javascript variable pointers inside functions

Question

So here's a strange one:

var a = true;
function f(v){ v = !v; }        //can this function actually change a?
f(a);   //a is still true..

The function obviously changes only the value if it's internal v variable.

I came across this working on an application that should change the values of properties of DOM Elements dynamically at run time. So ideally there should be a function that takes a property as an argument and reassigns it, right?

for clarity - I am actually trying to assign new values to DOM element's attributes at runtime: I have a reflector application that iterates through members of objects and I want it to be able to change the value of properties that it comes across, so the f function would belong to a BooleanSwitch constructor function that builds a booleanSwitch object for every boolean type property in the reflected object. the booleanSwitch can be a simple button on screen. I want to be able to change the value of the actual property of the object at hand just by clicking the button.. can anyone think of a way to do this?

Any thoughts are appriciated..

Are you sure the function you came across didn't have a return statement, or accept an Object argument? — I Hate Lazy
– I Hate Lazy, Commented Oct 28, 2012 at 0:14
...and are you sure the function was named switch? This would not be a valid function name. — I Hate Lazy
– I Hate Lazy, Commented Oct 28, 2012 at 0:18
Sounds like an XY problem. What are you actually trying to do? — Matt Ball
– Matt Ball, Commented Oct 28, 2012 at 0:24
Please update your code as already pointed out. Your question is not a problem according to the code, as long as you are not trying to assign any value to variable a inside the function body, you're just assigning the variable v passed as an argument. — dbf
– dbf, Commented Oct 28, 2012 at 0:27
@..607 I came across the problem,not the code.but ur right aboat the switch keyword, I've fixed that. — Ido Ofir
– Ido Ofir, Commented Oct 28, 2012 at 10:29

Andre Meinhold · Accepted Answer · 2012-10-28 00:13:21Z

3

Primitive values are passed by value. You do pass a boolean value into that function, so it can't change the value of the variable a, declared outside.

answered Oct 28, 2012 at 0:13

Andre Meinhold

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Comments

Niall · Accepted Answer · 2012-10-28 00:16:58Z

1

Objects (arrays, functions) are passed as reference. All other types (int, string etc) are copied.

In your example, 'v' is a copy of 'a', so 'a' does not change.

You need to return a value from the switch function, and assign it to a. eg:

function switch(v)
{
//do something
return v;
}

a = switch(a);

answered Oct 28, 2012 at 0:16

Niall

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1 Comment

Ido Ofir Over a year ago

yes, this would obviously work, but my real problem is that I don't know what 'a' is. this would only be known at run time. so I can't hardcode the assignment into 'a', I've edited the question for clarity..thnx.

Isen · Accepted Answer · 2012-10-28 11:27:10Z

1

The function must return a value.

function switchValue(v){ 
  return !v;
}        

var a = true;
switchValue(a);

This is because of JavaScript referencing feature. If you pass in the variable and dont return it, the variable will only change locally. You have to specifically return it, if you want it to be changed.

edited Oct 28, 2012 at 11:27

answered Oct 28, 2012 at 0:22

Isen

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2 Comments

dbf Over a year ago

And function switch magically knows where the variable a is? And this code will not run ..

Isen Over a year ago

change the function name to something else other than switch(), for example switchValue(v)

Ido Ofir · Accepted Answer · 2012-10-28 13:01:35Z

so just figured out a nice way to do this:

booleans, as other primitive values are passed by value, but objects are passed by reference.. the boolean value passed to function f can be passed by reference if it belongs to an object. so:

var a = true;
function f(v, z){
    v[z] = !v[z];
{
f(window, "a");

which looks a bit strange and is still hard coded, but can be made to work dynamically with properties when iterating through objects.

in my real application it looks something like this:

function BooleanSwitch(obj, property){
    var btn = document.createElement("button");
    btn.onclick = function(){
        obj[property] = !obj[property];
    }
    return btn;
}

function reflect(obj){
    for(member in obj){
        switch(typeof(obj[member])){
            case "boolean":
                var booleanSwitch = BooleanSwitch(obj, member);
                document.body.appendChild(booleanSwitch);  //or whatever..
                break;
            case "string": //..other stuff..
        }
    }
}

reflect(document);

this will create a boolean switch for each enumerable boolean property of the document object. when you click this button it changes the actual property of the document.

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