Why does this Javascript example copy the variable value instead of pass by reference?

Question

In Javascript The Good Parts, it states:

alt text

So I would expect the following code example to output 1001 since "objects are never copied but passed around by reference", so why does it output 0000?

var page_item = {
  id_code : 'welcome',
  title : 'Welcome',
  access_groups : {
      developer : '0010',
      administrator : '0100'
  }
};
page_item.access_groups.member = '0000';
var member = page_item.access_groups.member;
member = '1001';

$('p#test').html(page_item.access_groups.member); //should be "1001" but is "0000"

Added:

@Gareth @David, thanks, this is what I was trying to show in this example, works:

var page_item = {
  id_code : 'welcome',
  title : 'Welcome',
  access_groups : {
      developer : '0010',
      administrator : '0100'
  }
};
var page_item2 = page_item;
page_item2.access_groups.developer = '1001';

$('p#test').html(page_item.access_groups.developer); //is '1001'

This is the same in Python, and easily explained there (x.y = ... is a method call on x's __dict__, x = ... is not a method call). @Potential answerers: Is there a similar reasoning in JS? — user395760
– user395760, Commented Dec 14, 2010 at 15:49
@delnan - x.y = is not a method call in Javascript. (Well, it is possible in ECMAScript to define setter and getter methods for object properties but that's not the paradigm which caused the confusion in this question) — Gareth
– Gareth, Commented Dec 14, 2010 at 15:54

Gareth · Accepted Answer · 2010-12-14 15:41:15Z

4

Don't think of pass-by-reference in the C++ context, because it's not the same.

var member = page_item.access_groups.member // Sets member to this value
member = '1001'; // Now sets it to another value

If there was a method on strings which changed them, then this:

member.removeLastLetter();

would alter page_item.access_groups.member. However, with your = you are changing the variable's reference, not the object it previously referenced

answered Dec 14, 2010 at 15:41

Gareth

139k36 gold badges152 silver badges158 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Sam Over a year ago

I was thinking about whether this is actually correct or not, and a key thing to note is that String is an object too. After I realized that, it made more sense to me.

Quentin · Accepted Answer · 2010-12-14 15:40:41Z

1

Because page_item.access_groups.member is a string and not an Object.

answered Dec 14, 2010 at 15:40

Quentin

948k136 gold badges1.3k silver badges1.4k bronze badges

1 Comment

Gareth Over a year ago

Even if page_item.access_groups.member = {}, a following line var member = page_item.access_groups.member; member = {foo:1001} wouldn't update the page_item.access_groups.member object

Mike · Accepted Answer · 2010-12-14 15:53:26Z

0

This is probably getting bashed by JS-Gurus but basically it goes down like this:

Objects are passed by reference.
Strings (numbers, etc... basically 1 dimensional variables) are passed by value.

I did try and understand the lengthy explanations on data types but I seriously needed some work done and haven't gotten time to look at it more closely.

answered Dec 14, 2010 at 15:53

Mike

2,5973 gold badges23 silver badges36 bronze badges

1 Comment

Gareth Over a year ago

PBR and PBV are basically myths in javascript. The main point is, if you have a reference to anything (string or otherwise) in a variable var x, as soon as you write x = ... you are discarding your initial reference and creating a new one, as opposed to performing the an operation on the value you had previously referenced.

Collectives™ on Stack Overflow

Why does this Javascript example copy the variable value instead of pass by reference?

Added:

3 Answers 3

1 Comment

1 Comment

1 Comment

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

Added:

3 Answers 3

1 Comment

1 Comment

1 Comment

Your Answer

Sign up or log in

Post as a guest

Linked

Related