reference type vs. primitive type

[ThetaClear]

Hi, I know that objects in javascript, like in many other languages, holds the reference to the object itself and primitive types like number bool holds the actual value. I have something like this:

function manager()
{

var anObject = {prop1: 1, prop2: 2};

.
.
.

return
{
getMyObject: function(){

return anObject;
}

}

My question is why when I do something like this:

var m = new manager();
var o = m.getMyObject();
o = {a: 10, b: 20};

The actual value in 'o' which is a reference type is not changed when I do this:

var o2 = m.getMyObject();

what is actually set in the variable 'o'? and how can I make it persist in the above

[Richard Deeming]

Your code currently does the following:

1. Store the object returned from getMyObject in a variable called o;
2. Store a new object in the o variable;
3. Store the object returned from getMyObject in a variable called o2;

It should be fairly obvious why o and o2 do not point to the same object - step 2 makes o point to a new, different object. If you want your changes to the object to be persisted for future calls of the getMyObject function, you need to change the properties of the object returned from that function:

var m = new manager();
var o = m.getMyObject();

o.prop1 = 10;
o.prop2 = 20;

var o2 = m.getMyObject();
// o2 contains { prop1: 10, prop2: 20 }

---

"These people looked deep within my soul and assigned me a number based on the order in which I joined." - Homer

[ThetaClear]

Thanks for your reply Richard, I understood the sequence but there's still something not clear for me. A variable which holds a reference, basically holds a pointer to an object. If I set variable 'a' which holds a reference to object1 to a new variable called 'b', both of them will have a pointer that should point to the same place i.e. object1. Maybe I'm confusing it with a language like c# but if I set a new object to variable 'a' called 'object2', I'm creating a new pointer to that new object. I expect the place in memory where the original pointer which pointed to 'object1' will change and point to object2 for both 'a' and 'b'. But I see that's not happening. 'a' points to the new object while 'b' still points to the older one.

[Richard Deeming]

No. If variables a and b point to object1, and you change variable a to point to object2, variable b will not be affected. If you're more comfortable with C#, consider the following contrived example:

string a = string.Empty;
string b = string.Empty;

a = "Hello";

What value do you think will be in b after you run this code? If the language followed your idea, since both variables start out pointing to the same string, when you change a to point to the string "Hello", it would automatically change b as well. That would make it virtually impossible to write a program, since all variables and fields of the same type would automatically be pointing to the same instance of that type. Think of it like postal addresses: person a lives at address1, and person b also lives at address1. If you change the street-name for address1, that change affects both people. If person a moves to address2, that doesn't necessarily mean that person b moves as well.

---

"These people looked deep within my soul and assigned me a number based on the order in which I joined." - Homer

[ThetaClear]

Great, that was a good explanation. Thanks Richard!

[twseitex]

constructor of "ype "m" is function manager(){...} m = new constructor (...) m is a pointer ! e.g. new Function(...) creates a pointer of a function new Function() is a member of Javascript predefines types (predefined constructor) so manager() runs like new Function() ? function manager() { var anObject = {prop1: 1, prop2: 2}; // local var !!!! // does not exists after the end of constructor // use global var ! . . . return { getMyObject: function(){ return anObject; } // local function !!!! // does not exists after the end of constructor // use global var to save the pointer. }

[User 11056480]

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