Why is !true ? 'false' : 'true' returning 'true'?

Question

Why is it that when I do

(!true) ? 'false' : 'true'

it returns 'true'?

FYI, if you're happen to want the string 'true' or 'false' from a variable, you can also type: `!var+''` (or `!!var+''` if you want it flipped) — , Apr 08 '11 at 06:14
@Buhake, maybe JM at Work was confused by the opening parenthesis without a closing one and did not want to accept? — Amelse Etomer, Jul 17 '13 at 20:10
@Sebastian Langer, Aaah! Well spotted, 2 years after posting the original answer! Thanks! :-) — Buhake Sindi, Jul 17 '13 at 20:11
It is funny, that we now have the least and the highest scoring answers showing the same code. One of the answers should remove the parentheses at all to show some variety. :-) It would still be valid code. — Amelse Etomer, Jul 17 '13 at 22:01
@djangofan Because the OP of the question hasn't been online at StackOverflow since the end of 2011. Odds are this question will remain unanswered. — , Aug 21 '13 at 15:22

score 66 · Answer 1 · edited Aug 24 '13 at 05:15

66

It simply means

if (!true) {
  return 'false';
} else {
  return 'true';
}

!true (not true) means false, so the else is returned.

edited Aug 24 '13 at 05:15

Cole Tobin

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answered Apr 08 '11 at 06:11

Buhake Sindi

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score 23 · Answer 2 · answered Apr 08 '11 at 06:12

23

The syntax of A ? B : C means that if A is TRUE, then return the value B. Else return value C. Since A is FALSE, it returns the value C which happens to be true.

answered Apr 08 '11 at 06:12

score 21 · Answer 3 · edited Jul 17 '13 at 20:20

21

Because (!true) is false, and then the right side of the : is chosen.

edited Jul 17 '13 at 20:20

hopper

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answered Apr 08 '11 at 06:11

Daniel

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4

True is false and false is true. WAT – Cole Tobin Aug 21 '13 at 15:07

score 17 · Answer 4 · answered Apr 08 '11 at 06:12

17

Because the above is equivalent to:

if (false) {
    return 'false';
} else {
    return 'true';
}

Though perhaps the confusion is coming from the difference between:

if (false) // which is false

And

if (false == false) // which is true

answered Apr 08 '11 at 06:12

David Tang

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I think you mean 'true' there ;) – Apr 08 '11 at 06:13
you mean `else { return 'true'; }` :) – hage Apr 08 '11 at 06:14
I also believe that's exactly the case – Ilya Saunkin Apr 08 '11 at 06:56

StKiller · Answer 5 · 2019-02-09T04:39:28.853

7

This can be expanded to:

if(!true){
   return 'false';
} else {
   return 'true';
}

edited Feb 09 '19 at 04:39

answered Apr 08 '11 at 06:12

StKiller

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score 1 · Answer 6 · edited Aug 24 '13 at 05:14

1

if(!true) is equivalent to if(!true= true) which is equivalent to if(false=true) which is false. Therefore return (true) which is on the false side of the if statement.

edited Aug 24 '13 at 05:14

Cole Tobin

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answered Aug 21 '13 at 14:32

ndabenhle

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4

You can't set true equal to false. At least in sane languages. – Cole Tobin Aug 21 '13 at 15:08
2

I think you mean `if(!true==true)` – Carlos Muñoz Oct 31 '13 at 19:58
1

JS is a sane language: `ReferenceError: invalid assignment left-hand side` – Oriol Nov 02 '13 at 23:28

score 1 · Answer 7 · answered Aug 21 '13 at 15:41

The confusion lies here because of the use of string literals to represent boolean values. If you reverse the 'false' and 'true', it makes more sense:

(!true) ? 'true' : 'false'

Would return the string literal false, which is much different than a boolean value.

Your original statement (!true) ? 'false' : 'true' reads as

"If not true, then return the string literal true".

The statement I posted first reads as

"If not true, then return the string literal false".

Which, if you know the opposite (not) value of true is false, then it explains the logic illustrated.

score 0 · Answer 8 · answered Jan 25 '20 at 19:58

const test = true; // change this value to see the result change  
 
 if (!test) { // if test is NOT truthy
     console.log('false')
 } else { // if test === true
     console.log('true')
 }

// Since in the original question a ternary expression was used, the above code is equivalent to this:

!test ? console.log('false') : console.log('true');

Why is !true ? 'false' : 'true' returning 'true'?

8 Answers8

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