Javascript Variables as Object Pointers

Question

I have a question out of curiosity. So I looked into how JS handles variable assignment and I get it. How does variable assignment work in JavaScript?

But the same principle doesn't seem to exhibit itself in the following code I am working on:

var temp = playlist1[0];
playlist1[0] = playlist1[1];
playlist1[1] = temp;

I know this is a standard way to swap array elements. But if temp is pointing at playlist1[0], and playlist1[0]'s contents are changed to playlist1[1]'s then how come I don't end up with two playlist1[1] values in a row?

possible duplicate of [How does variable assignment work in JavaScript?](http://stackoverflow.com/questions/509579/how-does-variable-assignment-work-in-javascript) — Evan Trimboli, Jul 26 '13 at 03:21
I voted this as a duplicate because the answer in the question you linked is the answer to your question. — Evan Trimboli, Jul 26 '13 at 03:22
@EvanTrimboli I think the question is fair since it asks for further clarification on an existing question. The other question is old and probably not monitored anymore. — TGH, Jul 26 '13 at 05:38
OP argues that he asked this question to further his understanding of the topic. Upon initial interpretation (not coming from a place of 'knowing already') the linked question does not clearly answer the question. Upon final interpretation (coming from a place of 'knowing already') then the OP sees how the linked question may help; as a reference text and not a learning text. But OP maintains that this question adds value to the linked question and deepens the learning as to avoid a probable misconception of the variable as object pointer concept. — nemo, Jul 26 '13 at 17:54

score 12 · Accepted Answer · answered Jul 26 '13 at 05:40

Not only variables are object pointers. All values (that are not primitives) are object pointers. So temp is an object pointer. playlist1 is a object pointer to an array object whose elements are object pointers. e.g. playlist1[0] is an object pointer, playlist1[1] is an object pointer, etc.

But if temp is pointing at playlist1[0]

This doesn't make sense. temp is an object pointer. It points to an object. playlist1[0] is not an object; it's an object pointer. temp = playlist1[0]; makes the object pointer temp point to the same object as object pointer playlist1[0].

If you know C, it is equivalent to something like this:

Object *playlist1[10];

Object *temp = playlist1[0];
playlist1[0] = playlist1[1];
playlist1[1] = temp;

OP's takeaway: array elements are not objects, they too are pointers to objects. This answered my question and deepened my understanding of the mechanisms. — nemo, Jul 26 '13 at 17:50

TGH · Answer 2 · 2013-07-26T03:30:56.353

2

This is consistent with the answer in the referenced question: You are just changing which object the variable points to - not the data it used to point to. Meaning temp is unaffected by the move to have playlist1[1] point to playlist1[2]. Temp retains the original value it pointed to when playlis1[1] and temp both pointed to it. Only playlist1[1] is updated

edited Jul 26 '13 at 03:30

answered Jul 26 '13 at 03:20

TGH

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score 0 · Answer 3 · answered Jul 26 '13 at 03:39

Because those are still references to elements in the array and not the elements themselves. So in the line:

playlist[1]=playlist[2]

You are not changing anything about temp. Contrast that with something like (assuming array elements were objects):

playlist[1].x=playlist[2].x

That is actually assigning the value of the object in the array, and if temp pointed to playlist[1], then temp.x would equal playlist[2].x

bortunac · Answer 4 · 2015-09-11T12:15:51.370

say we have obj={l1:{l2:[1,2]},} and we want to address obj.l1.l2[1] using an array of levels like arr=["l1","l2",1] then :

Object.defineProperty(Object.prototype,'point',{
    value:function(arr){
        var rez=this;
        for(var s in arr){
            rez=rez[arr[s]];
            if(rez === undefined) return undefined;
        }
        return rez;
    }
});

So after defining "point" method (which is not enumerable to mess up everithing) we can use

obj.point(arr)

to get value 2

Javascript Variables as Object Pointers

4 Answers4