How to negate bool inside function in JS?

Question

I'm writing some script now and I have a problem when trying to negate boolean inside a function. I mean this:

var test = true;

function changeThisBoolPlease(asd){
  asd=!asd;
}

alert(test);
  changeThisBoolPlease(test);
alert(test);

alerts true, then true.

Any ideas? Isn't JS reference perfect?

EDIT:

Ok, this was only a part of my function:

function przesun(kolor, figury, castlings, x1, y1, x2, y2, strona) {
    kolor = nowaPozycjaKolor(kolor,x1, y1, x2, y2);
    figury = nowaPozycjaFigur(figury,x1, y1, x2, y2);
    strona = !strona;
}

Actually I cannot return this value. How to?

do you want to return `strona`? if so just return it. and assign the return. — robbmj, Dec 14 '13 at 22:42
please see my answer, I believe it will answer your question — robbmj, Dec 14 '13 at 22:44
The edit doesn't change anything. Changing the value of the arguments to the function just changes the arguments to the function. It has no effect on whatever variables (if any) were used when sending those values to the function. — T.J. Crowder, Dec 14 '13 at 22:44
@robbmj: When you answered the question, the OP received a notification. That's sufficient, don't comment saying "see my answer." — T.J. Crowder, Dec 14 '13 at 22:44
fair enough, my comment was more to let the OP know that even after their edit the answer does not need to change. — robbmj, Dec 14 '13 at 22:45
I don't want this function to return anything. I changed strona to object element. It has to be fully dynamic, so i won't use (for obvious reason) "not recommended" function ;) — adam, Dec 14 '13 at 22:47

robbmj · Answer 1 · 2013-12-14T22:40:12.510

4

You are just changing the value of asd in the example in your question.

try this

var test = true;

function changeThisBoolPlease(asd){
    return !asd;
}

alert(test);
test = changeThisBoolPlease(test);
alert(test);

Alternatively but not recommended, you could do it this way

var test = true;

function changeTestBoolPlease(){
    test = !test;
}

alert(test);
changeTestBoolPlease();
alert(test);

edited Dec 14 '13 at 22:40

answered Dec 14 '13 at 22:34

robbmj

16,085
8
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Adassko · Accepted Answer · 2013-12-14T22:48:27.973

2

Objects are not passed by reference but by value which is a reference (copy of reference)...

In your example you're not even passing an object but a primitive value type.

If you want a reference, then you need to wrap it in object element like:

var test = { val: true };

function changeThisBoolPlease(asd){
    asd.val=!asd.val;
}

alert(test.val);
changeThisBoolPlease(test);
alert(test.val);

edited Dec 14 '13 at 22:48

answered Dec 14 '13 at 22:35

Adassko

5,201
20
37

1

*"It's not passed by reference but by value which is a reference..."* No, the value is a boolean, not a reference. – T.J. Crowder Dec 14 '13 at 22:40
yes, you are right in this case but I assumed that `true` was only an example (which it was) and in reality he's passing an object. Edited – Adassko Dec 14 '13 at 22:47

score 1 · Answer 3 · answered Dec 14 '13 at 22:35

1

It's a scoping issue. Just return and set:

var test = true;

function changeThisBoolPlease(asd){
    return !asd;
}

alert(test);
test = changeThisBoolPlease(test);
alert(test);

answered Dec 14 '13 at 22:35

crad

433
3
6

How to negate bool inside function in JS?

3 Answers3