Pass by reference or pass by value

Question

Does object in javascript pass by reference? If yes why this code not working.

function change(myObj)
{
 myObj={};
 myObj.a=2;
}
 o={a:1};
change(o);
alert(o.a); //alert 1

but when I do

function change(myObj)
{

 myObj.a=2;
}
 o={a:1};
change(o);
alert(o.a); //alert 2

http://stackoverflow.com/questions/9437981/object-pass-by-reference — Pedro del Sol, Oct 24 '12 at 11:43

Vikdor · Answer 1 · 2012-10-24T11:49:21.583

3

The reference to the object is passed to methods. In your first example you are modifying the reference to point to a different object instead of trying to change the object pointed to by the reference and so, the changes are not visible outside that method.

In your second example, you are changing the state of the object pointed to by myobj reference and hence are visible outside the method.

edited Oct 24 '12 at 11:49

answered Oct 24 '12 at 11:42

Vikdor

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no, like references in java. In C, if you pass a pointer to a method, change the pointer to point to a different memory location, that change is visible in the called method. – Vikdor Oct 24 '12 at 12:03

score 3 · Accepted Answer · answered Oct 24 '12 at 17:26

I'll explain this thoroughly

o={a:1};
first you set global variable o to be a reference of new anonymous object that have attribute variable a with value = 1 name this object {a:1} as '1A'

change(o);
now you call function change and javascript check of typeof variable o and it's 'object'
actually it's should be 'reference that pointed to object' so the reference of object {a:1} is pass into function change by the way if variable is primitive it will pass only by value

function change(myObj){
now function change create variable myObj with typeof 'undefined' by default and then change to 'object' because it got reference parameter and now the variable myObj is a reference variable that pointed to object '1A' => {a:1} and myObj is visible only in function change and global variable o maybe still point to object '1A' => {a:1} if myObj is just a copy of reference to object '1A' => {a:1} by language design

myObj={};
now the reference variable myObj is point to new anonymous empty object {} name this object as '2B'

myObj.a=2; }
now you use reference myObj to set object '2B' => {} to have new attribute a with value = 2 and end the scope of function that mean global can't see object '2B' => {a:2}

alert(o.a); //alert 1
variable still point or may be point back to object {a:1} ,reference o can't lose it point,
because object '2B' => {a:2} can't be seen outside function change
and will be destroyed by garbage collection because it's lost the reference
and object '1A' => {a:1} can't be destroyed by garbage collection
because variable o still point at it that why you call o you receive object '1A' => {a:1}

sorry for my bad grammar but I try my best to make it easy to read.

Lews Therin · Answer 3 · 2012-10-24T12:02:08.783

In this case you are overwriting the reference to actual object.

function change(myObj)
{
 myObj={}; //overwriting the reference in myObj here.
 myObj.a=2;
}
 o={a:1};
change(o);
alert(o.a); //alert 1

Think of it this way:

function change(myObj = referenceToObjectPassedIn)
 myObj = new Object() ; 
  // referenceToObjectPassedIn lost in limbo
 myObject.a = 2;   //modifying newly created object.

To solve your problem you will have to return the new object.

score 0 · Answer 4 · answered Oct 24 '12 at 11:58

In example 1 you change what myObj references, so you end up changing a different object then the one you intended. When

alert(o.a);

is executed, o.a hasn't been changed since the object declaration, so 1 is returned. I hope that helps a bit. Also read this and it should clear some things up, helped me out when I was first learning JavaScript: http://www.snook.ca/archives/javascript/javascript_pass/.

Pass by reference or pass by value

4 Answers4