Javascript variable lifecycle, will the variable be copied when it is returned?

Question

It is hard to describe the situation without code. My modification made some answers irrelevant. I past the original code here and simplified version below:

function Entity(){
  var value = {};

  return {
    'value': value
  };
}

var e1 = Entity();
var e2 = Entity();
alert(e1.value === e2.value);

I thought it should return true. But in fact, it returns false. Is 'value' copied when it is returned from the function Entity?

update I think I know the reason now. each time Entity function is called the expression "var value={};" will generate a new Object. Thanks.

possible duplicate of [How do you determine equality for two JavaScript objects?](http://stackoverflow.com/questions/201183/how-do-you-determine-equality-for-two-javascript-objects) — jeremy, Mar 30 '13 at 03:27
I know that {0,1} doesn't equal to {0,1}. My question is why e1.value and e2.value (in the simplified version) are not referencing to the same instance in the function. — Devs love ZenUML, Mar 30 '13 at 04:26
Because the reference is recreated in each scope you create (i.e., a new "`value`" each time). Look [here](http://jsfiddle.net/k2hx6/): both `b` and `c` contain a reference to the original `a`. However, [here](http://jsfiddle.net/k2hx6/1/), both are referring to a different "`a`" — jeremy, Mar 30 '13 at 04:34
Also - do you mind getting rid of the original version in your code above? It's just confusing to readers. — jeremy, Mar 30 '13 at 04:38

score 1 · Accepted Answer · edited Mar 30 '13 at 05:03

1

value is not copied when its returned, but a new Object is created whenever you run the Entity function.

You can observe the "new Object" creation with simple code like

console.log({} === {}) //should give false

var a = {};
console.log(a === a); //should give true

And you can check that things don't get copyed on return by assigning to more variables when running your function

var a,b,c;

function Entity(){
   var value = {};
   a = value;
   b = value;
   return value;
}

c = Entity();

console.log(a==b, a==c, b==c); //should all give true

edited Mar 30 '13 at 05:03

Devs love ZenUML

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answered Mar 30 '13 at 03:25

hugomg

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1

It might also be worth mentioning that in general distinct Objects are never equal (in strict or abstract comparison), and that only numbers, strings and booleans can be compared. – Asad Saeeduddin Mar 30 '13 at 03:27

score 0 · Answer 2 · answered Mar 30 '13 at 03:24

0

[elements...] syntax creates a new array. In javascript, === operator compares arrays by reference (not by contents), so the result is false

answered Mar 30 '13 at 03:24

glebm

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Do you mind delete your answer? Because after I revised the question, it is not applicable now. Thanks. – Devs love ZenUML Mar 30 '13 at 04:45

plalx · Answer 3 · 2013-03-30T03:43:32.833

Your function is currently creating a new object each time it is getting called.

Keeping the same interface as in your example, to share the same array, you could do the following ->

Entity = (function () {
    //this variable is accessible only in the scope of this function
    var messageBoxes = ['hello'];

    //we return a function that return an object that will make the private messageBoxes variable accessible through it's messageBoxes property.
    return function () {
        return { messageBoxes: messageBoxes };
    };
})(); //note that this function will be self-executing


alert(Entity().messageBoxes === Entity().messageBoxes);

However, you would not be gaining anything here since you would be making the private messageBoxes variable publicly accessible directly.

Javascript variable lifecycle, will the variable be copied when it is returned?

3 Answers3