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10 Challenging JavaScript Quiz Questions and Answers

Nick Scialli (he/him) on August 27, 2020

The following questions are intended to be challenging and instructive. If you know exactly how to answer each one, that's great, but if you get so...

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pentacular • Aug 28 '20

identity (===)

Just noting that === is not an identity operator -- it's the strict equality comparison operator.

We can easily tell that it is not an identity operator because, e.g., NaN === NaN is false.

b and a are pointing to the same

It's a bit problematic to say this, since Javascript doesn't have pointers.

It's sufficient to say that a and b have the same value, so they are the same object, and so provide access to the same properties. The rest then follows as you've said.

Ahm A • Aug 29 '20 • Edited

the equality operator (==) will attempt to make the data types the same before proceeding

the identity operator (===) requires both data types to be the same, as a prerequisite.

pentacular • Aug 30 '20

We can tell that === is not an identity operator since it may return false when comparing a value with itself.

e.g. NaN === NaN.

Ahm A • Aug 30 '20 • Edited

NaN === NaN // false, The identity evaluation algorithm (IEA) rule 4
Operands are the same type (numbers), but the IEA rule 4 indicates than nothing is equal with a NaN.
The result is false.

The identity evaluation algorithm (IEA) ===:

1. If both operands have different types, they are not strictly equal
2. If both operands are null, they are strictly equal
3. If both operands are undefined, they are strictly equal
4. If one or both operands are NaN, they are not strictly equal
5. If both operands are true or both false, they are strictly equal
6. If both operands are numbers and have the same value, they are strictly equal
7. If both operands are strings and have the same value, they are strictly equal
8. If both operands have reference to the same object or function, they are strictly equal
9. In all other cases operands are not strictly equal.

Also,NaNs are the only non-reflexive value, i.e., if x !== x, then x is a NaN

more details:
dmitripavlutin.com/the-legend-of-j...

pentacular • Aug 30 '20

The page you've referenced is mostly correct, but they're a bit confused on terminology, and have made things rather more complicated that necessary.

See: ecma-international.org/publication...

7.2.16 Strict Equality Comparison
The comparison x === y, where x and y are values, produces true or false. Such a comparison is performed as follows:

If Type(x) is different from Type(y), return false.

If Type(x) is Number or BigInt, then a. Return ! Type(x)::equal(x, y).

Return ! SameValueNonNumeric(x, y).

Now, what this means is that some things with the same identity compare false with ===, and some things with different identities compare true with ===.

e.g.

NaN === NaN is false

-0 === 0 is true

Which means that it is not checking the identity of the operands -- it is computing a kind of equality (using the strict equality comparison).

So, === is not an identity operator, and for these reasons is not called an identity operator in the language specification.

It's just some confused people on the internet who are making that into a popular mistake. :)

Thomas Ghysels • Aug 27 '20

Some instructive feedback for question 10: initialize greatest to -Infinity (or arr[0])

cayhorstmann • Aug 29 '20

These are very nice questions that probe JavaScript fundamentals and not ephemeral trivia. I suggest you change #4 and #6 so that they don't require knowledge about the behavior of == vs. ===. (Nobody should use == with JavaScript in 2020.) If you think that makes #4 too easy, perhaps you could add whether Object.keys(obj).map(x => parseInt(x)) === Object.values(obj). And as a bonus puzzler, why isn't Object.keys(obj).map(parseInt) the same as Object.keys(obj).map(x => parseInt(x))?

Charles Allen • Sep 4 '20

Love the bonus question! That's quite the trap.

pris stratton • Aug 27 '20

Very cool I love these. I am going to try the quiz in the link =)

Jon Ege Ronnenberg • Sep 1 '20 • Edited

Nice quiz. I enjoyed it more than other JS quizzes I have seen.
In Question 2 though, since it is about performance, it might be useful to know that a var declaration in a for loop is more performant than a let declaration. As long as your for loop is scoped to a function, you should not get any nasty surprises.

An example could be:

function each (f, array) {
  for (var i = 0, max = array.length; i < max; ++i) {
    f(array[i], i);
  }
}

Madza • Aug 28 '20

Here are 156 others:
github.com/lydiahallie/javascript-...

Naresh Poonia • Aug 27 '20

I'm learning Javascript, I have bookmarked this post.
I would like to come here later and check how many I can answer.
Thank you for this post

Ali Nazari • Sep 4 '20

Don't forget to check these articles:
dev.to/macmacky/70-javascript-inte...
github.com/lydiahallie/javascript-...

YogeshDharya • Mar 16 '23

Hey Nick ! I really liked the blog and wanted to tell you that as of now(16th March 2023 22:02 IST) my Like made your existing likes sum up to 200 . Than u so much for your work . Kindly keep Sharing your knowledge with us