JavaScript Quiz

I will post some questions in JavaScript from today onwards share your own solutions without running these code on your console.

1. null === undefined

2. null == undefined

3. 2+"4"

4. 2-"3"

5. ""+2

6. +"2"

Comments

[lxr]

false
true
string:24
number:-1
string:2
number:2

[Fouzeddin]

false
true
6
-1
"2"
2

[Oyeyemi Clement]

1. False
2. True
3. "24"
4. 1
5. 2
6. "2"

Well, let me run to my console to confirm that I know this nice language with weird behavior

[Sai gowtham]

sure....

[Lea Rosema (she/her)]

I guess :)

1. false
2. true
3. "24"
4. -1
5. "2"
6. 2

[Josh Cheek]

You have to wrap it in parens b/c JS treats toplevel curlies as a "block" rather than an object. But, of course, if you begin a line with parens, then it will become a function call on the previous line, if one exists, so for sanity's sake, begin any paren line with a semicolon (same for brackets).

Personally, I think it's really iffy to say that {} and [] are falsy values. I've always understood "falsy" to mean you can drop it into a conditional and it will behave like false would have:

$ node -p ';[false, null, undefined, 0, NaN, "", [], {}].map(v => [v?"X":"√︎",v])'
[ [ '√︎', false ],
  [ '√︎', null ],
  [ '√︎', undefined ],
  [ '√︎', 0 ],
  [ '√︎', NaN ],
  [ '√︎', '' ],
  [ 'X', [] ],
  [ 'X', {} ] ]

[Josh Cheek]

I scored 4 out of 6. But if I wasn't able to run them as I went, it would be much lower.

Also, I feel like you should add "2"+4 right after 2+"4", b/c I was definitely wondering if the casting decision was based on which one was first. Eg in Ruby, + is a method call, so 2+"4" would be calling Integer#+ and "2"+4 would be calling String#+, thus the ordering implies different behaviour, even though + in math is commutative. I know that's not how JS works, but it means that I would at least be entertaining the possibility of non-commutativity.

null === undefined
// expected: false
// actual:   false
// score:    1/1

null == undefined
// expected: false... but less confidently :/
// actual:   true
// score:    1/2

2+"4"
// expected: Jesus >.< uhm... 6? If not 6, than 24, pretty sure
//           one of them gets cast, just not sure which.
// actual:   fuuuuuuck, 24 >.<
// score:    1/3

2-"3"
// expected: Based on the above, I'm really hoping it's -1
// actual:   -1
// score:    2/4

""+2
// expected: Oh fuck. uhhhh. maybe it comes in as zero, or maybe
//           it gets treated like a character string instead of
//           a number string, which maybe casts the 2 to a string
//           ...wait, yeah, it's definitely "2"!
// actual:   "2"
// score:    3/5

+"2"
// expected: Probably 2, right? I'd expect it to cast the "2" to
//           a number and then make it "positive" or something.
// actual:   2
// score:    4/6

[Casey Webb]

Oddly, node -e "if ([] == false) console.log('truthy')" logs truthy, but node -e "if ({} == false) console.log('truthy')" doesn't.

Oh Javascript...

The moral of the story: always use ===.

[Valentin Baca]

Got 3 and 6 wrong. Neat quiz!

[Casey Webb]

I can't think of any case where those would be falsy. What exactly did you experiment with that made it appear that way?

Sanity check...

node -e "if (({})) console.log('truthy')" // requires wrapper to not interpret as block
node -e "if ([]) console.log('truthy')"

Logs truthy twice

[Luis Uriel Leaño]

1. false
2. true
3. 24
4. NaN
5. 2
6. 2

[Kyle Blake]

Unless I'm missing some trickery in the original comment, [] and {} are truthy values in JavaScript.

[ｅｎｄａｎ]

1. false
2. true
3. "24"
5. "2"
6. "2"

??? Just a guess

[John Demian]

Uhhh I love these!

I've got one for you!

Without running this in the console let me know how much is

0.1 + 0.2

Hint: it's a number bigger than the one you are thinking of right now.

[bruno jennrich]

0.30000000000000004

but this would happen on any IEEE floating point machine

[isabolic99]

1. false
2. true
3. "42"
4. -1
5. "2"
6. 2

[isabolic99]

got 6 wrong

[André Michel Andy]

Before running these statements:

false
true
24
-1
NaN
2

After running them

false
true
24
-1
"2"
2

4/6 😅