 45).

29. Does fetch reject its promise for a 404 response?

• Yes, for every 4xx and 5xx
• No — check response.ok yourself
• Only for 500 errors
• Only in Node.js

fetch rejects only on network failure (lesson 49).

30. In what order does this print? setTimeout(() => log("A")); Promise.resolve().then(() => log("B")); log("C");

• A, B, C
• C, A, B
• C, B, A
• B, C, A

Synchronous code first, then microtasks (promises), then tasks (timers) (lesson 51).